6593758: RFE: Enhance GC ergonomics to dynamically choose ParallelGCThreads

Summary: Select number of GC threads dynamically based on heap usage and number of Java threads
Reviewed-by: johnc, ysr, jcoomes

39 files changed, 1521 insertions, 229 deletions

diff --git a/src/share/vm/gc_implementation/concurrentMarkSweep/compactibleFreeListSpace.cpp b/src/share/vm/gc_implementation/concurrentMarkSweep/compactibleFreeListSpace.cpp
index eeabe9739..2f9604350 100644
--- a/src/share/vm/gc_implementation/concurrentMarkSweep/compactibleFreeListSpace.cpp
+++ b/src/share/vm/gc_implementation/concurrentMarkSweep/compactibleFreeListSpace.cpp
@@ -668,12 +668,16 @@ public:
 
 // We de-virtualize the block-related calls below, since we know that our
 // space is a CompactibleFreeListSpace.
+
 #define FreeListSpace_DCTOC__walk_mem_region_with_cl_DEFN(ClosureType)          \
 void FreeListSpace_DCTOC::walk_mem_region_with_cl(MemRegion mr,                 \
                                                  HeapWord* bottom,              \
                                                  HeapWord* top,                 \
                                                  ClosureType* cl) {             \
-   if (SharedHeap::heap()->n_par_threads() > 0) {                               \
+   bool is_par = SharedHeap::heap()->n_par_threads() > 0;                       \
+   if (is_par) {                                                                \
+     assert(SharedHeap::heap()->n_par_threads() ==                              \
+            SharedHeap::heap()->workers()->active_workers(), "Mismatch");       \
      walk_mem_region_with_cl_par(mr, bottom, top, cl);                          \
    } else {                                                                     \
      walk_mem_region_with_cl_nopar(mr, bottom, top, cl);                        \
@@ -1925,6 +1929,9 @@ CompactibleFreeListSpace::splitChunkAndReturnRemainder(FreeChunk* chunk,
   if (rem_size < SmallForDictionary) {
     bool is_par = (SharedHeap::heap()->n_par_threads() > 0);
     if (is_par) _indexedFreeListParLocks[rem_size]->lock();
+    assert(!is_par ||
+           (SharedHeap::heap()->n_par_threads() ==
+            SharedHeap::heap()->workers()->active_workers()), "Mismatch");
     returnChunkToFreeList(ffc);
     split(size, rem_size);
     if (is_par) _indexedFreeListParLocks[rem_size]->unlock();
diff --git a/src/share/vm/gc_implementation/concurrentMarkSweep/concurrentMarkSweepGeneration.cpp b/src/share/vm/gc_implementation/concurrentMarkSweep/concurrentMarkSweepGeneration.cpp
index 54bbb24b0..162b991ec 100644
--- a/src/share/vm/gc_implementation/concurrentMarkSweep/concurrentMarkSweepGeneration.cpp
+++ b/src/share/vm/gc_implementation/concurrentMarkSweep/concurrentMarkSweepGeneration.cpp
@@ -4244,9 +4244,11 @@ void CMSConcMarkingTask::coordinator_yield() {
 
 bool CMSCollector::do_marking_mt(bool asynch) {
   assert(ConcGCThreads > 0 && conc_workers() != NULL, "precondition");
-  // In the future this would be determined ergonomically, based
-  // on #cpu's, # active mutator threads (and load), and mutation rate.
-  int num_workers = ConcGCThreads;
+  int num_workers = AdaptiveSizePolicy::calc_active_conc_workers(
+                                       conc_workers()->total_workers(),
+                                       conc_workers()->active_workers(),
+                                       Threads::number_of_non_daemon_threads());
+  conc_workers()->set_active_workers(num_workers);
 
   CompactibleFreeListSpace* cms_space  = _cmsGen->cmsSpace();
   CompactibleFreeListSpace* perm_space = _permGen->cmsSpace();
@@ -5062,6 +5064,8 @@ class CMSParRemarkTask: public AbstractGangTask {
   ParallelTaskTerminator _term;
 
  public:
+  // A value of 0 passed to n_workers will cause the number of
+  // workers to be taken from the active workers in the work gang.
   CMSParRemarkTask(CMSCollector* collector,
                    CompactibleFreeListSpace* cms_space,
                    CompactibleFreeListSpace* perm_space,
@@ -5544,7 +5548,15 @@ void CMSCollector::do_remark_parallel() {
   GenCollectedHeap* gch = GenCollectedHeap::heap();
   FlexibleWorkGang* workers = gch->workers();
   assert(workers != NULL, "Need parallel worker threads.");
-  int n_workers = workers->total_workers();
+  // Choose to use the number of GC workers most recently set
+  // into "active_workers".  If active_workers is not set, set it
+  // to ParallelGCThreads.
+  int n_workers = workers->active_workers();
+  if (n_workers == 0) {
+    assert(n_workers > 0, "Should have been set during scavenge");
+    n_workers = ParallelGCThreads;
+    workers->set_active_workers(n_workers);
+  }
   CompactibleFreeListSpace* cms_space  = _cmsGen->cmsSpace();
   CompactibleFreeListSpace* perm_space = _permGen->cmsSpace();
 
@@ -5884,8 +5896,17 @@ void CMSCollector::refProcessingWork(bool asynch, bool clear_all_soft_refs) {
       // and a different number of discovered lists may have Ref objects.
       // That is OK as long as the Reference lists are balanced (see
       // balance_all_queues() and balance_queues()).
-
-      rp->set_active_mt_degree(ParallelGCThreads);
+      GenCollectedHeap* gch = GenCollectedHeap::heap();
+      int active_workers = ParallelGCThreads;
+      FlexibleWorkGang* workers = gch->workers();
+      if (workers != NULL) {
+        active_workers = workers->active_workers();
+        // The expectation is that active_workers will have already
+        // been set to a reasonable value.  If it has not been set,
+        // investigate.
+        assert(active_workers > 0, "Should have been set during scavenge");
+      }
+      rp->set_active_mt_degree(active_workers);
       CMSRefProcTaskExecutor task_executor(*this);
       rp->process_discovered_references(&_is_alive_closure,
                                         &cmsKeepAliveClosure,
diff --git a/src/share/vm/gc_implementation/g1/collectionSetChooser.cpp b/src/share/vm/gc_implementation/g1/collectionSetChooser.cpp
index 0b785656f..544b5a867 100644
--- a/src/share/vm/gc_implementation/g1/collectionSetChooser.cpp
+++ b/src/share/vm/gc_implementation/g1/collectionSetChooser.cpp
@@ -255,7 +255,18 @@ void
 CollectionSetChooser::
 prepareForAddMarkedHeapRegionsPar(size_t n_regions, size_t chunkSize) {
   _first_par_unreserved_idx = 0;
-  size_t max_waste = ParallelGCThreads * chunkSize;
+  int n_threads = ParallelGCThreads;
+  if (UseDynamicNumberOfGCThreads) {
+    assert(G1CollectedHeap::heap()->workers()->active_workers() > 0,
+      "Should have been set earlier");
+    // This is defensive code. As the assertion above says, the number
+    // of active threads should be > 0, but in case there is some path
+    // or some improperly initialized variable with leads to no
+    // active threads, protect against that in a product build.
+    n_threads = MAX2(G1CollectedHeap::heap()->workers()->active_workers(),
+                     1);
+  }
+  size_t max_waste = n_threads * chunkSize;
   // it should be aligned with respect to chunkSize
   size_t aligned_n_regions =
                      (n_regions + (chunkSize - 1)) / chunkSize * chunkSize;
@@ -265,6 +276,11 @@ prepareForAddMarkedHeapRegionsPar(size_t n_regions, size_t chunkSize) {
 
 jint
 CollectionSetChooser::getParMarkedHeapRegionChunk(jint n_regions) {
+  // Don't do this assert because this can be called at a point
+  // where the loop up stream will not execute again but might
+  // try to claim more chunks (loop test has not been done yet).
+  // assert(_markedRegions.length() > _first_par_unreserved_idx,
+  //  "Striding beyond the marked regions");
   jint res = Atomic::add(n_regions, &_first_par_unreserved_idx);
   assert(_markedRegions.length() > res + n_regions - 1,
          "Should already have been expanded");
diff --git a/src/share/vm/gc_implementation/g1/concurrentMark.cpp b/src/share/vm/gc_implementation/g1/concurrentMark.cpp
index 0cf789171..97512a0cd 100644
--- a/src/share/vm/gc_implementation/g1/concurrentMark.cpp
+++ b/src/share/vm/gc_implementation/g1/concurrentMark.cpp
@@ -458,12 +458,17 @@ bool ConcurrentMark::not_yet_marked(oop obj) const {
 #pragma warning( disable:4355 ) // 'this' : used in base member initializer list
 #endif // _MSC_VER
 
+size_t ConcurrentMark::scale_parallel_threads(size_t n_par_threads) {
+  return MAX2((n_par_threads + 2) / 4, (size_t)1);
+}
+
 ConcurrentMark::ConcurrentMark(ReservedSpace rs,
                                int max_regions) :
   _markBitMap1(rs, MinObjAlignment - 1),
   _markBitMap2(rs, MinObjAlignment - 1),
 
   _parallel_marking_threads(0),
+  _max_parallel_marking_threads(0),
   _sleep_factor(0.0),
   _marking_task_overhead(1.0),
   _cleanup_sleep_factor(0.0),
@@ -554,15 +559,17 @@ ConcurrentMark::ConcurrentMark(ReservedSpace rs,
   if (ParallelGCThreads == 0) {
     // if we are not running with any parallel GC threads we will not
     // spawn any marking threads either
-    _parallel_marking_threads =   0;
-    _sleep_factor             = 0.0;
-    _marking_task_overhead    = 1.0;
+    _parallel_marking_threads =       0;
+    _max_parallel_marking_threads =   0;
+    _sleep_factor             =     0.0;
+    _marking_task_overhead    =     1.0;
   } else {
     if (ConcGCThreads > 0) {
       // notice that ConcGCThreads overwrites G1MarkingOverheadPercent
       // if both are set
 
       _parallel_marking_threads = ConcGCThreads;
+      _max_parallel_marking_threads = _parallel_marking_threads;
       _sleep_factor             = 0.0;
       _marking_task_overhead    = 1.0;
     } else if (G1MarkingOverheadPercent > 0) {
@@ -583,10 +590,12 @@ ConcurrentMark::ConcurrentMark(ReservedSpace rs,
                          (1.0 - marking_task_overhead) / marking_task_overhead;
 
       _parallel_marking_threads = (size_t) marking_thread_num;
+      _max_parallel_marking_threads = _parallel_marking_threads;
       _sleep_factor             = sleep_factor;
       _marking_task_overhead    = marking_task_overhead;
     } else {
-      _parallel_marking_threads = MAX2((ParallelGCThreads + 2) / 4, (size_t)1);
+      _parallel_marking_threads = scale_parallel_threads(ParallelGCThreads);
+      _max_parallel_marking_threads = _parallel_marking_threads;
       _sleep_factor             = 0.0;
       _marking_task_overhead    = 1.0;
     }
@@ -609,7 +618,7 @@ ConcurrentMark::ConcurrentMark(ReservedSpace rs,
 
     guarantee(parallel_marking_threads() > 0, "peace of mind");
     _parallel_workers = new FlexibleWorkGang("G1 Parallel Marking Threads",
-         (int) _parallel_marking_threads, false, true);
+         (int) _max_parallel_marking_threads, false, true);
     if (_parallel_workers == NULL) {
       vm_exit_during_initialization("Failed necessary allocation.");
     } else {
@@ -1106,6 +1115,33 @@ public:
   ~CMConcurrentMarkingTask() { }
 };
 
+// Calculates the number of active workers for a concurrent
+// phase.
+int ConcurrentMark::calc_parallel_marking_threads() {
+
+  size_t n_conc_workers;
+  if (!G1CollectedHeap::use_parallel_gc_threads()) {
+    n_conc_workers = 1;
+  } else {
+    if (!UseDynamicNumberOfGCThreads ||
+        (!FLAG_IS_DEFAULT(ConcGCThreads) &&
+         !ForceDynamicNumberOfGCThreads)) {
+      n_conc_workers = max_parallel_marking_threads();
+    } else {
+      n_conc_workers =
+        AdaptiveSizePolicy::calc_default_active_workers(
+                                     max_parallel_marking_threads(),
+                                     1, /* Minimum workers */
+                                     parallel_marking_threads(),
+                                     Threads::number_of_non_daemon_threads());
+      // Don't scale down "n_conc_workers" by scale_parallel_threads() because
+      // that scaling has already gone into "_max_parallel_marking_threads".
+    }
+  }
+  assert(n_conc_workers > 0, "Always need at least 1");
+  return (int) MAX2(n_conc_workers, (size_t) 1);
+}
+
 void ConcurrentMark::markFromRoots() {
   // we might be tempted to assert that:
   // assert(asynch == !SafepointSynchronize::is_at_safepoint(),
@@ -1116,9 +1152,20 @@ void ConcurrentMark::markFromRoots() {
 
   _restart_for_overflow = false;
 
-  size_t active_workers = MAX2((size_t) 1, parallel_marking_threads());
+  // Parallel task terminator is set in "set_phase()".
   force_overflow_conc()->init();
-  set_phase(active_workers, true /* concurrent */);
+
+  // _g1h has _n_par_threads
+
+  _parallel_marking_threads = calc_parallel_marking_threads();
+  assert(parallel_marking_threads() <= max_parallel_marking_threads(),
+    "Maximum number of marking threads exceeded");
+  _parallel_workers->set_active_workers((int)_parallel_marking_threads);
+  // Don't set _n_par_threads because it affects MT in proceess_strong_roots()
+  // and the decisions on that MT processing is made elsewhere.
+
+  assert( _parallel_workers->active_workers() > 0, "Should have been set");
+  set_phase(_parallel_workers->active_workers(), true /* concurrent */);
 
   CMConcurrentMarkingTask markingTask(this, cmThread());
   if (parallel_marking_threads() > 0) {
@@ -1181,6 +1228,7 @@ void ConcurrentMark::checkpointRootsFinal(bool clear_all_soft_refs) {
                                        true /* expected_active */);
 
     if (VerifyDuringGC) {
+
       HandleMark hm;  // handle scope
       gclog_or_tty->print(" VerifyDuringGC:(after)");
       Universe::heap()->prepare_for_verify();
@@ -1463,12 +1511,20 @@ public:
   G1ParFinalCountTask(G1CollectedHeap* g1h, CMBitMap* bm,
                       BitMap* region_bm, BitMap* card_bm)
     : AbstractGangTask("G1 final counting"), _g1h(g1h),
-      _bm(bm), _region_bm(region_bm), _card_bm(card_bm) {
-    if (ParallelGCThreads > 0) {
-      _n_workers = _g1h->workers()->total_workers();
+    _bm(bm), _region_bm(region_bm), _card_bm(card_bm),
+    _n_workers(0)
+  {
+    // Use the value already set as the number of active threads
+    // in the call to run_task().  Needed for the allocation of
+    // _live_bytes and _used_bytes.
+    if (G1CollectedHeap::use_parallel_gc_threads()) {
+      assert( _g1h->workers()->active_workers() > 0,
+        "Should have been previously set");
+      _n_workers = _g1h->workers()->active_workers();
     } else {
       _n_workers = 1;
     }
+
     _live_bytes = NEW_C_HEAP_ARRAY(size_t, _n_workers);
     _used_bytes = NEW_C_HEAP_ARRAY(size_t, _n_workers);
   }
@@ -1485,6 +1541,7 @@ public:
     calccl.no_yield();
     if (G1CollectedHeap::use_parallel_gc_threads()) {
       _g1h->heap_region_par_iterate_chunked(&calccl, i,
+                                            (int) _n_workers,
                                             HeapRegion::FinalCountClaimValue);
     } else {
       _g1h->heap_region_iterate(&calccl);
@@ -1600,6 +1657,7 @@ public:
                                            &hrrs_cleanup_task);
     if (G1CollectedHeap::use_parallel_gc_threads()) {
       _g1h->heap_region_par_iterate_chunked(&g1_note_end, i,
+                                            _g1h->workers()->active_workers(),
                                             HeapRegion::NoteEndClaimValue);
     } else {
       _g1h->heap_region_iterate(&g1_note_end);
@@ -1707,6 +1765,9 @@ void ConcurrentMark::cleanup() {
 
   HeapRegionRemSet::reset_for_cleanup_tasks();
 
+  g1h->set_par_threads();
+  size_t n_workers = g1h->n_par_threads();
+
   // Do counting once more with the world stopped for good measure.
   G1ParFinalCountTask g1_par_count_task(g1h, nextMarkBitMap(),
                                         &_region_bm, &_card_bm);
@@ -1715,9 +1776,10 @@ void ConcurrentMark::cleanup() {
                                                HeapRegion::InitialClaimValue),
            "sanity check");
 
-    int n_workers = g1h->workers()->total_workers();
-    g1h->set_par_threads(n_workers);
+    assert(g1h->n_par_threads() == (int) n_workers,
+      "Should not have been reset");
     g1h->workers()->run_task(&g1_par_count_task);
+    // Done with the parallel phase so reset to 0.
     g1h->set_par_threads(0);
 
     assert(g1h->check_heap_region_claim_values(
@@ -1767,8 +1829,7 @@ void ConcurrentMark::cleanup() {
   double note_end_start = os::elapsedTime();
   G1ParNoteEndTask g1_par_note_end_task(g1h, &_cleanup_list);
   if (G1CollectedHeap::use_parallel_gc_threads()) {
-    int n_workers = g1h->workers()->total_workers();
-    g1h->set_par_threads(n_workers);
+    g1h->set_par_threads((int)n_workers);
     g1h->workers()->run_task(&g1_par_note_end_task);
     g1h->set_par_threads(0);
 
@@ -1797,8 +1858,7 @@ void ConcurrentMark::cleanup() {
     double rs_scrub_start = os::elapsedTime();
     G1ParScrubRemSetTask g1_par_scrub_rs_task(g1h, &_region_bm, &_card_bm);
     if (G1CollectedHeap::use_parallel_gc_threads()) {
-      int n_workers = g1h->workers()->total_workers();
-      g1h->set_par_threads(n_workers);
+      g1h->set_par_threads((int)n_workers);
       g1h->workers()->run_task(&g1_par_scrub_rs_task);
       g1h->set_par_threads(0);
 
@@ -1816,7 +1876,7 @@ void ConcurrentMark::cleanup() {
 
   // this will also free any regions totally full of garbage objects,
   // and sort the regions.
-  g1h->g1_policy()->record_concurrent_mark_cleanup_end();
+  g1h->g1_policy()->record_concurrent_mark_cleanup_end((int)n_workers);
 
   // Statistics.
   double end = os::elapsedTime();
@@ -2187,7 +2247,7 @@ void ConcurrentMark::weakRefsWork(bool clear_all_soft_refs) {
 
     // We use the work gang from the G1CollectedHeap and we utilize all
     // the worker threads.
-    int active_workers = g1h->workers() ? g1h->workers()->total_workers() : 1;
+    int active_workers = g1h->workers() ? g1h->workers()->active_workers() : 1;
     active_workers = MAX2(MIN2(active_workers, (int)_max_task_num), 1);
 
     G1CMRefProcTaskExecutor par_task_executor(g1h, this,
@@ -2270,7 +2330,9 @@ public:
   }
 
   CMRemarkTask(ConcurrentMark* cm) :
-    AbstractGangTask("Par Remark"), _cm(cm) { }
+    AbstractGangTask("Par Remark"), _cm(cm) {
+    _cm->terminator()->reset_for_reuse(cm->_g1h->workers()->active_workers());
+  }
 };
 
 void ConcurrentMark::checkpointRootsFinalWork() {
@@ -2282,16 +2344,21 @@ void ConcurrentMark::checkpointRootsFinalWork() {
 
   if (G1CollectedHeap::use_parallel_gc_threads()) {
     G1CollectedHeap::StrongRootsScope srs(g1h);
-    // this is remark, so we'll use up all available threads
-    int active_workers = ParallelGCThreads;
+    // this is remark, so we'll use up all active threads
+    int active_workers = g1h->workers()->active_workers();
+    if (active_workers == 0) {
+      assert(active_workers > 0, "Should have been set earlier");
+      active_workers = ParallelGCThreads;
+      g1h->workers()->set_active_workers(active_workers);
+    }
     set_phase(active_workers, false /* concurrent */);
+    // Leave _parallel_marking_threads at it's
+    // value originally calculated in the ConcurrentMark
+    // constructor and pass values of the active workers
+    // through the gang in the task.
 
     CMRemarkTask remarkTask(this);
-    // We will start all available threads, even if we decide that the
-    // active_workers will be fewer. The extra ones will just bail out
-    // immediately.
-    int n_workers = g1h->workers()->total_workers();
-    g1h->set_par_threads(n_workers);
+    g1h->set_par_threads(active_workers);
     g1h->workers()->run_task(&remarkTask);
     g1h->set_par_threads(0);
   } else {
diff --git a/src/share/vm/gc_implementation/g1/concurrentMark.hpp b/src/share/vm/gc_implementation/g1/concurrentMark.hpp
index c724594f4..ff8b39e8b 100644
--- a/src/share/vm/gc_implementation/g1/concurrentMark.hpp
+++ b/src/share/vm/gc_implementation/g1/concurrentMark.hpp
@@ -375,7 +375,9 @@ protected:
   ConcurrentMarkThread* _cmThread;   // the thread doing the work
   G1CollectedHeap*      _g1h;        // the heap.
   size_t                _parallel_marking_threads; // the number of marking
-                                                   // threads we'll use
+                                                   // threads we're use
+  size_t                _max_parallel_marking_threads; // max number of marking
+                                                   // threads we'll ever use
   double                _sleep_factor; // how much we have to sleep, with
                                        // respect to the work we just did, to
                                        // meet the marking overhead goal
@@ -473,7 +475,7 @@ protected:
 
   double*   _accum_task_vtime;   // accumulated task vtime
 
-  WorkGang* _parallel_workers;
+  FlexibleWorkGang* _parallel_workers;
 
   ForceOverflowSettings _force_overflow_conc;
   ForceOverflowSettings _force_overflow_stw;
@@ -504,6 +506,7 @@ protected:
 
   // accessor methods
   size_t parallel_marking_threads() { return _parallel_marking_threads; }
+  size_t max_parallel_marking_threads() { return _max_parallel_marking_threads;}
   double sleep_factor()             { return _sleep_factor; }
   double marking_task_overhead()    { return _marking_task_overhead;}
   double cleanup_sleep_factor()     { return _cleanup_sleep_factor; }
@@ -709,6 +712,14 @@ public:
   CMBitMapRO* prevMarkBitMap() const { return _prevMarkBitMap; }
   CMBitMap*   nextMarkBitMap() const { return _nextMarkBitMap; }
 
+  // Returns the number of GC threads to be used in a concurrent
+  // phase based on the number of GC threads being used in a STW
+  // phase.
+  size_t scale_parallel_threads(size_t n_par_threads);
+
+  // Calculates the number of GC threads to be used in a concurrent phase.
+  int calc_parallel_marking_threads();
+
   // The following three are interaction between CM and
   // G1CollectedHeap
 
diff --git a/src/share/vm/gc_implementation/g1/g1CollectedHeap.cpp b/src/share/vm/gc_implementation/g1/g1CollectedHeap.cpp
index a1a8e0405..b0861f8e9 100644
--- a/src/share/vm/gc_implementation/g1/g1CollectedHeap.cpp
+++ b/src/share/vm/gc_implementation/g1/g1CollectedHeap.cpp
@@ -66,6 +66,18 @@ size_t G1CollectedHeap::_humongous_object_threshold_in_words = 0;
 // apply to TLAB allocation, which is not part of this interface: it
 // is done by clients of this interface.)
 
+// Notes on implementation of parallelism in different tasks.
+//
+// G1ParVerifyTask uses heap_region_par_iterate_chunked() for parallelism.
+// The number of GC workers is passed to heap_region_par_iterate_chunked().
+// It does use run_task() which sets _n_workers in the task.
+// G1ParTask executes g1_process_strong_roots() ->
+// SharedHeap::process_strong_roots() which calls eventuall to
+// CardTableModRefBS::par_non_clean_card_iterate_work() which uses
+// SequentialSubTasksDone.  SharedHeap::process_strong_roots() also
+// directly uses SubTasksDone (_process_strong_tasks field in SharedHeap).
+//
+
 // Local to this file.
 
 class RefineCardTableEntryClosure: public CardTableEntryClosure {
@@ -1156,6 +1168,7 @@ public:
   void work(int i) {
     RebuildRSOutOfRegionClosure rebuild_rs(_g1, i);
     _g1->heap_region_par_iterate_chunked(&rebuild_rs, i,
+                                          _g1->workers()->active_workers(),
                                          HeapRegion::RebuildRSClaimValue);
   }
 };
@@ -1360,12 +1373,32 @@ bool G1CollectedHeap::do_collection(bool explicit_gc,
     }
 
     // Rebuild remembered sets of all regions.
-
     if (G1CollectedHeap::use_parallel_gc_threads()) {
+      int n_workers =
+        AdaptiveSizePolicy::calc_active_workers(workers()->total_workers(),
+                                       workers()->active_workers(),
+                                       Threads::number_of_non_daemon_threads());
+      assert(UseDynamicNumberOfGCThreads ||
+             n_workers == workers()->total_workers(),
+             "If not dynamic should be using all the  workers");
+      workers()->set_active_workers(n_workers);
+      // Set parallel threads in the heap (_n_par_threads) only
+      // before a parallel phase and always reset it to 0 after
+      // the phase so that the number of parallel threads does
+      // no get carried forward to a serial phase where there
+      // may be code that is "possibly_parallel".
+      set_par_threads(n_workers);
+
       ParRebuildRSTask rebuild_rs_task(this);
       assert(check_heap_region_claim_values(
              HeapRegion::InitialClaimValue), "sanity check");
-      set_par_threads(workers()->total_workers());
+      assert(UseDynamicNumberOfGCThreads ||
+             workers()->active_workers() == workers()->total_workers(),
+        "Unless dynamic should use total workers");
+      // Use the most recent number of  active workers
+      assert(workers()->active_workers() > 0,
+        "Active workers not properly set");
+      set_par_threads(workers()->active_workers());
       workers()->run_task(&rebuild_rs_task);
       set_par_threads(0);
       assert(check_heap_region_claim_values(
@@ -2477,11 +2510,17 @@ void G1CollectedHeap::heap_region_iterate_from(HeapRegion* r,
 void
 G1CollectedHeap::heap_region_par_iterate_chunked(HeapRegionClosure* cl,
                                                  int worker,
+                                                 int no_of_par_workers,
                                                  jint claim_value) {
   const size_t regions = n_regions();
-  const size_t worker_num = (G1CollectedHeap::use_parallel_gc_threads() ? ParallelGCThreads : 1);
+  const size_t max_workers = (G1CollectedHeap::use_parallel_gc_threads() ?
+                             no_of_par_workers :
+                             1);
+  assert(UseDynamicNumberOfGCThreads ||
+         no_of_par_workers == workers()->total_workers(),
+         "Non dynamic should use fixed number of workers");
   // try to spread out the starting points of the workers
-  const size_t start_index = regions / worker_num * (size_t) worker;
+  const size_t start_index = regions / max_workers * (size_t) worker;
 
   // each worker will actually look at all regions
   for (size_t count = 0; count < regions; ++count) {
@@ -2920,6 +2959,7 @@ public:
     HandleMark hm;
     VerifyRegionClosure blk(_allow_dirty, true, _vo);
     _g1h->heap_region_par_iterate_chunked(&blk, worker_i,
+                                          _g1h->workers()->active_workers(),
                                           HeapRegion::ParVerifyClaimValue);
     if (blk.failures()) {
       _failures = true;
@@ -2937,6 +2977,10 @@ void G1CollectedHeap::verify(bool allow_dirty,
   if (SafepointSynchronize::is_at_safepoint() || ! UseTLAB) {
     if (!silent) { gclog_or_tty->print("Roots (excluding permgen) "); }
     VerifyRootsClosure rootsCl(vo);
+
+    assert(Thread::current()->is_VM_thread(),
+      "Expected to be executed serially by the VM thread at this point");
+
     CodeBlobToOopClosure blobsCl(&rootsCl, /*do_marking=*/ false);
 
     // We apply the relevant closures to all the oops in the
@@ -2981,7 +3025,10 @@ void G1CollectedHeap::verify(bool allow_dirty,
              "sanity check");
 
       G1ParVerifyTask task(this, allow_dirty, vo);
-      int n_workers = workers()->total_workers();
+      assert(UseDynamicNumberOfGCThreads ||
+        workers()->active_workers() == workers()->total_workers(),
+        "If not dynamic should be using all the workers");
+      int n_workers = workers()->active_workers();
       set_par_threads(n_workers);
       workers()->run_task(&task);
       set_par_threads(0);
@@ -2989,6 +3036,8 @@ void G1CollectedHeap::verify(bool allow_dirty,
         failures = true;
       }
 
+      // Checks that the expected amount of parallel work was done.
+      // The implication is that n_workers is > 0.
       assert(check_heap_region_claim_values(HeapRegion::ParVerifyClaimValue),
              "sanity check");
 
@@ -3402,6 +3451,10 @@ G1CollectedHeap::do_collection_pause_at_safepoint(double target_pause_time_ms) {
     assert(check_young_list_well_formed(),
       "young list should be well formed");
 
+    // Don't dynamically change the number of GC threads this early.  A value of
+    // 0 is used to indicate serial work.  When parallel work is done,
+    // it will be set.
+
     { // Call to jvmpi::post_class_unload_events must occur outside of active GC
       IsGCActiveMark x;
 
@@ -3615,7 +3668,8 @@ G1CollectedHeap::do_collection_pause_at_safepoint(double target_pause_time_ms) {
         double end_time_sec = os::elapsedTime();
         double pause_time_ms = (end_time_sec - start_time_sec) * MILLIUNITS;
         g1_policy()->record_pause_time_ms(pause_time_ms);
-        g1_policy()->record_collection_pause_end();
+        int active_gc_threads = workers()->active_workers();
+        g1_policy()->record_collection_pause_end(active_gc_threads);
 
         MemoryService::track_memory_usage();
 
@@ -4562,13 +4616,13 @@ protected:
   }
 
 public:
-  G1ParTask(G1CollectedHeap* g1h, int workers, RefToScanQueueSet *task_queues)
+  G1ParTask(G1CollectedHeap* g1h,
+            RefToScanQueueSet *task_queues)
     : AbstractGangTask("G1 collection"),
       _g1h(g1h),
       _queues(task_queues),
-      _terminator(workers, _queues),
-      _stats_lock(Mutex::leaf, "parallel G1 stats lock", true),
-      _n_workers(workers)
+      _terminator(0, _queues),
+      _stats_lock(Mutex::leaf, "parallel G1 stats lock", true)
   {}
 
   RefToScanQueueSet* queues() { return _queues; }
@@ -4577,6 +4631,20 @@ public:
     return queues()->queue(i);
   }
 
+  ParallelTaskTerminator* terminator() { return &_terminator; }
+
+  virtual void set_for_termination(int active_workers) {
+    // This task calls set_n_termination() in par_non_clean_card_iterate_work()
+    // in the young space (_par_seq_tasks) in the G1 heap
+    // for SequentialSubTasksDone.
+    // This task also uses SubTasksDone in SharedHeap and G1CollectedHeap
+    // both of which need setting by set_n_termination().
+    _g1h->SharedHeap::set_n_termination(active_workers);
+    _g1h->set_n_termination(active_workers);
+    terminator()->reset_for_reuse(active_workers);
+    _n_workers = active_workers;
+  }
+
   void work(int i) {
     if (i >= _n_workers) return;  // no work needed this round
 
@@ -4861,12 +4929,12 @@ class G1STWRefProcTaskExecutor: public AbstractRefProcTaskExecutor {
 private:
   G1CollectedHeap*   _g1h;
   RefToScanQueueSet* _queues;
-  WorkGang*          _workers;
+  FlexibleWorkGang*  _workers;
   int                _active_workers;
 
 public:
   G1STWRefProcTaskExecutor(G1CollectedHeap* g1h,
-                        WorkGang* workers,
+                        FlexibleWorkGang* workers,
                         RefToScanQueueSet *task_queues,
                         int n_workers) :
     _g1h(g1h),
@@ -5122,11 +5190,13 @@ void G1CollectedHeap::process_discovered_references() {
   // referents points to another object which is also referenced by an
   // object discovered by the STW ref processor.
 
-  int n_workers = (G1CollectedHeap::use_parallel_gc_threads() ?
-                        workers()->total_workers() : 1);
+  int active_workers = (G1CollectedHeap::use_parallel_gc_threads() ?
+                        workers()->active_workers() : 1);
 
-  set_par_threads(n_workers);
-  G1ParPreserveCMReferentsTask keep_cm_referents(this, n_workers, _task_queues);
+  assert(active_workers == workers()->active_workers(),
+         "Need to reset active_workers");
+  set_par_threads(active_workers);
+  G1ParPreserveCMReferentsTask keep_cm_referents(this, active_workers, _task_queues);
 
   if (G1CollectedHeap::use_parallel_gc_threads()) {
     workers()->run_task(&keep_cm_referents);
@@ -5192,7 +5262,6 @@ void G1CollectedHeap::process_discovered_references() {
                                       NULL);
   } else {
     // Parallel reference processing
-    int active_workers = (ParallelGCThreads > 0 ? workers()->total_workers() : 1);
     assert(rp->num_q() == active_workers, "sanity");
     assert(active_workers <= rp->max_num_q(), "sanity");
 
@@ -5225,7 +5294,9 @@ void G1CollectedHeap::enqueue_discovered_references() {
   } else {
     // Parallel reference enqueuing
 
-    int active_workers = (ParallelGCThreads > 0 ? workers()->total_workers() : 1);
+    int active_workers = (ParallelGCThreads > 0 ? workers()->active_workers() : 1);
+    assert(active_workers == workers()->active_workers(),
+           "Need to reset active_workers");
     assert(rp->num_q() == active_workers, "sanity");
     assert(active_workers <= rp->max_num_q(), "sanity");
 
@@ -5252,9 +5323,24 @@ void G1CollectedHeap::evacuate_collection_set() {
   concurrent_g1_refine()->set_use_cache(false);
   concurrent_g1_refine()->clear_hot_cache_claimed_index();
 
-  int n_workers = (ParallelGCThreads > 0 ? workers()->total_workers() : 1);
-  set_par_threads(n_workers);
-  G1ParTask g1_par_task(this, n_workers, _task_queues);
+  int n_workers;
+  if (G1CollectedHeap::use_parallel_gc_threads()) {
+    n_workers =
+      AdaptiveSizePolicy::calc_active_workers(workers()->total_workers(),
+                                     workers()->active_workers(),
+                                     Threads::number_of_non_daemon_threads());
+    assert(UseDynamicNumberOfGCThreads ||
+           n_workers == workers()->total_workers(),
+           "If not dynamic should be using all the  workers");
+    set_par_threads(n_workers);
+  } else {
+    assert(n_par_threads() == 0,
+           "Should be the original non-parallel value");
+    n_workers = 1;
+  }
+  workers()->set_active_workers(n_workers);
+
+  G1ParTask g1_par_task(this, _task_queues);
 
   init_for_evac_failure(NULL);
 
@@ -5267,6 +5353,10 @@ void G1CollectedHeap::evacuate_collection_set() {
     // The individual threads will set their evac-failure closures.
     StrongRootsScope srs(this);
     if (ParallelGCVerbose) G1ParScanThreadState::print_termination_stats_hdr();
+    // These tasks use ShareHeap::_process_strong_tasks
+    assert(UseDynamicNumberOfGCThreads ||
+           workers()->active_workers() == workers()->total_workers(),
+           "If not dynamic should be using all the  workers");
     workers()->run_task(&g1_par_task);
   } else {
     StrongRootsScope srs(this);
@@ -5275,6 +5365,7 @@ void G1CollectedHeap::evacuate_collection_set() {
 
   double par_time = (os::elapsedTime() - start_par) * 1000.0;
   g1_policy()->record_par_time(par_time);
+
   set_par_threads(0);
 
   // Process any discovered reference objects - we have
@@ -5905,6 +5996,21 @@ HeapRegion* MutatorAllocRegion::allocate_new_region(size_t word_size,
   return _g1h->new_mutator_alloc_region(word_size, force);
 }
 
+void G1CollectedHeap::set_par_threads() {
+  // Don't change the number of workers.  Use the value previously set
+  // in the workgroup.
+  int n_workers = workers()->active_workers();
+    assert(UseDynamicNumberOfGCThreads ||
+           n_workers == workers()->total_workers(),
+      "Otherwise should be using the total number of workers");
+  if (n_workers == 0) {
+    assert(false, "Should have been set in prior evacuation pause.");
+    n_workers = ParallelGCThreads;
+    workers()->set_active_workers(n_workers);
+  }
+  set_par_threads(n_workers);
+}
+
 void MutatorAllocRegion::retire_region(HeapRegion* alloc_region,
                                        size_t allocated_bytes) {
   _g1h->retire_mutator_alloc_region(alloc_region, allocated_bytes);
diff --git a/src/share/vm/gc_implementation/g1/g1CollectedHeap.hpp b/src/share/vm/gc_implementation/g1/g1CollectedHeap.hpp
index 1dc3ff166..c67075118 100644
--- a/src/share/vm/gc_implementation/g1/g1CollectedHeap.hpp
+++ b/src/share/vm/gc_implementation/g1/g1CollectedHeap.hpp
@@ -987,6 +987,16 @@ public:
 
   void set_par_threads(int t) {
     SharedHeap::set_par_threads(t);
+    // Done in SharedHeap but oddly there are
+    // two _process_strong_tasks's in a G1CollectedHeap
+    // so do it here too.
+    _process_strong_tasks->set_n_threads(t);
+  }
+
+  // Set _n_par_threads according to a policy TBD.
+  void set_par_threads();
+
+  void set_n_termination(int t) {
     _process_strong_tasks->set_n_threads(t);
   }
 
@@ -1276,6 +1286,7 @@ public:
   // i.e., that a closure never attempt to abort a traversal.
   void heap_region_par_iterate_chunked(HeapRegionClosure* blk,
                                        int worker,
+                                       int no_of_par_workers,
                                        jint claim_value);
 
   // It resets all the region claim values to the default.
diff --git a/src/share/vm/gc_implementation/g1/g1CollectorPolicy.cpp b/src/share/vm/gc_implementation/g1/g1CollectorPolicy.cpp
index 2182983a8..1491dd4d2 100644
--- a/src/share/vm/gc_implementation/g1/g1CollectorPolicy.cpp
+++ b/src/share/vm/gc_implementation/g1/g1CollectorPolicy.cpp
@@ -1024,7 +1024,7 @@ void G1CollectorPolicy::print_par_stats(int level,
   double total = 0.0;
   LineBuffer buf(level);
   buf.append("[%s (ms):", str);
-  for (uint i = 0; i < ParallelGCThreads; ++i) {
+  for (uint i = 0; i < no_of_gc_threads(); ++i) {
     double val = data[i];
     if (val < min)
       min = val;
@@ -1034,7 +1034,7 @@ void G1CollectorPolicy::print_par_stats(int level,
     buf.append("  %3.1lf", val);
   }
   buf.append_and_print_cr("");
-  double avg = total / (double) ParallelGCThreads;
+  double avg = total / (double) no_of_gc_threads();
   buf.append_and_print_cr(" Avg: %5.1lf, Min: %5.1lf, Max: %5.1lf, Diff: %5.1lf]",
     avg, min, max, max - min);
 }
@@ -1046,7 +1046,7 @@ void G1CollectorPolicy::print_par_sizes(int level,
   double total = 0.0;
   LineBuffer buf(level);
   buf.append("[%s :", str);
-  for (uint i = 0; i < ParallelGCThreads; ++i) {
+  for (uint i = 0; i < no_of_gc_threads(); ++i) {
     double val = data[i];
     if (val < min)
       min = val;
@@ -1056,7 +1056,7 @@ void G1CollectorPolicy::print_par_sizes(int level,
     buf.append(" %d", (int) val);
   }
   buf.append_and_print_cr("");
-  double avg = total / (double) ParallelGCThreads;
+  double avg = total / (double) no_of_gc_threads();
   buf.append_and_print_cr(" Sum: %d, Avg: %d, Min: %d, Max: %d, Diff: %d]",
     (int)total, (int)avg, (int)min, (int)max, (int)max - (int)min);
 }
@@ -1076,10 +1076,10 @@ void G1CollectorPolicy::print_stats(int level,
 double G1CollectorPolicy::avg_value(double* data) {
   if (G1CollectedHeap::use_parallel_gc_threads()) {
     double ret = 0.0;
-    for (uint i = 0; i < ParallelGCThreads; ++i) {
+    for (uint i = 0; i < no_of_gc_threads(); ++i) {
       ret += data[i];
     }
-    return ret / (double) ParallelGCThreads;
+    return ret / (double) no_of_gc_threads();
   } else {
     return data[0];
   }
@@ -1088,7 +1088,7 @@ double G1CollectorPolicy::avg_value(double* data) {
 double G1CollectorPolicy::max_value(double* data) {
   if (G1CollectedHeap::use_parallel_gc_threads()) {
     double ret = data[0];
-    for (uint i = 1; i < ParallelGCThreads; ++i) {
+    for (uint i = 1; i < no_of_gc_threads(); ++i) {
       if (data[i] > ret) {
         ret = data[i];
       }
@@ -1102,7 +1102,7 @@ double G1CollectorPolicy::max_value(double* data) {
 double G1CollectorPolicy::sum_of_values(double* data) {
   if (G1CollectedHeap::use_parallel_gc_threads()) {
     double sum = 0.0;
-    for (uint i = 0; i < ParallelGCThreads; i++) {
+    for (uint i = 0; i < no_of_gc_threads(); i++) {
       sum += data[i];
     }
     return sum;
@@ -1115,7 +1115,7 @@ double G1CollectorPolicy::max_sum(double* data1, double* data2) {
   double ret = data1[0] + data2[0];
 
   if (G1CollectedHeap::use_parallel_gc_threads()) {
-    for (uint i = 1; i < ParallelGCThreads; ++i) {
+    for (uint i = 1; i < no_of_gc_threads(); ++i) {
       double data = data1[i] + data2[i];
       if (data > ret) {
         ret = data;
@@ -1128,7 +1128,7 @@ double G1CollectorPolicy::max_sum(double* data1, double* data2) {
 // Anything below that is considered to be zero
 #define MIN_TIMER_GRANULARITY 0.0000001
 
-void G1CollectorPolicy::record_collection_pause_end() {
+void G1CollectorPolicy::record_collection_pause_end(int no_of_gc_threads) {
   double end_time_sec = os::elapsedTime();
   double elapsed_ms = _last_pause_time_ms;
   bool parallel = G1CollectedHeap::use_parallel_gc_threads();
@@ -1140,6 +1140,7 @@ void G1CollectorPolicy::record_collection_pause_end() {
   assert(cur_used_bytes == _g1->recalculate_used(), "It should!");
   bool last_pause_included_initial_mark = false;
   bool update_stats = !_g1->evacuation_failed();
+  set_no_of_gc_threads(no_of_gc_threads);
 
 #ifndef PRODUCT
   if (G1YoungSurvRateVerbose) {
@@ -2304,6 +2305,7 @@ public:
     ParKnownGarbageHRClosure parKnownGarbageCl(_hrSorted, _chunk_size, i);
     // Back to zero for the claim value.
     _g1->heap_region_par_iterate_chunked(&parKnownGarbageCl, i,
+                                         _g1->workers()->active_workers(),
                                          HeapRegion::InitialClaimValue);
     jint regions_added = parKnownGarbageCl.marked_regions_added();
     _hrSorted->incNumMarkedHeapRegions(regions_added);
@@ -2315,7 +2317,7 @@ public:
 };
 
 void
-G1CollectorPolicy::record_concurrent_mark_cleanup_end() {
+G1CollectorPolicy::record_concurrent_mark_cleanup_end(int no_of_gc_threads) {
   double start_sec;
   if (G1PrintParCleanupStats) {
     start_sec = os::elapsedTime();
@@ -2331,10 +2333,27 @@ G1CollectorPolicy::record_concurrent_mark_cleanup_end() {
 
   if (G1CollectedHeap::use_parallel_gc_threads()) {
     const size_t OverpartitionFactor = 4;
-    const size_t MinWorkUnit = 8;
-    const size_t WorkUnit =
-      MAX2(_g1->n_regions() / (ParallelGCThreads * OverpartitionFactor),
-           MinWorkUnit);
+    size_t WorkUnit;
+    // The use of MinChunkSize = 8 in the original code
+    // causes some assertion failures when the total number of
+    // region is less than 8.  The code here tries to fix that.
+    // Should the original code also be fixed?
+    if (no_of_gc_threads > 0) {
+      const size_t MinWorkUnit =
+        MAX2(_g1->n_regions() / no_of_gc_threads, (size_t) 1U);
+      WorkUnit =
+        MAX2(_g1->n_regions() / (no_of_gc_threads * OverpartitionFactor),
+             MinWorkUnit);
+    } else {
+      assert(no_of_gc_threads > 0,
+        "The active gc workers should be greater than 0");
+      // In a product build do something reasonable to avoid a crash.
+      const size_t MinWorkUnit =
+        MAX2(_g1->n_regions() / ParallelGCThreads, (size_t) 1U);
+      WorkUnit =
+        MAX2(_g1->n_regions() / (ParallelGCThreads * OverpartitionFactor),
+             MinWorkUnit);
+    }
     _collectionSetChooser->prepareForAddMarkedHeapRegionsPar(_g1->n_regions(),
                                                              WorkUnit);
     ParKnownGarbageTask parKnownGarbageTask(_collectionSetChooser,
diff --git a/src/share/vm/gc_implementation/g1/g1CollectorPolicy.hpp b/src/share/vm/gc_implementation/g1/g1CollectorPolicy.hpp
index b339dfb86..579384ba6 100644
--- a/src/share/vm/gc_implementation/g1/g1CollectorPolicy.hpp
+++ b/src/share/vm/gc_implementation/g1/g1CollectorPolicy.hpp
@@ -89,6 +89,9 @@ private:
   // has been set, or 1 otherwise
   int _parallel_gc_threads;
 
+  // The number of GC threads currently active.
+  uintx _no_of_gc_threads;
+
   enum SomePrivateConstants {
     NumPrevPausesForHeuristics = 10
   };
@@ -280,6 +283,9 @@ private:
                                     double update_rs_processed_buffers,
                                     double goal_ms);
 
+  uintx no_of_gc_threads() { return _no_of_gc_threads; }
+  void set_no_of_gc_threads(uintx v) { _no_of_gc_threads = v; }
+
   double _pause_time_target_ms;
   double _recorded_young_cset_choice_time_ms;
   double _recorded_non_young_cset_choice_time_ms;
@@ -287,6 +293,7 @@ private:
   size_t _max_pending_cards;
 
 public:
+  // Accessors
 
   void set_region_eden(HeapRegion* hr, int young_index_in_cset) {
     hr->set_young();
@@ -737,13 +744,13 @@ public:
   void record_concurrent_mark_remark_end();
 
   void record_concurrent_mark_cleanup_start();
-  void record_concurrent_mark_cleanup_end();
+  void record_concurrent_mark_cleanup_end(int no_of_gc_threads);
   void record_concurrent_mark_cleanup_completed();
 
   void record_concurrent_pause();
   void record_concurrent_pause_end();
 
-  void record_collection_pause_end();
+  void record_collection_pause_end(int no_of_gc_threads);
   void print_heap_transition();
 
   // Record the fact that a full collection occurred.
diff --git a/src/share/vm/gc_implementation/g1/g1RemSet.cpp b/src/share/vm/gc_implementation/g1/g1RemSet.cpp
index e8e5eebd7..dd644efbe 100644
--- a/src/share/vm/gc_implementation/g1/g1RemSet.cpp
+++ b/src/share/vm/gc_implementation/g1/g1RemSet.cpp
@@ -218,7 +218,7 @@ public:
 
 HeapRegion* G1RemSet::calculateStartRegion(int worker_i) {
   HeapRegion* result = _g1p->collection_set();
-  if (ParallelGCThreads > 0) {
+  if (G1CollectedHeap::use_parallel_gc_threads()) {
     size_t cs_size = _g1p->cset_region_length();
     int n_workers = _g1->workers()->total_workers();
     size_t cs_spans = cs_size / n_workers;
@@ -430,8 +430,10 @@ void G1RemSet::prepare_for_oops_into_collection_set_do() {
   DirtyCardQueueSet& dcqs = JavaThread::dirty_card_queue_set();
   dcqs.concatenate_logs();
 
-  if (ParallelGCThreads > 0) {
-    _seq_task->set_n_threads((int)n_workers());
+  if (G1CollectedHeap::use_parallel_gc_threads()) {
+    // Don't set the number of workers here.  It will be set
+    // when the task is run
+    // _seq_task->set_n_termination((int)n_workers());
   }
   guarantee( _cards_scanned == NULL, "invariant" );
   _cards_scanned = NEW_C_HEAP_ARRAY(size_t, n_workers());
@@ -578,7 +580,10 @@ void G1RemSet::scrub(BitMap* region_bm, BitMap* card_bm) {
 void G1RemSet::scrub_par(BitMap* region_bm, BitMap* card_bm,
                                 int worker_num, int claim_val) {
   ScrubRSClosure scrub_cl(region_bm, card_bm);
-  _g1->heap_region_par_iterate_chunked(&scrub_cl, worker_num, claim_val);
+  _g1->heap_region_par_iterate_chunked(&scrub_cl,
+                                       worker_num,
+                                       (int) n_workers(),
+                                       claim_val);
 }
 
 
diff --git a/src/share/vm/gc_implementation/parNew/parCardTableModRefBS.cpp b/src/share/vm/gc_implementation/parNew/parCardTableModRefBS.cpp
index ea75a5944..5155ca934 100644
--- a/src/share/vm/gc_implementation/parNew/parCardTableModRefBS.cpp
+++ b/src/share/vm/gc_implementation/parNew/parCardTableModRefBS.cpp
@@ -33,6 +33,7 @@
 #include "runtime/java.hpp"
 #include "runtime/mutexLocker.hpp"
 #include "runtime/virtualspace.hpp"
+#include "runtime/vmThread.hpp"
 
 void CardTableModRefBS::non_clean_card_iterate_parallel_work(Space* sp, MemRegion mr,
                                                              OopsInGenClosure* cl,
@@ -42,6 +43,11 @@ void CardTableModRefBS::non_clean_card_iterate_parallel_work(Space* sp, MemRegio
   assert((n_threads == 1 && ParallelGCThreads == 0) ||
          n_threads <= (int)ParallelGCThreads,
          "# worker threads != # requested!");
+  assert(!Thread::current()->is_VM_thread() || (n_threads == 1), "There is only 1 VM thread");
+  assert(UseDynamicNumberOfGCThreads ||
+         !FLAG_IS_DEFAULT(ParallelGCThreads) ||
+         n_threads == (int)ParallelGCThreads,
+         "# worker threads != # requested!");
   // Make sure the LNC array is valid for the space.
   jbyte**   lowest_non_clean;
   uintptr_t lowest_non_clean_base_chunk_index;
@@ -52,6 +58,8 @@ void CardTableModRefBS::non_clean_card_iterate_parallel_work(Space* sp, MemRegio
 
   int n_strides = n_threads * ParGCStridesPerThread;
   SequentialSubTasksDone* pst = sp->par_seq_tasks();
+  // Sets the condition for completion of the subtask (how many threads
+  // need to finish in order to be done).
   pst->set_n_threads(n_threads);
   pst->set_n_tasks(n_strides);
 
diff --git a/src/share/vm/gc_implementation/parNew/parNewGeneration.cpp b/src/share/vm/gc_implementation/parNew/parNewGeneration.cpp
index 578a4e0a8..1c20f87c1 100644
--- a/src/share/vm/gc_implementation/parNew/parNewGeneration.cpp
+++ b/src/share/vm/gc_implementation/parNew/parNewGeneration.cpp
@@ -305,7 +305,7 @@ public:
 
   inline ParScanThreadState& thread_state(int i);
 
-  void reset(bool promotion_failed);
+  void reset(int active_workers, bool promotion_failed);
   void flush();
 
   #if TASKQUEUE_STATS
@@ -322,6 +322,9 @@ private:
   ParallelTaskTerminator& _term;
   ParNewGeneration&       _gen;
   Generation&             _next_gen;
+ public:
+  bool is_valid(int id) const { return id < length(); }
+  ParallelTaskTerminator* terminator() { return &_term; }
 };
 
 
@@ -351,9 +354,9 @@ inline ParScanThreadState& ParScanThreadStateSet::thread_state(int i)
 }
 
 
-void ParScanThreadStateSet::reset(bool promotion_failed)
+void ParScanThreadStateSet::reset(int active_threads, bool promotion_failed)
 {
-  _term.reset_for_reuse();
+  _term.reset_for_reuse(active_threads);
   if (promotion_failed) {
     for (int i = 0; i < length(); ++i) {
       thread_state(i).print_and_clear_promotion_failure_size();
@@ -569,6 +572,24 @@ ParNewGenTask::ParNewGenTask(ParNewGeneration* gen, Generation* next_gen,
     _state_set(state_set)
   {}
 
+// Reset the terminator for the given number of
+// active threads.
+void ParNewGenTask::set_for_termination(int active_workers) {
+  _state_set->reset(active_workers, _gen->promotion_failed());
+  // Should the heap be passed in?  There's only 1 for now so
+  // grab it instead.
+  GenCollectedHeap* gch = GenCollectedHeap::heap();
+  gch->set_n_termination(active_workers);
+}
+
+// The "i" passed to this method is the part of the work for
+// this thread.  It is not the worker ID.  The "i" is derived
+// from _started_workers which is incremented in internal_note_start()
+// called in GangWorker loop() and which is called under the
+// which is  called under the protection of the gang monitor and is
+// called after a task is started.  So "i" is based on
+// first-come-first-served.
+
 void ParNewGenTask::work(int i) {
   GenCollectedHeap* gch = GenCollectedHeap::heap();
   // Since this is being done in a separate thread, need new resource
@@ -581,6 +602,8 @@ void ParNewGenTask::work(int i) {
   Generation* old_gen = gch->next_gen(_gen);
 
   ParScanThreadState& par_scan_state = _state_set->thread_state(i);
+  assert(_state_set->is_valid(i), "Should not have been called");
+
   par_scan_state.set_young_old_boundary(_young_old_boundary);
 
   par_scan_state.start_strong_roots();
@@ -733,7 +756,9 @@ public:
 
 private:
   virtual void work(int i);
-
+  virtual void set_for_termination(int active_workers) {
+    _state_set.terminator()->reset_for_reuse(active_workers);
+  }
 private:
   ParNewGeneration&      _gen;
   ProcessTask&           _task;
@@ -789,18 +814,20 @@ void ParNewRefProcTaskExecutor::execute(ProcessTask& task)
   GenCollectedHeap* gch = GenCollectedHeap::heap();
   assert(gch->kind() == CollectedHeap::GenCollectedHeap,
          "not a generational heap");
-  WorkGang* workers = gch->workers();
+  FlexibleWorkGang* workers = gch->workers();
   assert(workers != NULL, "Need parallel worker threads.");
+  _state_set.reset(workers->active_workers(), _generation.promotion_failed());
   ParNewRefProcTaskProxy rp_task(task, _generation, *_generation.next_gen(),
                                  _generation.reserved().end(), _state_set);
   workers->run_task(&rp_task);
-  _state_set.reset(_generation.promotion_failed());
+  _state_set.reset(0 /* bad value in debug if not reset */,
+                   _generation.promotion_failed());
 }
 
 void ParNewRefProcTaskExecutor::execute(EnqueueTask& task)
 {
   GenCollectedHeap* gch = GenCollectedHeap::heap();
-  WorkGang* workers = gch->workers();
+  FlexibleWorkGang* workers = gch->workers();
   assert(workers != NULL, "Need parallel worker threads.");
   ParNewRefEnqueueTaskProxy enq_task(task);
   workers->run_task(&enq_task);
@@ -856,7 +883,13 @@ void ParNewGeneration::collect(bool   full,
   assert(gch->kind() == CollectedHeap::GenCollectedHeap,
     "not a CMS generational heap");
   AdaptiveSizePolicy* size_policy = gch->gen_policy()->size_policy();
-  WorkGang* workers = gch->workers();
+  FlexibleWorkGang* workers = gch->workers();
+  assert(workers != NULL, "Need workgang for parallel work");
+  int active_workers =
+      AdaptiveSizePolicy::calc_active_workers(workers->total_workers(),
+                                   workers->active_workers(),
+                                   Threads::number_of_non_daemon_threads());
+  workers->set_active_workers(active_workers);
   _next_gen = gch->next_gen(this);
   assert(_next_gen != NULL,
     "This must be the youngest gen, and not the only gen");
@@ -894,13 +927,19 @@ void ParNewGeneration::collect(bool   full,
 
   gch->save_marks();
   assert(workers != NULL, "Need parallel worker threads.");
-  ParallelTaskTerminator _term(workers->total_workers(), task_queues());
-  ParScanThreadStateSet thread_state_set(workers->total_workers(),
+  int n_workers = active_workers;
+
+  // Set the correct parallelism (number of queues) in the reference processor
+  ref_processor()->set_active_mt_degree(n_workers);
+
+  // Always set the terminator for the active number of workers
+  // because only those workers go through the termination protocol.
+  ParallelTaskTerminator _term(n_workers, task_queues());
+  ParScanThreadStateSet thread_state_set(workers->active_workers(),
                                          *to(), *this, *_next_gen, *task_queues(),
                                          _overflow_stacks, desired_plab_sz(), _term);
 
   ParNewGenTask tsk(this, _next_gen, reserved().end(), &thread_state_set);
-  int n_workers = workers->total_workers();
   gch->set_par_threads(n_workers);
   gch->rem_set()->prepare_for_younger_refs_iterate(true);
   // It turns out that even when we're using 1 thread, doing the work in a
@@ -914,7 +953,8 @@ void ParNewGeneration::collect(bool   full,
     GenCollectedHeap::StrongRootsScope srs(gch);
     tsk.work(0);
   }
-  thread_state_set.reset(promotion_failed());
+  thread_state_set.reset(0 /* Bad value in debug if not reset */,
+                         promotion_failed());
 
   // Process (weak) reference objects found during scavenge.
   ReferenceProcessor* rp = ref_processor();
@@ -927,6 +967,8 @@ void ParNewGeneration::collect(bool   full,
   EvacuateFollowersClosureGeneral evacuate_followers(gch, _level,
     &scan_without_gc_barrier, &scan_with_gc_barrier);
   rp->setup_policy(clear_all_soft_refs);
+  // Can  the mt_degree be set later (at run_task() time would be best)?
+  rp->set_active_mt_degree(active_workers);
   if (rp->processing_is_mt()) {
     ParNewRefProcTaskExecutor task_executor(*this, thread_state_set);
     rp->process_discovered_references(&is_alive, &keep_alive,
diff --git a/src/share/vm/gc_implementation/parNew/parNewGeneration.hpp b/src/share/vm/gc_implementation/parNew/parNewGeneration.hpp
index 296eb8254..ccd6268a6 100644
--- a/src/share/vm/gc_implementation/parNew/parNewGeneration.hpp
+++ b/src/share/vm/gc_implementation/parNew/parNewGeneration.hpp
@@ -240,6 +240,10 @@ public:
   HeapWord* young_old_boundary() { return _young_old_boundary; }
 
   void work(int i);
+
+  // Reset the terminator in ParScanThreadStateSet for
+  // "active_workers" threads.
+  virtual void set_for_termination(int active_workers);
 };
 
 class KeepAliveClosure: public DefNewGeneration::KeepAliveClosure {
diff --git a/src/share/vm/gc_implementation/parallelScavenge/cardTableExtension.cpp b/src/share/vm/gc_implementation/parallelScavenge/cardTableExtension.cpp
index 5b06d3126..6e42facf2 100644
--- a/src/share/vm/gc_implementation/parallelScavenge/cardTableExtension.cpp
+++ b/src/share/vm/gc_implementation/parallelScavenge/cardTableExtension.cpp
@@ -223,7 +223,8 @@ void CardTableExtension::scavenge_contents_parallel(ObjectStartArray* start_arra
                                                     MutableSpace* sp,
                                                     HeapWord* space_top,
                                                     PSPromotionManager* pm,
-                                                    uint stripe_number) {
+                                                    uint stripe_number,
+                                                    uint stripe_total) {
   int ssize = 128; // Naked constant!  Work unit = 64k.
   int dirty_card_count = 0;
 
@@ -231,7 +232,11 @@ void CardTableExtension::scavenge_contents_parallel(ObjectStartArray* start_arra
   jbyte* start_card = byte_for(sp->bottom());
   jbyte* end_card   = byte_for(sp_top - 1) + 1;
   oop* last_scanned = NULL; // Prevent scanning objects more than once
-  for (jbyte* slice = start_card; slice < end_card; slice += ssize*ParallelGCThreads) {
+  // The width of the stripe ssize*stripe_total must be
+  // consistent with the number of stripes so that the complete slice
+  // is covered.
+  size_t slice_width = ssize * stripe_total;
+  for (jbyte* slice = start_card; slice < end_card; slice += slice_width) {
     jbyte* worker_start_card = slice + stripe_number * ssize;
     if (worker_start_card >= end_card)
       return; // We're done.
diff --git a/src/share/vm/gc_implementation/parallelScavenge/cardTableExtension.hpp b/src/share/vm/gc_implementation/parallelScavenge/cardTableExtension.hpp
index 00d6673e1..27f917d23 100644
--- a/src/share/vm/gc_implementation/parallelScavenge/cardTableExtension.hpp
+++ b/src/share/vm/gc_implementation/parallelScavenge/cardTableExtension.hpp
@@ -69,7 +69,8 @@ class CardTableExtension : public CardTableModRefBS {
                                   MutableSpace* sp,
                                   HeapWord* space_top,
                                   PSPromotionManager* pm,
-                                  uint stripe_number);
+                                  uint stripe_number,
+                                  uint stripe_total);
 
   // Verification
   static void verify_all_young_refs_imprecise();
diff --git a/src/share/vm/gc_implementation/parallelScavenge/gcTaskManager.cpp b/src/share/vm/gc_implementation/parallelScavenge/gcTaskManager.cpp
index c4c512041..33641b6b1 100644
--- a/src/share/vm/gc_implementation/parallelScavenge/gcTaskManager.cpp
+++ b/src/share/vm/gc_implementation/parallelScavenge/gcTaskManager.cpp
@@ -25,6 +25,7 @@
 #include "precompiled.hpp"
 #include "gc_implementation/parallelScavenge/gcTaskManager.hpp"
 #include "gc_implementation/parallelScavenge/gcTaskThread.hpp"
+#include "gc_implementation/shared/adaptiveSizePolicy.hpp"
 #include "memory/allocation.hpp"
 #include "memory/allocation.inline.hpp"
 #include "runtime/mutex.hpp"
@@ -181,6 +182,7 @@ void GCTaskQueue::enqueue(GCTask* task) {
   }
   set_insert_end(task);
   increment_length();
+  verify_length();
   if (TraceGCTaskQueue) {
     print("after:");
   }
@@ -192,7 +194,7 @@ void GCTaskQueue::enqueue(GCTaskQueue* list) {
     tty->print_cr("[" INTPTR_FORMAT "]"
                   " GCTaskQueue::enqueue(list: "
                   INTPTR_FORMAT ")",
-                  this);
+                  this, list);
     print("before:");
     list->print("list:");
   }
@@ -211,14 +213,15 @@ void GCTaskQueue::enqueue(GCTaskQueue* list) {
     list->remove_end()->set_older(insert_end());
     insert_end()->set_newer(list->remove_end());
     set_insert_end(list->insert_end());
+    set_length(length() + list_length);
     // empty the argument list.
   }
-  set_length(length() + list_length);
   list->initialize();
   if (TraceGCTaskQueue) {
     print("after:");
     list->print("list:");
   }
+  verify_length();
 }
 
 // Dequeue one task.
@@ -288,6 +291,7 @@ GCTask* GCTaskQueue::remove() {
   decrement_length();
   assert(result->newer() == NULL, "shouldn't be on queue");
   assert(result->older() == NULL, "shouldn't be on queue");
+  verify_length();
   return result;
 }
 
@@ -311,22 +315,40 @@ GCTask* GCTaskQueue::remove(GCTask* task) {
   result->set_newer(NULL);
   result->set_older(NULL);
   decrement_length();
+  verify_length();
   return result;
 }
 
 NOT_PRODUCT(
+// Count the elements in the queue and verify the length against
+// that count.
+void GCTaskQueue::verify_length() const {
+  uint count = 0;
+  for (GCTask* element = insert_end();
+       element != NULL;
+       element = element->older()) {
+
+    count++;
+  }
+  assert(count == length(), "Length does not match queue");
+}
+
 void GCTaskQueue::print(const char* message) const {
   tty->print_cr("[" INTPTR_FORMAT "] GCTaskQueue:"
                 "  insert_end: " INTPTR_FORMAT
                 "  remove_end: " INTPTR_FORMAT
+                "  length:       %d"
                 "  %s",
-                this, insert_end(), remove_end(), message);
+                this, insert_end(), remove_end(), length(), message);
+  uint count = 0;
   for (GCTask* element = insert_end();
        element != NULL;
        element = element->older()) {
     element->print("    ");
+    count++;
     tty->cr();
   }
+  tty->print("Total tasks: %d", count);
 }
 )
 
@@ -351,12 +373,16 @@ SynchronizedGCTaskQueue::~SynchronizedGCTaskQueue() {
 //
 GCTaskManager::GCTaskManager(uint workers) :
   _workers(workers),
+  _active_workers(0),
+  _idle_workers(0),
   _ndc(NULL) {
   initialize();
 }
 
 GCTaskManager::GCTaskManager(uint workers, NotifyDoneClosure* ndc) :
   _workers(workers),
+  _active_workers(0),
+  _idle_workers(0),
   _ndc(ndc) {
   initialize();
 }
@@ -373,6 +399,7 @@ void GCTaskManager::initialize() {
   GCTaskQueue* unsynchronized_queue = GCTaskQueue::create_on_c_heap();
   _queue = SynchronizedGCTaskQueue::create(unsynchronized_queue, lock());
   _noop_task = NoopGCTask::create_on_c_heap();
+  _idle_inactive_task = WaitForBarrierGCTask::create_on_c_heap();
   _resource_flag = NEW_C_HEAP_ARRAY(bool, workers());
   {
     // Set up worker threads.
@@ -418,6 +445,8 @@ GCTaskManager::~GCTaskManager() {
   assert(queue()->is_empty(), "still have queued work");
   NoopGCTask::destroy(_noop_task);
   _noop_task = NULL;
+  WaitForBarrierGCTask::destroy(_idle_inactive_task);
+  _idle_inactive_task = NULL;
   if (_thread != NULL) {
     for (uint i = 0; i < workers(); i += 1) {
       GCTaskThread::destroy(thread(i));
@@ -442,6 +471,86 @@ GCTaskManager::~GCTaskManager() {
   }
 }
 
+void GCTaskManager::set_active_gang() {
+  _active_workers =
+    AdaptiveSizePolicy::calc_active_workers(workers(),
+                                 active_workers(),
+                                 Threads::number_of_non_daemon_threads());
+
+  assert(!all_workers_active() || active_workers() == ParallelGCThreads,
+         err_msg("all_workers_active() is  incorrect: "
+                 "active %d  ParallelGCThreads %d", active_workers(),
+                 ParallelGCThreads));
+  if (TraceDynamicGCThreads) {
+    gclog_or_tty->print_cr("GCTaskManager::set_active_gang(): "
+                           "all_workers_active()  %d  workers %d  "
+                           "active  %d  ParallelGCThreads %d ",
+                           all_workers_active(), workers(),  active_workers(),
+                           ParallelGCThreads);
+  }
+}
+
+// Create IdleGCTasks for inactive workers.
+// Creates tasks in a ResourceArea and assumes
+// an appropriate ResourceMark.
+void GCTaskManager::task_idle_workers() {
+  {
+    int more_inactive_workers = 0;
+    {
+      // Stop any idle tasks from exiting their IdleGCTask's
+      // and get the count for additional IdleGCTask's under
+      // the GCTaskManager's monitor so that the "more_inactive_workers"
+      // count is correct.
+      MutexLockerEx ml(monitor(), Mutex::_no_safepoint_check_flag);
+      _idle_inactive_task->set_should_wait(true);
+      // active_workers are a number being requested.  idle_workers
+      // are the number currently idle.  If all the workers are being
+      // requested to be active but some are already idle, reduce
+      // the number of active_workers to be consistent with the
+      // number of idle_workers.  The idle_workers are stuck in
+      // idle tasks and will no longer be release (since a new GC
+      // is starting).  Try later to release enough idle_workers
+      // to allow the desired number of active_workers.
+      more_inactive_workers =
+        workers() - active_workers() - idle_workers();
+      if (more_inactive_workers < 0) {
+        int reduced_active_workers = active_workers() + more_inactive_workers;
+        set_active_workers(reduced_active_workers);
+        more_inactive_workers = 0;
+      }
+      if (TraceDynamicGCThreads) {
+        gclog_or_tty->print_cr("JT: %d  workers %d  active  %d  "
+                                "idle %d  more %d",
+                                Threads::number_of_non_daemon_threads(),
+                                workers(),
+                                active_workers(),
+                                idle_workers(),
+                                more_inactive_workers);
+      }
+    }
+    GCTaskQueue* q = GCTaskQueue::create();
+    for(uint i = 0; i < (uint) more_inactive_workers; i++) {
+      q->enqueue(IdleGCTask::create_on_c_heap());
+      increment_idle_workers();
+    }
+    assert(workers() == active_workers() + idle_workers(),
+      "total workers should equal active + inactive");
+    add_list(q);
+    // GCTaskQueue* q was created in a ResourceArea so a
+    // destroy() call is not needed.
+  }
+}
+
+void  GCTaskManager::release_idle_workers() {
+  {
+    MutexLockerEx ml(monitor(),
+      Mutex::_no_safepoint_check_flag);
+    _idle_inactive_task->set_should_wait(false);
+    monitor()->notify_all();
+  // Release monitor
+  }
+}
+
 void GCTaskManager::print_task_time_stamps() {
   for(uint i=0; i<ParallelGCThreads; i++) {
     GCTaskThread* t = thread(i);
@@ -510,6 +619,13 @@ void GCTaskManager::add_list(GCTaskQueue* list) {
   // Release monitor().
 }
 
+// GC workers wait in get_task() for new work to be added
+// to the GCTaskManager's queue.  When new work is added,
+// a notify is sent to the waiting GC workers which then
+// compete to get tasks.  If a GC worker wakes up and there
+// is no work on the queue, it is given a noop_task to execute
+// and then loops to find more work.
+
 GCTask* GCTaskManager::get_task(uint which) {
   GCTask* result = NULL;
   // Grab the queue lock.
@@ -558,8 +674,10 @@ GCTask* GCTaskManager::get_task(uint which) {
                   which, result, GCTask::Kind::to_string(result->kind()));
     tty->print_cr("     %s", result->name());
   }
-  increment_busy_workers();
-  increment_delivered_tasks();
+  if (!result->is_idle_task()) {
+    increment_busy_workers();
+    increment_delivered_tasks();
+  }
   return result;
   // Release monitor().
 }
@@ -622,6 +740,7 @@ uint GCTaskManager::increment_busy_workers() {
 
 uint GCTaskManager::decrement_busy_workers() {
   assert(queue()->own_lock(), "don't own the lock");
+  assert(_busy_workers > 0, "About to make a mistake");
   _busy_workers -= 1;
   return _busy_workers;
 }
@@ -643,11 +762,28 @@ void GCTaskManager::note_release(uint which) {
   set_resource_flag(which, false);
 }
 
+// "list" contains tasks that are ready to execute.  Those
+// tasks are added to the GCTaskManager's queue of tasks and
+// then the GC workers are notified that there is new work to
+// do.
+//
+// Typically different types of tasks can be added to the "list".
+// For example in PSScavenge OldToYoungRootsTask, SerialOldToYoungRootsTask,
+// ScavengeRootsTask, and StealTask tasks are all added to the list
+// and then the GC workers are notified of new work.  The tasks are
+// handed out in the order in which they are added to the list
+// (although execution is not necessarily in that order).  As long
+// as any tasks are running the GCTaskManager will wait for execution
+// to complete.  GC workers that execute a stealing task remain in
+// the stealing task until all stealing tasks have completed.  The load
+// balancing afforded by the stealing tasks work best if the stealing
+// tasks are added last to the list.
+
 void GCTaskManager::execute_and_wait(GCTaskQueue* list) {
   WaitForBarrierGCTask* fin = WaitForBarrierGCTask::create();
   list->enqueue(fin);
   add_list(list);
-  fin->wait_for();
+  fin->wait_for(true /* reset */);
   // We have to release the barrier tasks!
   WaitForBarrierGCTask::destroy(fin);
 }
@@ -692,6 +828,72 @@ void NoopGCTask::destruct() {
 }
 
 //
+// IdleGCTask
+//
+
+IdleGCTask* IdleGCTask::create() {
+  IdleGCTask* result = new IdleGCTask(false);
+  return result;
+}
+
+IdleGCTask* IdleGCTask::create_on_c_heap() {
+  IdleGCTask* result = new(ResourceObj::C_HEAP) IdleGCTask(true);
+  return result;
+}
+
+void IdleGCTask::do_it(GCTaskManager* manager, uint which) {
+  WaitForBarrierGCTask* wait_for_task = manager->idle_inactive_task();
+  if (TraceGCTaskManager) {
+    tty->print_cr("[" INTPTR_FORMAT "]"
+                  " IdleGCTask:::do_it()"
+      "  should_wait: %s",
+      this, wait_for_task->should_wait() ? "true" : "false");
+  }
+  MutexLockerEx ml(manager->monitor(), Mutex::_no_safepoint_check_flag);
+  if (TraceDynamicGCThreads) {
+    gclog_or_tty->print_cr("--- idle %d", which);
+  }
+  // Increment has to be done when the idle tasks are created.
+  // manager->increment_idle_workers();
+  manager->monitor()->notify_all();
+  while (wait_for_task->should_wait()) {
+    if (TraceGCTaskManager) {
+      tty->print_cr("[" INTPTR_FORMAT "]"
+                    " IdleGCTask::do_it()"
+        "  [" INTPTR_FORMAT "] (%s)->wait()",
+        this, manager->monitor(), manager->monitor()->name());
+    }
+    manager->monitor()->wait(Mutex::_no_safepoint_check_flag, 0);
+  }
+  manager->decrement_idle_workers();
+  if (TraceDynamicGCThreads) {
+    gclog_or_tty->print_cr("--- release %d", which);
+  }
+  if (TraceGCTaskManager) {
+    tty->print_cr("[" INTPTR_FORMAT "]"
+                  " IdleGCTask::do_it() returns"
+      "  should_wait: %s",
+      this, wait_for_task->should_wait() ? "true" : "false");
+  }
+  // Release monitor().
+}
+
+void IdleGCTask::destroy(IdleGCTask* that) {
+  if (that != NULL) {
+    that->destruct();
+    if (that->is_c_heap_obj()) {
+      FreeHeap(that);
+    }
+  }
+}
+
+void IdleGCTask::destruct() {
+  // This has to know it's superclass structure, just like the constructor.
+  this->GCTask::destruct();
+  // Nothing else to do.
+}
+
+//
 // BarrierGCTask
 //
 
@@ -768,7 +970,8 @@ WaitForBarrierGCTask* WaitForBarrierGCTask::create() {
 }
 
 WaitForBarrierGCTask* WaitForBarrierGCTask::create_on_c_heap() {
-  WaitForBarrierGCTask* result = new WaitForBarrierGCTask(true);
+  WaitForBarrierGCTask* result =
+    new (ResourceObj::C_HEAP) WaitForBarrierGCTask(true);
   return result;
 }
 
@@ -849,7 +1052,7 @@ void WaitForBarrierGCTask::do_it(GCTaskManager* manager, uint which) {
   }
 }
 
-void WaitForBarrierGCTask::wait_for() {
+void WaitForBarrierGCTask::wait_for(bool reset) {
   if (TraceGCTaskManager) {
     tty->print_cr("[" INTPTR_FORMAT "]"
                   " WaitForBarrierGCTask::wait_for()"
@@ -869,7 +1072,9 @@ void WaitForBarrierGCTask::wait_for() {
       monitor()->wait(Mutex::_no_safepoint_check_flag, 0);
     }
     // Reset the flag in case someone reuses this task.
-    set_should_wait(true);
+    if (reset) {
+      set_should_wait(true);
+    }
     if (TraceGCTaskManager) {
       tty->print_cr("[" INTPTR_FORMAT "]"
                     " WaitForBarrierGCTask::wait_for() returns"
diff --git a/src/share/vm/gc_implementation/parallelScavenge/gcTaskManager.hpp b/src/share/vm/gc_implementation/parallelScavenge/gcTaskManager.hpp
index 3bd3af1e3..65a8458d3 100644
--- a/src/share/vm/gc_implementation/parallelScavenge/gcTaskManager.hpp
+++ b/src/share/vm/gc_implementation/parallelScavenge/gcTaskManager.hpp
@@ -45,6 +45,7 @@ class BarrierGCTask;
 class ReleasingBarrierGCTask;
 class NotifyingBarrierGCTask;
 class WaitForBarrierGCTask;
+class IdleGCTask;
 // A free list of Monitor*'s.
 class MonitorSupply;
 
@@ -64,7 +65,8 @@ public:
       unknown_task,
       ordinary_task,
       barrier_task,
-      noop_task
+      noop_task,
+      idle_task
     };
     static const char* to_string(kind value);
   };
@@ -108,6 +110,9 @@ public:
   bool is_noop_task() const {
     return kind()==Kind::noop_task;
   }
+  bool is_idle_task() const {
+    return kind()==Kind::idle_task;
+  }
   void print(const char* message) const PRODUCT_RETURN;
 protected:
   // Constructors: Only create subclasses.
@@ -153,6 +158,7 @@ public:
     assert(((insert_end() == NULL && remove_end() == NULL) ||
             (insert_end() != NULL && remove_end() != NULL)),
            "insert_end and remove_end don't match");
+    assert((insert_end() != NULL) || (_length == 0), "Not empty");
     return insert_end() == NULL;
   }
   uint length() const {
@@ -204,6 +210,8 @@ protected:
   GCTask* remove();                     // Remove from remove end.
   GCTask* remove(GCTask* task);         // Remove from the middle.
   void print(const char* message) const PRODUCT_RETURN;
+  // Debug support
+  void verify_length() const PRODUCT_RETURN;
 };
 
 // A GCTaskQueue that can be synchronized.
@@ -285,12 +293,76 @@ protected:
   }
 };
 
+// Dynamic number of GC threads
+//
+//  GC threads wait in get_task() for work (i.e., a task) to perform.
+// When the number of GC threads was static, the number of tasks
+// created to do a job was equal to or greater than the maximum
+// number of GC threads (ParallelGCThreads).  The job might be divided
+// into a number of tasks greater than the number of GC threads for
+// load balancing (i.e., over partitioning).  The last task to be
+// executed by a GC thread in a job is a work stealing task.  A
+// GC  thread that gets a work stealing task continues to execute
+// that task until the job is done.  In the static number of GC theads
+// case, tasks are added to a queue (FIFO).  The work stealing tasks are
+// the last to be added.  Once the tasks are added, the GC threads grab
+// a task and go.  A single thread can do all the non-work stealing tasks
+// and then execute a work stealing and wait for all the other GC threads
+// to execute their work stealing task.
+//  In the dynamic number of GC threads implementation, idle-tasks are
+// created to occupy the non-participating or "inactive" threads.  An
+// idle-task makes the GC thread wait on a barrier that is part of the
+// GCTaskManager.  The GC threads that have been "idled" in a IdleGCTask
+// are released once all the active GC threads have finished their work
+// stealing tasks.  The GCTaskManager does not wait for all the "idled"
+// GC threads to resume execution. When those GC threads do resume
+// execution in the course of the thread scheduling, they call get_tasks()
+// as all the other GC threads do.  Because all the "idled" threads are
+// not required to execute in order to finish a job, it is possible for
+// a GC thread to still be "idled" when the next job is started.  Such
+// a thread stays "idled" for the next job.  This can result in a new
+// job not having all the expected active workers.  For example if on
+// job requests 4 active workers out of a total of 10 workers so the
+// remaining 6 are "idled", if the next job requests 6 active workers
+// but all 6 of the "idled" workers are still idle, then the next job
+// will only get 4 active workers.
+//  The implementation for the parallel old compaction phase has an
+// added complication.  In the static case parold partitions the chunks
+// ready to be filled into stacks, one for each GC thread.  A GC thread
+// executing a draining task (drains the stack of ready chunks)
+// claims a stack according to it's id (the unique ordinal value assigned
+// to each GC thread).  In the dynamic case not all GC threads will
+// actively participate so stacks with ready to fill chunks can only be
+// given to the active threads.  An initial implementation chose stacks
+// number 1-n to get the ready chunks and required that GC threads
+// 1-n be the active workers.  This was undesirable because it required
+// certain threads to participate.  In the final implementation a
+// list of stacks equal in number to the active workers are filled
+// with ready chunks.  GC threads that participate get a stack from
+// the task (DrainStacksCompactionTask), empty the stack, and then add it to a
+// recycling list at the end of the task.  If the same GC thread gets
+// a second task, it gets a second stack to drain and returns it.  The
+// stacks are added to a recycling list so that later stealing tasks
+// for this tasks can get a stack from the recycling list.  Stealing tasks
+// use the stacks in its work in a way similar to the draining tasks.
+// A thread is not guaranteed to get anything but a stealing task and
+// a thread that only gets a stealing task has to get a stack. A failed
+// implementation tried to have the GC threads keep the stack they used
+// during a draining task for later use in the stealing task but that didn't
+// work because as noted a thread is not guaranteed to get a draining task.
+//
+// For PSScavenge and ParCompactionManager the GC threads are
+// held in the GCTaskThread** _thread array in GCTaskManager.
+
+
 class GCTaskManager : public CHeapObj {
  friend class ParCompactionManager;
  friend class PSParallelCompact;
  friend class PSScavenge;
  friend class PSRefProcTaskExecutor;
  friend class RefProcTaskExecutor;
+ friend class GCTaskThread;
+ friend class IdleGCTask;
 private:
   // Instance state.
   NotifyDoneClosure*        _ndc;               // Notify on completion.
@@ -298,6 +370,7 @@ private:
   Monitor*                  _monitor;           // Notification of changes.
   SynchronizedGCTaskQueue*  _queue;             // Queue of tasks.
   GCTaskThread**            _thread;            // Array of worker threads.
+  uint                      _active_workers;    // Number of active workers.
   uint                      _busy_workers;      // Number of busy workers.
   uint                      _blocking_worker;   // The worker that's blocking.
   bool*                     _resource_flag;     // Array of flag per threads.
@@ -307,6 +380,8 @@ private:
   uint                      _emptied_queue;     // Times we emptied the queue.
   NoopGCTask*               _noop_task;         // The NoopGCTask instance.
   uint                      _noop_tasks;        // Count of noop tasks.
+  WaitForBarrierGCTask*     _idle_inactive_task;// Task for inactive workers
+  volatile uint             _idle_workers;      // Number of idled workers
 public:
   // Factory create and destroy methods.
   static GCTaskManager* create(uint workers) {
@@ -324,6 +399,9 @@ public:
   uint busy_workers() const {
     return _busy_workers;
   }
+  volatile uint idle_workers() const {
+    return _idle_workers;
+  }
   //     Pun between Monitor* and Mutex*
   Monitor* monitor() const {
     return _monitor;
@@ -331,6 +409,9 @@ public:
   Monitor * lock() const {
     return _monitor;
   }
+  WaitForBarrierGCTask* idle_inactive_task() {
+    return _idle_inactive_task;
+  }
   // Methods.
   //     Add the argument task to be run.
   void add_task(GCTask* task);
@@ -350,6 +431,10 @@ public:
   bool should_release_resources(uint which); // Predicate.
   //     Note the release of resources by the argument worker.
   void note_release(uint which);
+  //     Create IdleGCTasks for inactive workers and start workers
+  void task_idle_workers();
+  //     Release the workers in IdleGCTasks
+  void release_idle_workers();
   // Constants.
   //     A sentinel worker identifier.
   static uint sentinel_worker() {
@@ -375,6 +460,15 @@ protected:
   uint workers() const {
     return _workers;
   }
+  void set_active_workers(uint v) {
+    assert(v <= _workers, "Trying to set more workers active than there are");
+    _active_workers = MIN2(v, _workers);
+    assert(v != 0, "Trying to set active workers to 0");
+    _active_workers = MAX2(1U, _active_workers);
+  }
+  // Sets the number of threads that will be used in a collection
+  void set_active_gang();
+
   NotifyDoneClosure* notify_done_closure() const {
     return _ndc;
   }
@@ -457,8 +551,21 @@ protected:
   void reset_noop_tasks() {
     _noop_tasks = 0;
   }
+  void increment_idle_workers() {
+    _idle_workers++;
+  }
+  void decrement_idle_workers() {
+    _idle_workers--;
+  }
   // Other methods.
   void initialize();
+
+ public:
+  // Return true if all workers are currently active.
+  bool all_workers_active() { return workers() == active_workers(); }
+  uint active_workers() const {
+    return _active_workers;
+  }
 };
 
 //
@@ -475,6 +582,8 @@ public:
   static NoopGCTask* create();
   static NoopGCTask* create_on_c_heap();
   static void destroy(NoopGCTask* that);
+
+  virtual char* name() { return (char *)"noop task"; }
   // Methods from GCTask.
   void do_it(GCTaskManager* manager, uint which) {
     // Nothing to do.
@@ -518,6 +627,8 @@ protected:
   }
   // Destructor-like method.
   void destruct();
+
+  virtual char* name() { return (char *)"barrier task"; }
   // Methods.
   //     Wait for this to be the only task running.
   void do_it_internal(GCTaskManager* manager, uint which);
@@ -586,11 +697,13 @@ protected:
 // the BarrierGCTask is done.
 // This may cover many of the uses of NotifyingBarrierGCTasks.
 class WaitForBarrierGCTask : public BarrierGCTask {
+  friend class GCTaskManager;
+  friend class IdleGCTask;
 private:
   // Instance state.
-  Monitor*   _monitor;                  // Guard and notify changes.
-  bool       _should_wait;              // true=>wait, false=>proceed.
-  const bool _is_c_heap_obj;            // Was allocated on the heap.
+  Monitor*      _monitor;                  // Guard and notify changes.
+  volatile bool _should_wait;              // true=>wait, false=>proceed.
+  const bool    _is_c_heap_obj;            // Was allocated on the heap.
 public:
   virtual char* name() { return (char *) "waitfor-barrier-task"; }
 
@@ -600,7 +713,10 @@ public:
   static void destroy(WaitForBarrierGCTask* that);
   // Methods.
   void     do_it(GCTaskManager* manager, uint which);
-  void     wait_for();
+  void     wait_for(bool reset);
+  void set_should_wait(bool value) {
+    _should_wait = value;
+  }
 protected:
   // Constructor.  Clients use factory, but there might be subclasses.
   WaitForBarrierGCTask(bool on_c_heap);
@@ -613,12 +729,36 @@ protected:
   bool should_wait() const {
     return _should_wait;
   }
-  void set_should_wait(bool value) {
-    _should_wait = value;
+  bool is_c_heap_obj() {
+    return _is_c_heap_obj;
   }
+};
+
+// Task that is used to idle a GC task when fewer than
+// the maximum workers are wanted.
+class IdleGCTask : public GCTask {
+  const bool    _is_c_heap_obj;            // Was allocated on the heap.
+ public:
   bool is_c_heap_obj() {
     return _is_c_heap_obj;
   }
+  // Factory create and destroy methods.
+  static IdleGCTask* create();
+  static IdleGCTask* create_on_c_heap();
+  static void destroy(IdleGCTask* that);
+
+  virtual char* name() { return (char *)"idle task"; }
+  // Methods from GCTask.
+  virtual void do_it(GCTaskManager* manager, uint which);
+protected:
+  // Constructor.
+  IdleGCTask(bool on_c_heap) :
+    GCTask(GCTask::Kind::idle_task),
+    _is_c_heap_obj(on_c_heap) {
+    // Nothing to do.
+  }
+  // Destructor-like method.
+  void destruct();
 };
 
 class MonitorSupply : public AllStatic {
diff --git a/src/share/vm/gc_implementation/parallelScavenge/gcTaskThread.cpp b/src/share/vm/gc_implementation/parallelScavenge/gcTaskThread.cpp
index 41c90f9ec..235039dd2 100644
--- a/src/share/vm/gc_implementation/parallelScavenge/gcTaskThread.cpp
+++ b/src/share/vm/gc_implementation/parallelScavenge/gcTaskThread.cpp
@@ -93,6 +93,11 @@ void GCTaskThread::print_on(outputStream* st) const {
   st->cr();
 }
 
+// GC workers get tasks from the GCTaskManager and execute
+// them in this method.  If there are no tasks to execute,
+// the GC workers wait in the GCTaskManager's get_task()
+// for tasks to be enqueued for execution.
+
 void GCTaskThread::run() {
   // Set up the thread for stack overflow support
   this->record_stack_base_and_size();
@@ -124,7 +129,6 @@ void GCTaskThread::run() {
     for (; /* break */; ) {
       // This will block until there is a task to be gotten.
       GCTask* task = manager()->get_task(which());
-
       // In case the update is costly
       if (PrintGCTaskTimeStamps) {
         timer.update();
@@ -134,18 +138,28 @@ void GCTaskThread::run() {
       char* name = task->name();
 
       task->do_it(manager(), which());
-      manager()->note_completion(which());
 
-      if (PrintGCTaskTimeStamps) {
-        assert(_time_stamps != NULL, "Sanity (PrintGCTaskTimeStamps set late?)");
+      if (!task->is_idle_task()) {
+        manager()->note_completion(which());
 
-        timer.update();
+        if (PrintGCTaskTimeStamps) {
+          assert(_time_stamps != NULL,
+            "Sanity (PrintGCTaskTimeStamps set late?)");
+
+          timer.update();
 
-        GCTaskTimeStamp* time_stamp = time_stamp_at(_time_stamp_index++);
+          GCTaskTimeStamp* time_stamp = time_stamp_at(_time_stamp_index++);
 
-        time_stamp->set_name(name);
-        time_stamp->set_entry_time(entry_time);
-        time_stamp->set_exit_time(timer.ticks());
+          time_stamp->set_name(name);
+          time_stamp->set_entry_time(entry_time);
+          time_stamp->set_exit_time(timer.ticks());
+        }
+      } else {
+        // idle tasks complete outside the normal accounting
+        // so that a task can complete without waiting for idle tasks.
+        // They have to be terminated separately.
+        IdleGCTask::destroy((IdleGCTask*)task);
+        set_is_working(true);
       }
 
       // Check if we should release our inner resources.
diff --git a/src/share/vm/gc_implementation/parallelScavenge/gcTaskThread.hpp b/src/share/vm/gc_implementation/parallelScavenge/gcTaskThread.hpp
index 7fc907bf0..c8406545e 100644
--- a/src/share/vm/gc_implementation/parallelScavenge/gcTaskThread.hpp
+++ b/src/share/vm/gc_implementation/parallelScavenge/gcTaskThread.hpp
@@ -35,6 +35,7 @@ class GCTaskTimeStamp;
 class GCTaskManager;
 
 class GCTaskThread : public WorkerThread {
+  friend class GCTaskManager;
 private:
   // Instance state.
   GCTaskManager* _manager;              // Manager for worker.
@@ -45,6 +46,8 @@ private:
 
   GCTaskTimeStamp* time_stamp_at(uint index);
 
+  bool _is_working;                     // True if participating in GC tasks
+
  public:
   // Factory create and destroy methods.
   static GCTaskThread* create(GCTaskManager* manager,
@@ -84,6 +87,7 @@ protected:
   uint processor_id() const {
     return _processor_id;
   }
+  void set_is_working(bool v) { _is_working = v; }
 };
 
 class GCTaskTimeStamp : public CHeapObj
diff --git a/src/share/vm/gc_implementation/parallelScavenge/pcTasks.cpp b/src/share/vm/gc_implementation/parallelScavenge/pcTasks.cpp
index 75ee0b3a9..c577daf9c 100644
--- a/src/share/vm/gc_implementation/parallelScavenge/pcTasks.cpp
+++ b/src/share/vm/gc_implementation/parallelScavenge/pcTasks.cpp
@@ -152,15 +152,16 @@ void RefProcTaskExecutor::execute(ProcessTask& task)
 {
   ParallelScavengeHeap* heap = PSParallelCompact::gc_heap();
   uint parallel_gc_threads = heap->gc_task_manager()->workers();
+  uint active_gc_threads = heap->gc_task_manager()->active_workers();
   RegionTaskQueueSet* qset = ParCompactionManager::region_array();
-  ParallelTaskTerminator terminator(parallel_gc_threads, qset);
+  ParallelTaskTerminator terminator(active_gc_threads, qset);
   GCTaskQueue* q = GCTaskQueue::create();
   for(uint i=0; i<parallel_gc_threads; i++) {
     q->enqueue(new RefProcTaskProxy(task, i));
   }
   if (task.marks_oops_alive()) {
     if (parallel_gc_threads>1) {
-      for (uint j=0; j<parallel_gc_threads; j++) {
+      for (uint j=0; j<active_gc_threads; j++) {
         q->enqueue(new StealMarkingTask(&terminator));
       }
     }
@@ -216,7 +217,6 @@ void StealMarkingTask::do_it(GCTaskManager* manager, uint which) {
 // StealRegionCompactionTask
 //
 
-
 StealRegionCompactionTask::StealRegionCompactionTask(ParallelTaskTerminator* t):
   _terminator(t) {}
 
@@ -229,6 +229,32 @@ void StealRegionCompactionTask::do_it(GCTaskManager* manager, uint which) {
   ParCompactionManager* cm =
     ParCompactionManager::gc_thread_compaction_manager(which);
 
+
+  // If not all threads are active, get a draining stack
+  // from the list.  Else, just use this threads draining stack.
+  uint which_stack_index;
+  bool use_all_workers = manager->all_workers_active();
+  if (use_all_workers) {
+    which_stack_index = which;
+    assert(manager->active_workers() == ParallelGCThreads,
+           err_msg("all_workers_active has been incorrectly set: "
+                   " active %d  ParallelGCThreads %d", manager->active_workers(),
+                   ParallelGCThreads));
+  } else {
+    which_stack_index = ParCompactionManager::pop_recycled_stack_index();
+  }
+
+  cm->set_region_stack_index(which_stack_index);
+  cm->set_region_stack(ParCompactionManager::region_list(which_stack_index));
+  if (TraceDynamicGCThreads) {
+    gclog_or_tty->print_cr("StealRegionCompactionTask::do_it "
+                           "region_stack_index %d region_stack = 0x%x "
+                           " empty (%d) use all workers %d",
+    which_stack_index, ParCompactionManager::region_list(which_stack_index),
+    cm->region_stack()->is_empty(),
+    use_all_workers);
+  }
+
   // Has to drain stacks first because there may be regions on
   // preloaded onto the stack and this thread may never have
   // done a draining task.  Are the draining tasks needed?
@@ -285,6 +311,50 @@ void DrainStacksCompactionTask::do_it(GCTaskManager* manager, uint which) {
   ParCompactionManager* cm =
     ParCompactionManager::gc_thread_compaction_manager(which);
 
+  uint which_stack_index;
+  bool use_all_workers = manager->all_workers_active();
+  if (use_all_workers) {
+    which_stack_index = which;
+    assert(manager->active_workers() == ParallelGCThreads,
+           err_msg("all_workers_active has been incorrectly set: "
+                   " active %d  ParallelGCThreads %d", manager->active_workers(),
+                   ParallelGCThreads));
+  } else {
+    which_stack_index = stack_index();
+  }
+
+  cm->set_region_stack(ParCompactionManager::region_list(which_stack_index));
+  if (TraceDynamicGCThreads) {
+    gclog_or_tty->print_cr("DrainStacksCompactionTask::do_it which = %d "
+                           "which_stack_index = %d/empty(%d) "
+                           "use all workers %d",
+                           which, which_stack_index,
+                           cm->region_stack()->is_empty(),
+                           use_all_workers);
+  }
+
+  cm->set_region_stack_index(which_stack_index);
+
   // Process any regions already in the compaction managers stacks.
   cm->drain_region_stacks();
+
+  assert(cm->region_stack()->is_empty(), "Not empty");
+
+  if (!use_all_workers) {
+    // Always give up the region stack.
+    assert(cm->region_stack() ==
+           ParCompactionManager::region_list(cm->region_stack_index()),
+           "region_stack and region_stack_index are inconsistent");
+    ParCompactionManager::push_recycled_stack_index(cm->region_stack_index());
+
+    if (TraceDynamicGCThreads) {
+      void* old_region_stack = (void*) cm->region_stack();
+      int old_region_stack_index = cm->region_stack_index();
+      gclog_or_tty->print_cr("Pushing region stack 0x%x/%d",
+        old_region_stack, old_region_stack_index);
+    }
+
+    cm->set_region_stack(NULL);
+    cm->set_region_stack_index((uint)max_uintx);
+  }
 }
diff --git a/src/share/vm/gc_implementation/parallelScavenge/psCompactionManager.cpp b/src/share/vm/gc_implementation/parallelScavenge/psCompactionManager.cpp
index 24b9ddea0..c94de3a59 100644
--- a/src/share/vm/gc_implementation/parallelScavenge/psCompactionManager.cpp
+++ b/src/share/vm/gc_implementation/parallelScavenge/psCompactionManager.cpp
@@ -39,6 +39,9 @@
 
 PSOldGen*            ParCompactionManager::_old_gen = NULL;
 ParCompactionManager**  ParCompactionManager::_manager_array = NULL;
+
+RegionTaskQueue**              ParCompactionManager::_region_list = NULL;
+
 OopTaskQueueSet*     ParCompactionManager::_stack_array = NULL;
 ParCompactionManager::ObjArrayTaskQueueSet*
   ParCompactionManager::_objarray_queues = NULL;
@@ -46,8 +49,14 @@ ObjectStartArray*    ParCompactionManager::_start_array = NULL;
 ParMarkBitMap*       ParCompactionManager::_mark_bitmap = NULL;
 RegionTaskQueueSet*  ParCompactionManager::_region_array = NULL;
 
+uint*                 ParCompactionManager::_recycled_stack_index = NULL;
+int                   ParCompactionManager::_recycled_top = -1;
+int                   ParCompactionManager::_recycled_bottom = -1;
+
 ParCompactionManager::ParCompactionManager() :
-    _action(CopyAndUpdate) {
+    _action(CopyAndUpdate),
+    _region_stack(NULL),
+    _region_stack_index((uint)max_uintx) {
 
   ParallelScavengeHeap* heap = (ParallelScavengeHeap*)Universe::heap();
   assert(heap->kind() == CollectedHeap::ParallelScavengeHeap, "Sanity");
@@ -57,7 +66,10 @@ ParCompactionManager::ParCompactionManager() :
 
   marking_stack()->initialize();
   _objarray_stack.initialize();
-  region_stack()->initialize();
+}
+
+ParCompactionManager::~ParCompactionManager() {
+  delete _recycled_stack_index;
 }
 
 void ParCompactionManager::initialize(ParMarkBitMap* mbm) {
@@ -72,6 +84,19 @@ void ParCompactionManager::initialize(ParMarkBitMap* mbm) {
   _manager_array = NEW_C_HEAP_ARRAY(ParCompactionManager*, parallel_gc_threads+1 );
   guarantee(_manager_array != NULL, "Could not allocate manager_array");
 
+  _region_list = NEW_C_HEAP_ARRAY(RegionTaskQueue*,
+                                         parallel_gc_threads+1);
+  guarantee(_region_list != NULL, "Could not initialize promotion manager");
+
+  _recycled_stack_index = NEW_C_HEAP_ARRAY(uint, parallel_gc_threads);
+
+  // parallel_gc-threads + 1 to be consistent with the number of
+  // compaction managers.
+  for(uint i=0; i<parallel_gc_threads + 1; i++) {
+    _region_list[i] = new RegionTaskQueue();
+    region_list(i)->initialize();
+  }
+
   _stack_array = new OopTaskQueueSet(parallel_gc_threads);
   guarantee(_stack_array != NULL, "Could not allocate stack_array");
   _objarray_queues = new ObjArrayTaskQueueSet(parallel_gc_threads);
@@ -85,7 +110,7 @@ void ParCompactionManager::initialize(ParMarkBitMap* mbm) {
     guarantee(_manager_array[i] != NULL, "Could not create ParCompactionManager");
     stack_array()->register_queue(i, _manager_array[i]->marking_stack());
     _objarray_queues->register_queue(i, &_manager_array[i]->_objarray_stack);
-    region_array()->register_queue(i, _manager_array[i]->region_stack());
+    region_array()->register_queue(i, region_list(i));
   }
 
   // The VMThread gets its own ParCompactionManager, which is not available
@@ -97,6 +122,29 @@ void ParCompactionManager::initialize(ParMarkBitMap* mbm) {
     "Not initialized?");
 }
 
+int ParCompactionManager::pop_recycled_stack_index() {
+  assert(_recycled_bottom <= _recycled_top, "list is empty");
+  // Get the next available index
+  if (_recycled_bottom < _recycled_top) {
+    uint cur, next, last;
+    do {
+      cur = _recycled_bottom;
+      next = cur + 1;
+      last = Atomic::cmpxchg(next, &_recycled_bottom, cur);
+    } while (cur != last);
+    return _recycled_stack_index[next];
+  } else {
+    return -1;
+  }
+}
+
+void ParCompactionManager::push_recycled_stack_index(uint v) {
+  // Get the next available index
+  int cur = Atomic::add(1, &_recycled_top);
+  _recycled_stack_index[cur] = v;
+  assert(_recycled_bottom <= _recycled_top, "list top and bottom are wrong");
+}
+
 bool ParCompactionManager::should_update() {
   assert(action() != NotValid, "Action is not set");
   return (action() == ParCompactionManager::Update) ||
@@ -121,6 +169,15 @@ bool ParCompactionManager::should_reset_only() {
   return action() == ParCompactionManager::ResetObjects;
 }
 
+void ParCompactionManager::region_list_push(uint list_index,
+                                            size_t region_index) {
+  region_list(list_index)->push(region_index);
+}
+
+void ParCompactionManager::verify_region_list_empty(uint list_index) {
+  assert(region_list(list_index)->is_empty(), "Not empty");
+}
+
 ParCompactionManager*
 ParCompactionManager::gc_thread_compaction_manager(int index) {
   assert(index >= 0 && index < (int)ParallelGCThreads, "index out of range");
diff --git a/src/share/vm/gc_implementation/parallelScavenge/psCompactionManager.hpp b/src/share/vm/gc_implementation/parallelScavenge/psCompactionManager.hpp
index cbf1b7d03..2438c1e4e 100644
--- a/src/share/vm/gc_implementation/parallelScavenge/psCompactionManager.hpp
+++ b/src/share/vm/gc_implementation/parallelScavenge/psCompactionManager.hpp
@@ -48,6 +48,7 @@ class ParCompactionManager : public CHeapObj {
   friend class StealRegionCompactionTask;
   friend class UpdateAndFillClosure;
   friend class RefProcTaskExecutor;
+  friend class IdleGCTask;
 
  public:
 
@@ -85,7 +86,31 @@ private:
   // Is there a way to reuse the _marking_stack for the
   // saving empty regions?  For now just create a different
   // type of TaskQueue.
-  RegionTaskQueue               _region_stack;
+  RegionTaskQueue*             _region_stack;
+
+  static RegionTaskQueue**     _region_list;
+  // Index in _region_list for current _region_stack.
+  uint _region_stack_index;
+
+  // Indexes of recycled region stacks/overflow stacks
+  // Stacks of regions to be compacted are embedded in the tasks doing
+  // the compaction.  A thread that executes the task extracts the
+  // region stack and drains it.  These threads keep these region
+  // stacks for use during compaction task stealing.  If a thread
+  // gets a second draining task, it pushed its current region stack
+  // index into the array _recycled_stack_index and gets a new
+  // region stack from the task.  A thread that is executing a
+  // compaction stealing task without ever having executing a
+  // draining task, will get a region stack from _recycled_stack_index.
+  //
+  // Array of indexes into the array of region stacks.
+  static uint*                    _recycled_stack_index;
+  // The index into _recycled_stack_index of the last region stack index
+  // pushed.  If -1, there are no entries into _recycled_stack_index.
+  static int                      _recycled_top;
+  // The index into _recycled_stack_index of the last region stack index
+  // popped.  If -1, there has not been any entry popped.
+  static int                      _recycled_bottom;
 
   Stack<Klass*>                 _revisit_klass_stack;
   Stack<DataLayout*>            _revisit_mdo_stack;
@@ -104,7 +129,6 @@ private:
   // Array of tasks.  Needed by the ParallelTaskTerminator.
   static RegionTaskQueueSet* region_array()      { return _region_array; }
   OverflowTaskQueue<oop>*  marking_stack()       { return &_marking_stack; }
-  RegionTaskQueue* region_stack()                { return &_region_stack; }
 
   // Pushes onto the marking stack.  If the marking stack is full,
   // pushes onto the overflow stack.
@@ -116,10 +140,33 @@ private:
   Action action() { return _action; }
   void set_action(Action v) { _action = v; }
 
+  RegionTaskQueue* region_stack()                { return _region_stack; }
+  void set_region_stack(RegionTaskQueue* v)       { _region_stack = v; }
+
   inline static ParCompactionManager* manager_array(int index);
 
+  inline static RegionTaskQueue* region_list(int index) {
+    return _region_list[index];
+  }
+
+  uint region_stack_index() { return _region_stack_index; }
+  void set_region_stack_index(uint v) { _region_stack_index = v; }
+
+  // Pop and push unique reusable stack index
+  static int pop_recycled_stack_index();
+  static void push_recycled_stack_index(uint v);
+  static void reset_recycled_stack_index() {
+    _recycled_bottom = _recycled_top = -1;
+  }
+
   ParCompactionManager();
+  ~ParCompactionManager();
 
+  // Pushes onto the region stack at the given index.  If the
+  // region stack is full,
+  // pushes onto the region overflow stack.
+  static void region_list_push(uint stack_index, size_t region_index);
+  static void verify_region_list_empty(uint stack_index);
   ParMarkBitMap* mark_bitmap() { return _mark_bitmap; }
 
   // Take actions in preparation for a compaction.
diff --git a/src/share/vm/gc_implementation/parallelScavenge/psParallelCompact.cpp b/src/share/vm/gc_implementation/parallelScavenge/psParallelCompact.cpp
index a62059c68..3646131c6 100644
--- a/src/share/vm/gc_implementation/parallelScavenge/psParallelCompact.cpp
+++ b/src/share/vm/gc_implementation/parallelScavenge/psParallelCompact.cpp
@@ -2045,6 +2045,11 @@ void PSParallelCompact::invoke_no_policy(bool maximum_heap_compaction) {
     ResourceMark rm;
     HandleMark hm;
 
+    // Set the number of GC threads to be used in this collection
+    gc_task_manager()->set_active_gang();
+    gc_task_manager()->task_idle_workers();
+    heap->set_par_threads(gc_task_manager()->active_workers());
+
     const bool is_system_gc = gc_cause == GCCause::_java_lang_system_gc;
 
     // This is useful for debugging but don't change the output the
@@ -2197,6 +2202,7 @@ void PSParallelCompact::invoke_no_policy(bool maximum_heap_compaction) {
     // Track memory usage and detect low memory
     MemoryService::track_memory_usage();
     heap->update_counters();
+    gc_task_manager()->release_idle_workers();
   }
 
 #ifdef ASSERT
@@ -2204,7 +2210,7 @@ void PSParallelCompact::invoke_no_policy(bool maximum_heap_compaction) {
     ParCompactionManager* const cm =
       ParCompactionManager::manager_array(int(i));
     assert(cm->marking_stack()->is_empty(),       "should be empty");
-    assert(cm->region_stack()->is_empty(),        "should be empty");
+    assert(ParCompactionManager::region_list(int(i))->is_empty(), "should be empty");
     assert(cm->revisit_klass_stack()->is_empty(), "should be empty");
   }
 #endif // ASSERT
@@ -2351,8 +2357,9 @@ void PSParallelCompact::marking_phase(ParCompactionManager* cm,
 
   ParallelScavengeHeap* heap = gc_heap();
   uint parallel_gc_threads = heap->gc_task_manager()->workers();
+  uint active_gc_threads = heap->gc_task_manager()->active_workers();
   TaskQueueSetSuper* qset = ParCompactionManager::region_array();
-  ParallelTaskTerminator terminator(parallel_gc_threads, qset);
+  ParallelTaskTerminator terminator(active_gc_threads, qset);
 
   PSParallelCompact::MarkAndPushClosure mark_and_push_closure(cm);
   PSParallelCompact::FollowStackClosure follow_stack_closure(cm);
@@ -2374,21 +2381,13 @@ void PSParallelCompact::marking_phase(ParCompactionManager* cm,
     q->enqueue(new MarkFromRootsTask(MarkFromRootsTask::jvmti));
     q->enqueue(new MarkFromRootsTask(MarkFromRootsTask::code_cache));
 
-    if (parallel_gc_threads > 1) {
-      for (uint j = 0; j < parallel_gc_threads; j++) {
+    if (active_gc_threads > 1) {
+      for (uint j = 0; j < active_gc_threads; j++) {
         q->enqueue(new StealMarkingTask(&terminator));
       }
     }
 
-    WaitForBarrierGCTask* fin = WaitForBarrierGCTask::create();
-    q->enqueue(fin);
-
-    gc_task_manager()->add_list(q);
-
-    fin->wait_for();
-
-    // We have to release the barrier tasks!
-    WaitForBarrierGCTask::destroy(fin);
+    gc_task_manager()->execute_and_wait(q);
   }
 
   // Process reference objects found during marking
@@ -2483,10 +2482,22 @@ void PSParallelCompact::enqueue_region_draining_tasks(GCTaskQueue* q,
 {
   TraceTime tm("drain task setup", print_phases(), true, gclog_or_tty);
 
-  const unsigned int task_count = MAX2(parallel_gc_threads, 1U);
-  for (unsigned int j = 0; j < task_count; j++) {
+  // Find the threads that are active
+  unsigned int which = 0;
+
+  const uint task_count = MAX2(parallel_gc_threads, 1U);
+  for (uint j = 0; j < task_count; j++) {
     q->enqueue(new DrainStacksCompactionTask(j));
+    ParCompactionManager::verify_region_list_empty(j);
+    // Set the region stacks variables to "no" region stack values
+    // so that they will be recognized and needing a region stack
+    // in the stealing tasks if they do not get one by executing
+    // a draining stack.
+    ParCompactionManager* cm = ParCompactionManager::manager_array(j);
+    cm->set_region_stack(NULL);
+    cm->set_region_stack_index((uint)max_uintx);
   }
+  ParCompactionManager::reset_recycled_stack_index();
 
   // Find all regions that are available (can be filled immediately) and
   // distribute them to the thread stacks.  The iteration is done in reverse
@@ -2495,8 +2506,10 @@ void PSParallelCompact::enqueue_region_draining_tasks(GCTaskQueue* q,
   const ParallelCompactData& sd = PSParallelCompact::summary_data();
 
   size_t fillable_regions = 0;   // A count for diagnostic purposes.
-  unsigned int which = 0;       // The worker thread number.
+  // A region index which corresponds to the tasks created above.
+  // "which" must be 0 <= which < task_count
 
+  which = 0;
   for (unsigned int id = to_space_id; id > perm_space_id; --id) {
     SpaceInfo* const space_info = _space_info + id;
     MutableSpace* const space = space_info->space();
@@ -2509,8 +2522,7 @@ void PSParallelCompact::enqueue_region_draining_tasks(GCTaskQueue* q,
 
     for (size_t cur = end_region - 1; cur >= beg_region; --cur) {
       if (sd.region(cur)->claim_unsafe()) {
-        ParCompactionManager* cm = ParCompactionManager::manager_array(which);
-        cm->push_region(cur);
+        ParCompactionManager::region_list_push(which, cur);
 
         if (TraceParallelOldGCCompactionPhase && Verbose) {
           const size_t count_mod_8 = fillable_regions & 7;
@@ -2521,8 +2533,10 @@ void PSParallelCompact::enqueue_region_draining_tasks(GCTaskQueue* q,
 
         NOT_PRODUCT(++fillable_regions;)
 
-        // Assign regions to threads in round-robin fashion.
+        // Assign regions to tasks in round-robin fashion.
         if (++which == task_count) {
+          assert(which <= parallel_gc_threads,
+            "Inconsistent number of workers");
           which = 0;
         }
       }
@@ -2642,26 +2656,19 @@ void PSParallelCompact::compact() {
   PSOldGen* old_gen = heap->old_gen();
   old_gen->start_array()->reset();
   uint parallel_gc_threads = heap->gc_task_manager()->workers();
+  uint active_gc_threads = heap->gc_task_manager()->active_workers();
   TaskQueueSetSuper* qset = ParCompactionManager::region_array();
-  ParallelTaskTerminator terminator(parallel_gc_threads, qset);
+  ParallelTaskTerminator terminator(active_gc_threads, qset);
 
   GCTaskQueue* q = GCTaskQueue::create();
-  enqueue_region_draining_tasks(q, parallel_gc_threads);
-  enqueue_dense_prefix_tasks(q, parallel_gc_threads);
-  enqueue_region_stealing_tasks(q, &terminator, parallel_gc_threads);
+  enqueue_region_draining_tasks(q, active_gc_threads);
+  enqueue_dense_prefix_tasks(q, active_gc_threads);
+  enqueue_region_stealing_tasks(q, &terminator, active_gc_threads);
 
   {
     TraceTime tm_pc("par compact", print_phases(), true, gclog_or_tty);
 
-    WaitForBarrierGCTask* fin = WaitForBarrierGCTask::create();
-    q->enqueue(fin);
-
-    gc_task_manager()->add_list(q);
-
-    fin->wait_for();
-
-    // We have to release the barrier tasks!
-    WaitForBarrierGCTask::destroy(fin);
+    gc_task_manager()->execute_and_wait(q);
 
 #ifdef  ASSERT
     // Verify that all regions have been processed before the deferred updates.
@@ -2729,6 +2736,9 @@ void
 PSParallelCompact::follow_weak_klass_links() {
   // All klasses on the revisit stack are marked at this point.
   // Update and follow all subklass, sibling and implementor links.
+  // Check all the stacks here even if not all the workers are active.
+  // There is no accounting which indicates which stacks might have
+  // contents to be followed.
   if (PrintRevisitStats) {
     gclog_or_tty->print_cr("#classes in system dictionary = %d",
                            SystemDictionary::number_of_classes());
diff --git a/src/share/vm/gc_implementation/parallelScavenge/psScavenge.cpp b/src/share/vm/gc_implementation/parallelScavenge/psScavenge.cpp
index 1094d1708..0cf826eca 100644
--- a/src/share/vm/gc_implementation/parallelScavenge/psScavenge.cpp
+++ b/src/share/vm/gc_implementation/parallelScavenge/psScavenge.cpp
@@ -181,28 +181,29 @@ class PSRefProcTaskExecutor: public AbstractRefProcTaskExecutor {
 void PSRefProcTaskExecutor::execute(ProcessTask& task)
 {
   GCTaskQueue* q = GCTaskQueue::create();
-  for(uint i=0; i<ParallelGCThreads; i++) {
+  GCTaskManager* manager = ParallelScavengeHeap::gc_task_manager();
+  for(uint i=0; i < manager->active_workers(); i++) {
     q->enqueue(new PSRefProcTaskProxy(task, i));
   }
-  ParallelTaskTerminator terminator(
-                 ParallelScavengeHeap::gc_task_manager()->workers(),
+  ParallelTaskTerminator terminator(manager->active_workers(),
                  (TaskQueueSetSuper*) PSPromotionManager::stack_array_depth());
-  if (task.marks_oops_alive() && ParallelGCThreads > 1) {
-    for (uint j=0; j<ParallelGCThreads; j++) {
+  if (task.marks_oops_alive() && manager->active_workers() > 1) {
+    for (uint j = 0; j < manager->active_workers(); j++) {
       q->enqueue(new StealTask(&terminator));
     }
   }
-  ParallelScavengeHeap::gc_task_manager()->execute_and_wait(q);
+  manager->execute_and_wait(q);
 }
 
 
 void PSRefProcTaskExecutor::execute(EnqueueTask& task)
 {
   GCTaskQueue* q = GCTaskQueue::create();
-  for(uint i=0; i<ParallelGCThreads; i++) {
+  GCTaskManager* manager = ParallelScavengeHeap::gc_task_manager();
+  for(uint i=0; i < manager->active_workers(); i++) {
     q->enqueue(new PSRefEnqueueTaskProxy(task, i));
   }
-  ParallelScavengeHeap::gc_task_manager()->execute_and_wait(q);
+  manager->execute_and_wait(q);
 }
 
 // This method contains all heap specific policy for invoking scavenge.
@@ -375,6 +376,14 @@ bool PSScavenge::invoke_no_policy() {
     // Release all previously held resources
     gc_task_manager()->release_all_resources();
 
+    // Set the number of GC threads to be used in this collection
+    gc_task_manager()->set_active_gang();
+    gc_task_manager()->task_idle_workers();
+    // Get the active number of workers here and use that value
+    // throughout the methods.
+    uint active_workers = gc_task_manager()->active_workers();
+    heap->set_par_threads(active_workers);
+
     PSPromotionManager::pre_scavenge();
 
     // We'll use the promotion manager again later.
@@ -385,8 +394,9 @@ bool PSScavenge::invoke_no_policy() {
 
       GCTaskQueue* q = GCTaskQueue::create();
 
-      for(uint i=0; i<ParallelGCThreads; i++) {
-        q->enqueue(new OldToYoungRootsTask(old_gen, old_top, i));
+      uint stripe_total = active_workers;
+      for(uint i=0; i < stripe_total; i++) {
+        q->enqueue(new OldToYoungRootsTask(old_gen, old_top, i, stripe_total));
       }
 
       q->enqueue(new SerialOldToYoungRootsTask(perm_gen, perm_top));
@@ -403,10 +413,10 @@ bool PSScavenge::invoke_no_policy() {
       q->enqueue(new ScavengeRootsTask(ScavengeRootsTask::code_cache));
 
       ParallelTaskTerminator terminator(
-                  gc_task_manager()->workers(),
+        active_workers,
                   (TaskQueueSetSuper*) promotion_manager->stack_array_depth());
-      if (ParallelGCThreads>1) {
-        for (uint j=0; j<ParallelGCThreads; j++) {
+      if (active_workers > 1) {
+        for (uint j = 0; j < active_workers; j++) {
           q->enqueue(new StealTask(&terminator));
         }
       }
@@ -419,6 +429,7 @@ bool PSScavenge::invoke_no_policy() {
     // Process reference objects discovered during scavenge
     {
       reference_processor()->setup_policy(false); // not always_clear
+      reference_processor()->set_active_mt_degree(active_workers);
       PSKeepAliveClosure keep_alive(promotion_manager);
       PSEvacuateFollowersClosure evac_followers(promotion_manager);
       if (reference_processor()->processing_is_mt()) {
@@ -622,6 +633,8 @@ bool PSScavenge::invoke_no_policy() {
     // Track memory usage and detect low memory
     MemoryService::track_memory_usage();
     heap->update_counters();
+
+    gc_task_manager()->release_idle_workers();
   }
 
   if (VerifyAfterGC && heap->total_collections() >= VerifyGCStartAt) {
@@ -804,6 +817,7 @@ void PSScavenge::initialize() {
 
   // Initialize ref handling object for scavenging.
   MemRegion mr = young_gen->reserved();
+
   _ref_processor =
     new ReferenceProcessor(mr,                         // span
                            ParallelRefProcEnabled && (ParallelGCThreads > 1), // mt processing
diff --git a/src/share/vm/gc_implementation/parallelScavenge/psTasks.cpp b/src/share/vm/gc_implementation/parallelScavenge/psTasks.cpp
index f3011a824..0800681f9 100644
--- a/src/share/vm/gc_implementation/parallelScavenge/psTasks.cpp
+++ b/src/share/vm/gc_implementation/parallelScavenge/psTasks.cpp
@@ -202,7 +202,8 @@ void OldToYoungRootsTask::do_it(GCTaskManager* manager, uint which) {
                                            _gen->object_space(),
                                            _gen_top,
                                            pm,
-                                           _stripe_number);
+                                           _stripe_number,
+                                           _stripe_total);
 
     // Do the real work
     pm->drain_stacks(false);
diff --git a/src/share/vm/gc_implementation/parallelScavenge/psTasks.hpp b/src/share/vm/gc_implementation/parallelScavenge/psTasks.hpp
index 7ae5b21cb..d31f653ac 100644
--- a/src/share/vm/gc_implementation/parallelScavenge/psTasks.hpp
+++ b/src/share/vm/gc_implementation/parallelScavenge/psTasks.hpp
@@ -135,16 +135,63 @@ class SerialOldToYoungRootsTask : public GCTask {
 // OldToYoungRootsTask
 //
 // This task is used to scan old to young roots in parallel
+//
+// A GC thread executing this tasks divides the generation (old gen)
+// into slices and takes a stripe in the slice as its part of the
+// work.
+//
+//      +===============+        slice 0
+//      |  stripe 0     |
+//      +---------------+
+//      |  stripe 1     |
+//      +---------------+
+//      |  stripe 2     |
+//      +---------------+
+//      |  stripe 3     |
+//      +===============+        slice 1
+//      |  stripe 0     |
+//      +---------------+
+//      |  stripe 1     |
+//      +---------------+
+//      |  stripe 2     |
+//      +---------------+
+//      |  stripe 3     |
+//      +===============+        slice 2
+//      ...
+//
+// A task is created for each stripe.  In this case there are 4 tasks
+// created.  A GC thread first works on its stripe within slice 0
+// and then moves to its stripe in the next slice until all stripes
+// exceed the top of the generation.  Note that having fewer GC threads
+// than stripes works because all the tasks are executed so all stripes
+// will be covered.  In this example if 4 tasks have been created to cover
+// all the stripes and there are only 3 threads, one of the threads will
+// get the tasks with the 4th stripe.  However, there is a dependence in
+// CardTableExtension::scavenge_contents_parallel() on the number
+// of tasks created.  In scavenge_contents_parallel the distance
+// to the next stripe is calculated based on the number of tasks.
+// If the stripe width is ssize, a task's next stripe is at
+// ssize * number_of_tasks (= slice_stride).  In this case after
+// finishing stripe 0 in slice 0, the thread finds the stripe 0 in slice1
+// by adding slice_stride to the start of stripe 0 in slice 0 to get
+// to the start of stride 0 in slice 1.
 
 class OldToYoungRootsTask : public GCTask {
  private:
   PSOldGen* _gen;
   HeapWord* _gen_top;
   uint _stripe_number;
+  uint _stripe_total;
 
  public:
-  OldToYoungRootsTask(PSOldGen *gen, HeapWord* gen_top, uint stripe_number) :
-    _gen(gen), _gen_top(gen_top), _stripe_number(stripe_number) { }
+  OldToYoungRootsTask(PSOldGen *gen,
+                      HeapWord* gen_top,
+                      uint stripe_number,
+                      uint stripe_total) :
+    _gen(gen),
+    _gen_top(gen_top),
+    _stripe_number(stripe_number),
+    _stripe_total(stripe_total) { }
 
   char* name() { return (char *)"old-to-young-roots-task"; }
 
diff --git a/src/share/vm/gc_implementation/shared/adaptiveSizePolicy.cpp b/src/share/vm/gc_implementation/shared/adaptiveSizePolicy.cpp
index 4c933f85b..2b5858b72 100644
--- a/src/share/vm/gc_implementation/shared/adaptiveSizePolicy.cpp
+++ b/src/share/vm/gc_implementation/shared/adaptiveSizePolicy.cpp
@@ -28,8 +28,10 @@
 #include "memory/collectorPolicy.hpp"
 #include "runtime/timer.hpp"
 #include "utilities/ostream.hpp"
+#include "utilities/workgroup.hpp"
 elapsedTimer AdaptiveSizePolicy::_minor_timer;
 elapsedTimer AdaptiveSizePolicy::_major_timer;
+bool AdaptiveSizePolicy::_debug_perturbation = false;
 
 // The throughput goal is implemented as
 //      _throughput_goal = 1 - ( 1 / (1 + gc_cost_ratio))
@@ -88,6 +90,134 @@ AdaptiveSizePolicy::AdaptiveSizePolicy(size_t init_eden_size,
   _young_gen_policy_is_ready = false;
 }
 
+//  If the number of GC threads was set on the command line,
+// use it.
+//  Else
+//    Calculate the number of GC threads based on the number of Java threads.
+//    Calculate the number of GC threads based on the size of the heap.
+//    Use the larger.
+
+int AdaptiveSizePolicy::calc_default_active_workers(uintx total_workers,
+                                            const uintx min_workers,
+                                            uintx active_workers,
+                                            uintx application_workers) {
+  // If the user has specifically set the number of
+  // GC threads, use them.
+
+  // If the user has turned off using a dynamic number of GC threads
+  // or the users has requested a specific number, set the active
+  // number of workers to all the workers.
+
+  uintx new_active_workers = total_workers;
+  uintx prev_active_workers = active_workers;
+  uintx active_workers_by_JT = 0;
+  uintx active_workers_by_heap_size = 0;
+
+  // Always use at least min_workers but use up to
+  // GCThreadsPerJavaThreads * application threads.
+  active_workers_by_JT =
+    MAX2((uintx) GCWorkersPerJavaThread * application_workers,
+         min_workers);
+
+  // Choose a number of GC threads based on the current size
+  // of the heap.  This may be complicated because the size of
+  // the heap depends on factors such as the thoughput goal.
+  // Still a large heap should be collected by more GC threads.
+  active_workers_by_heap_size =
+      MAX2((size_t) 2U, Universe::heap()->capacity() / HeapSizePerGCThread);
+
+  uintx max_active_workers =
+    MAX2(active_workers_by_JT, active_workers_by_heap_size);
+
+  // Limit the number of workers to the the number created,
+  // (workers()).
+  new_active_workers = MIN2(max_active_workers,
+                                (uintx) total_workers);
+
+  // Increase GC workers instantly but decrease them more
+  // slowly.
+  if (new_active_workers < prev_active_workers) {
+    new_active_workers =
+      MAX2(min_workers, (prev_active_workers + new_active_workers) / 2);
+  }
+
+  // Check once more that the number of workers is within the limits.
+  assert(min_workers <= total_workers, "Minimum workers not consistent with total workers");
+  assert(new_active_workers >= min_workers, "Minimum workers not observed");
+  assert(new_active_workers <= total_workers, "Total workers not observed");
+
+  if (ForceDynamicNumberOfGCThreads) {
+    // Assume this is debugging and jiggle the number of GC threads.
+    if (new_active_workers == prev_active_workers) {
+      if (new_active_workers < total_workers) {
+        new_active_workers++;
+      } else if (new_active_workers > min_workers) {
+        new_active_workers--;
+      }
+    }
+    if (new_active_workers == total_workers) {
+      if (_debug_perturbation) {
+        new_active_workers =  min_workers;
+      }
+      _debug_perturbation = !_debug_perturbation;
+    }
+    assert((new_active_workers <= (uintx) ParallelGCThreads) &&
+           (new_active_workers >= min_workers),
+      "Jiggled active workers too much");
+  }
+
+  if (TraceDynamicGCThreads) {
+     gclog_or_tty->print_cr("GCTaskManager::calc_default_active_workers() : "
+       "active_workers(): %d  new_acitve_workers: %d  "
+       "prev_active_workers: %d\n"
+       " active_workers_by_JT: %d  active_workers_by_heap_size: %d",
+       active_workers, new_active_workers, prev_active_workers,
+       active_workers_by_JT, active_workers_by_heap_size);
+  }
+  assert(new_active_workers > 0, "Always need at least 1");
+  return new_active_workers;
+}
+
+int AdaptiveSizePolicy::calc_active_workers(uintx total_workers,
+                                            uintx active_workers,
+                                            uintx application_workers) {
+  // If the user has specifically set the number of
+  // GC threads, use them.
+
+  // If the user has turned off using a dynamic number of GC threads
+  // or the users has requested a specific number, set the active
+  // number of workers to all the workers.
+
+  int new_active_workers;
+  if (!UseDynamicNumberOfGCThreads ||
+     (!FLAG_IS_DEFAULT(ParallelGCThreads) && !ForceDynamicNumberOfGCThreads)) {
+    new_active_workers = total_workers;
+  } else {
+    new_active_workers = calc_default_active_workers(total_workers,
+                                                     2, /* Minimum number of workers */
+                                                     active_workers,
+                                                     application_workers);
+  }
+  assert(new_active_workers > 0, "Always need at least 1");
+  return new_active_workers;
+}
+
+int AdaptiveSizePolicy::calc_active_conc_workers(uintx total_workers,
+                                                 uintx active_workers,
+                                                 uintx application_workers) {
+  if (!UseDynamicNumberOfGCThreads ||
+     (!FLAG_IS_DEFAULT(ConcGCThreads) && !ForceDynamicNumberOfGCThreads)) {
+    return ConcGCThreads;
+  } else {
+    int no_of_gc_threads = calc_default_active_workers(
+                             total_workers,
+                             1, /* Minimum number of workers */
+                             active_workers,
+                             application_workers);
+    return no_of_gc_threads;
+  }
+}
+
 bool AdaptiveSizePolicy::tenuring_threshold_change() const {
   return decrement_tenuring_threshold_for_gc_cost() ||
          increment_tenuring_threshold_for_gc_cost() ||
diff --git a/src/share/vm/gc_implementation/shared/adaptiveSizePolicy.hpp b/src/share/vm/gc_implementation/shared/adaptiveSizePolicy.hpp
index 4822eb4bf..dd1895e9e 100644
--- a/src/share/vm/gc_implementation/shared/adaptiveSizePolicy.hpp
+++ b/src/share/vm/gc_implementation/shared/adaptiveSizePolicy.hpp
@@ -187,6 +187,8 @@ class AdaptiveSizePolicy : public CHeapObj {
   julong _young_gen_change_for_minor_throughput;
   julong _old_gen_change_for_major_throughput;
 
+  static const uint GCWorkersPerJavaThread  = 2;
+
   // Accessors
 
   double gc_pause_goal_sec() const { return _gc_pause_goal_sec; }
@@ -331,6 +333,8 @@ class AdaptiveSizePolicy : public CHeapObj {
   // Return true if the policy suggested a change.
   bool tenuring_threshold_change() const;
 
+  static bool _debug_perturbation;
+
  public:
   AdaptiveSizePolicy(size_t init_eden_size,
                      size_t init_promo_size,
@@ -338,6 +342,31 @@ class AdaptiveSizePolicy : public CHeapObj {
                      double gc_pause_goal_sec,
                      uint gc_cost_ratio);
 
+  // Return number default  GC threads to use in the next GC.
+  static int calc_default_active_workers(uintx total_workers,
+                                         const uintx min_workers,
+                                         uintx active_workers,
+                                         uintx application_workers);
+
+  // Return number of GC threads to use in the next GC.
+  // This is called sparingly so as not to change the
+  // number of GC workers gratuitously.
+  //   For ParNew collections
+  //   For PS scavenge and ParOld collections
+  //   For G1 evacuation pauses (subject to update)
+  // Other collection phases inherit the number of
+  // GC workers from the calls above.  For example,
+  // a CMS parallel remark uses the same number of GC
+  // workers as the most recent ParNew collection.
+  static int calc_active_workers(uintx total_workers,
+                                 uintx active_workers,
+                                 uintx application_workers);
+
+  // Return number of GC threads to use in the next concurrent GC phase.
+  static int calc_active_conc_workers(uintx total_workers,
+                                      uintx active_workers,
+                                      uintx application_workers);
+
   bool is_gc_cms_adaptive_size_policy() {
     return kind() == _gc_cms_adaptive_size_policy;
   }
diff --git a/src/share/vm/memory/cardTableModRefBS.cpp b/src/share/vm/memory/cardTableModRefBS.cpp
index c6645015d..11b0e384d 100644
--- a/src/share/vm/memory/cardTableModRefBS.cpp
+++ b/src/share/vm/memory/cardTableModRefBS.cpp
@@ -460,9 +460,43 @@ void CardTableModRefBS::non_clean_card_iterate_possibly_parallel(Space* sp,
                                                                  OopsInGenClosure* cl,
                                                                  CardTableRS* ct) {
   if (!mr.is_empty()) {
-    int n_threads = SharedHeap::heap()->n_par_threads();
-    if (n_threads > 0) {
+    // Caller (process_strong_roots()) claims that all GC threads
+    // execute this call.  With UseDynamicNumberOfGCThreads now all
+    // active GC threads execute this call.  The number of active GC
+    // threads needs to be passed to par_non_clean_card_iterate_work()
+    // to get proper partitioning and termination.
+    //
+    // This is an example of where n_par_threads() is used instead
+    // of workers()->active_workers().  n_par_threads can be set to 0 to
+    // turn off parallelism.  For example when this code is called as
+    // part of verification and SharedHeap::process_strong_roots() is being
+    // used, then n_par_threads() may have been set to 0.  active_workers
+    // is not overloaded with the meaning that it is a switch to disable
+    // parallelism and so keeps the meaning of the number of
+    // active gc workers.  If parallelism has not been shut off by
+    // setting n_par_threads to 0, then n_par_threads should be
+    // equal to active_workers.  When a different mechanism for shutting
+    // off parallelism is used, then active_workers can be used in
+    // place of n_par_threads.
+    //  This is an example of a path where n_par_threads is
+    // set to 0 to turn off parallism.
+    //  [7] CardTableModRefBS::non_clean_card_iterate()
+    //  [8] CardTableRS::younger_refs_in_space_iterate()
+    //  [9] Generation::younger_refs_in_space_iterate()
+    //  [10] OneContigSpaceCardGeneration::younger_refs_iterate()
+    //  [11] CompactingPermGenGen::younger_refs_iterate()
+    //  [12] CardTableRS::younger_refs_iterate()
+    //  [13] SharedHeap::process_strong_roots()
+    //  [14] G1CollectedHeap::verify()
+    //  [15] Universe::verify()
+    //  [16] G1CollectedHeap::do_collection_pause_at_safepoint()
+    //
+    int n_threads =  SharedHeap::heap()->n_par_threads();
+    bool is_par = n_threads > 0;
+    if (is_par) {
 #ifndef SERIALGC
+      assert(SharedHeap::heap()->n_par_threads() ==
+             SharedHeap::heap()->workers()->active_workers(), "Mismatch");
       non_clean_card_iterate_parallel_work(sp, mr, cl, ct, n_threads);
 #else  // SERIALGC
       fatal("Parallel gc not supported here.");
@@ -489,6 +523,10 @@ void CardTableModRefBS::non_clean_card_iterate_possibly_parallel(Space* sp,
 // change their values in any manner.
 void CardTableModRefBS::non_clean_card_iterate_serial(MemRegion mr,
                                                       MemRegionClosure* cl) {
+  bool is_par = (SharedHeap::heap()->n_par_threads() > 0);
+  assert(!is_par ||
+          (SharedHeap::heap()->n_par_threads() ==
+          SharedHeap::heap()->workers()->active_workers()), "Mismatch");
   for (int i = 0; i < _cur_covered_regions; i++) {
     MemRegion mri = mr.intersection(_covered[i]);
     if (mri.word_size() > 0) {
diff --git a/src/share/vm/memory/cardTableRS.cpp b/src/share/vm/memory/cardTableRS.cpp
index 4b47da783..c152d6cb0 100644
--- a/src/share/vm/memory/cardTableRS.cpp
+++ b/src/share/vm/memory/cardTableRS.cpp
@@ -164,7 +164,13 @@ inline bool ClearNoncleanCardWrapper::clear_card_serial(jbyte* entry) {
 ClearNoncleanCardWrapper::ClearNoncleanCardWrapper(
   DirtyCardToOopClosure* dirty_card_closure, CardTableRS* ct) :
     _dirty_card_closure(dirty_card_closure), _ct(ct) {
+    // Cannot yet substitute active_workers for n_par_threads
+    // in the case where parallelism is being turned off by
+    // setting n_par_threads to 0.
     _is_par = (SharedHeap::heap()->n_par_threads() > 0);
+    assert(!_is_par ||
+           (SharedHeap::heap()->n_par_threads() ==
+            SharedHeap::heap()->workers()->active_workers()), "Mismatch");
 }
 
 void ClearNoncleanCardWrapper::do_MemRegion(MemRegion mr) {
diff --git a/src/share/vm/memory/sharedHeap.cpp b/src/share/vm/memory/sharedHeap.cpp
index 15d8eaa54..8094b6e7a 100644
--- a/src/share/vm/memory/sharedHeap.cpp
+++ b/src/share/vm/memory/sharedHeap.cpp
@@ -58,7 +58,6 @@ SharedHeap::SharedHeap(CollectorPolicy* policy_) :
   _perm_gen(NULL), _rem_set(NULL),
   _strong_roots_parity(0),
   _process_strong_tasks(new SubTasksDone(SH_PS_NumElements)),
-  _n_par_threads(0),
   _workers(NULL)
 {
   if (_process_strong_tasks == NULL || !_process_strong_tasks->valid()) {
@@ -80,6 +79,14 @@ SharedHeap::SharedHeap(CollectorPolicy* policy_) :
   }
 }
 
+int SharedHeap::n_termination() {
+  return _process_strong_tasks->n_threads();
+}
+
+void SharedHeap::set_n_termination(int t) {
+  _process_strong_tasks->set_n_threads(t);
+}
+
 bool SharedHeap::heap_lock_held_for_gc() {
   Thread* t = Thread::current();
   return    Heap_lock->owned_by_self()
@@ -144,6 +151,10 @@ void SharedHeap::process_strong_roots(bool activate_scope,
   StrongRootsScope srs(this, activate_scope);
   // General strong roots.
   assert(_strong_roots_parity != 0, "must have called prologue code");
+  // _n_termination for _process_strong_tasks should be set up stream
+  // in a method not running in a GC worker.  Otherwise the GC worker
+  // could be trying to change the termination condition while the task
+  // is executing in another GC worker.
   if (!_process_strong_tasks->is_task_claimed(SH_PS_Universe_oops_do)) {
     Universe::oops_do(roots);
     // Consider perm-gen discovered lists to be strong.
diff --git a/src/share/vm/memory/sharedHeap.hpp b/src/share/vm/memory/sharedHeap.hpp
index 55a9b569d..8e819c487 100644
--- a/src/share/vm/memory/sharedHeap.hpp
+++ b/src/share/vm/memory/sharedHeap.hpp
@@ -49,6 +49,62 @@ class FlexibleWorkGang;
 class CollectorPolicy;
 class KlassHandle;
 
+// Note on use of FlexibleWorkGang's for GC.
+// There are three places where task completion is determined.
+// In
+//    1) ParallelTaskTerminator::offer_termination() where _n_threads
+//    must be set to the correct value so that count of workers that
+//    have offered termination will exactly match the number
+//    working on the task.  Tasks such as those derived from GCTask
+//    use ParallelTaskTerminator's.  Tasks that want load balancing
+//    by work stealing use this method to gauge completion.
+//    2) SubTasksDone has a variable _n_threads that is used in
+//    all_tasks_completed() to determine completion.  all_tasks_complete()
+//    counts the number of tasks that have been done and then reset
+//    the SubTasksDone so that it can be used again.  When the number of
+//    tasks is set to the number of GC workers, then _n_threads must
+//    be set to the number of active GC workers. G1CollectedHeap,
+//    HRInto_G1RemSet, GenCollectedHeap and SharedHeap have SubTasksDone.
+//    This seems too many.
+//    3) SequentialSubTasksDone has an _n_threads that is used in
+//    a way similar to SubTasksDone and has the same dependency on the
+//    number of active GC workers.  CompactibleFreeListSpace and Space
+//    have SequentialSubTasksDone's.
+// Example of using SubTasksDone and SequentialSubTasksDone
+// G1CollectedHeap::g1_process_strong_roots() calls
+//  process_strong_roots(false, // no scoping; this is parallel code
+//                       collecting_perm_gen, so,
+//                       &buf_scan_non_heap_roots,
+//                       &eager_scan_code_roots,
+//                       &buf_scan_perm);
+//  which delegates to SharedHeap::process_strong_roots() and uses
+//  SubTasksDone* _process_strong_tasks to claim tasks.
+//  process_strong_roots() calls
+//      rem_set()->younger_refs_iterate(perm_gen(), perm_blk);
+//  to scan the card table and which eventually calls down into
+//  CardTableModRefBS::par_non_clean_card_iterate_work().  This method
+//  uses SequentialSubTasksDone* _pst to claim tasks.
+//  Both SubTasksDone and SequentialSubTasksDone call their method
+//  all_tasks_completed() to count the number of GC workers that have
+//  finished their work.  That logic is "when all the workers are
+//  finished the tasks are finished".
+//
+//  The pattern that appears  in the code is to set _n_threads
+//  to a value > 1 before a task that you would like executed in parallel
+//  and then to set it to 0 after that task has completed.  A value of
+//  0 is a "special" value in set_n_threads() which translates to
+//  setting _n_threads to 1.
+//
+//  Some code uses _n_terminiation to decide if work should be done in
+//  parallel.  The notorious possibly_parallel_oops_do() in threads.cpp
+//  is an example of such code.  Look for variable "is_par" for other
+//  examples.
+//
+//  The active_workers is not reset to 0 after a parallel phase.  It's
+//  value may be used in later phases and in one instance at least
+//  (the parallel remark) it has to be used (the parallel remark depends
+//  on the partitioning done in the previous parallel scavenge).
+
 class SharedHeap : public CollectedHeap {
   friend class VMStructs;
 
@@ -84,11 +140,6 @@ protected:
   // If we're doing parallel GC, use this gang of threads.
   FlexibleWorkGang* _workers;
 
-  // Number of parallel threads currently working on GC tasks.
-  // O indicates use sequential code; 1 means use parallel code even with
-  // only one thread, for performance testing purposes.
-  int _n_par_threads;
-
   // Full initialization is done in a concrete subtype's "initialize"
   // function.
   SharedHeap(CollectorPolicy* policy_);
@@ -107,6 +158,7 @@ public:
   CollectorPolicy *collector_policy() const { return _collector_policy; }
 
   void set_barrier_set(BarrierSet* bs);
+  SubTasksDone* process_strong_tasks() { return _process_strong_tasks; }
 
   // Does operations required after initialization has been done.
   virtual void post_initialize();
@@ -198,13 +250,6 @@ public:
 
   FlexibleWorkGang* workers() const { return _workers; }
 
-  // Sets the number of parallel threads that will be doing tasks
-  // (such as process strong roots) subsequently.
-  virtual void set_par_threads(int t);
-
-  // Number of threads currently working on GC tasks.
-  int n_par_threads() { return _n_par_threads; }
-
   // Invoke the "do_oop" method the closure "roots" on all root locations.
   // If "collecting_perm_gen" is false, then roots that may only contain
   // references to permGen objects are not scanned; instead, in that case,
@@ -240,6 +285,13 @@ public:
   virtual void gc_prologue(bool full) = 0;
   virtual void gc_epilogue(bool full) = 0;
 
+  // Sets the number of parallel threads that will be doing tasks
+  // (such as process strong roots) subsequently.
+  virtual void set_par_threads(int t);
+
+  int n_termination();
+  void set_n_termination(int t);
+
   //
   // New methods from CollectedHeap
   //
diff --git a/src/share/vm/runtime/arguments.cpp b/src/share/vm/runtime/arguments.cpp
index b8e514d5c..a3a68e7b8 100644
--- a/src/share/vm/runtime/arguments.cpp
+++ b/src/share/vm/runtime/arguments.cpp
@@ -1394,8 +1394,8 @@ void Arguments::set_parallel_gc_flags() {
   // If no heap maximum was requested explicitly, use some reasonable fraction
   // of the physical memory, up to a maximum of 1GB.
   if (UseParallelGC) {
-    FLAG_SET_ERGO(uintx, ParallelGCThreads,
-                  Abstract_VM_Version::parallel_worker_threads());
+    FLAG_SET_DEFAULT(ParallelGCThreads,
+                     Abstract_VM_Version::parallel_worker_threads());
 
     // If InitialSurvivorRatio or MinSurvivorRatio were not specified, but the
     // SurvivorRatio has been set, reset their default values to SurvivorRatio +
diff --git a/src/share/vm/runtime/globals.hpp b/src/share/vm/runtime/globals.hpp
index f330d44ea..3ef356ebe 100644
--- a/src/share/vm/runtime/globals.hpp
+++ b/src/share/vm/runtime/globals.hpp
@@ -1416,6 +1416,21 @@ class CommandLineFlags {
   product(uintx, ParallelGCThreads, 0,                                      \
           "Number of parallel threads parallel gc will use")                \
                                                                             \
+  product(bool, UseDynamicNumberOfGCThreads, false,                         \
+          "Dynamically choose the number of parallel threads "              \
+          "parallel gc will use")                                           \
+                                                                            \
+  diagnostic(bool, ForceDynamicNumberOfGCThreads, false,                    \
+          "Force dynamic selection of the number of"                        \
+          "parallel threads parallel gc will use to aid debugging")         \
+                                                                            \
+  product(uintx, HeapSizePerGCThread, ScaleForWordSize(64*M),               \
+          "Size of heap (bytes) per GC thread used in calculating the "     \
+          "number of GC threads")                                           \
+                                                                            \
+  product(bool, TraceDynamicGCThreads, false,                               \
+          "Trace the dynamic GC thread usage")                              \
+                                                                            \
   develop(bool, ParallelOldGCSplitALot, false,                              \
           "Provoke splitting (copying data from a young gen space to"       \
           "multiple destination spaces)")                                   \
@@ -2357,7 +2372,7 @@ class CommandLineFlags {
   develop(bool, TraceGCTaskQueue, false,                                    \
           "Trace actions of the GC task queues")                            \
                                                                             \
-  develop(bool, TraceGCTaskThread, false,                                   \
+  diagnostic(bool, TraceGCTaskThread, false,                                   \
           "Trace actions of the GC task threads")                           \
                                                                             \
   product(bool, PrintParallelOldGCPhaseTimes, false,                        \
diff --git a/src/share/vm/runtime/thread.cpp b/src/share/vm/runtime/thread.cpp
index a8cbf854a..da291036e 100644
--- a/src/share/vm/runtime/thread.cpp
+++ b/src/share/vm/runtime/thread.cpp
@@ -778,12 +778,12 @@ bool Thread::claim_oops_do_par_case(int strong_roots_parity) {
       return true;
     } else {
       guarantee(res == strong_roots_parity, "Or else what?");
-      assert(SharedHeap::heap()->n_par_threads() > 0,
-             "Should only fail when parallel.");
+      assert(SharedHeap::heap()->workers()->active_workers() > 0,
+         "Should only fail when parallel.");
       return false;
     }
   }
-  assert(SharedHeap::heap()->n_par_threads() > 0,
+  assert(SharedHeap::heap()->workers()->active_workers() > 0,
          "Should only fail when parallel.");
   return false;
 }
@@ -3939,7 +3939,15 @@ void Threads::possibly_parallel_oops_do(OopClosure* f, CodeBlobClosure* cf) {
   // root groups.  Overhead should be small enough to use all the time,
   // even in sequential code.
   SharedHeap* sh = SharedHeap::heap();
-  bool is_par = (sh->n_par_threads() > 0);
+  // Cannot yet substitute active_workers for n_par_threads
+  // because of G1CollectedHeap::verify() use of
+  // SharedHeap::process_strong_roots().  n_par_threads == 0 will
+  // turn off parallelism in process_strong_roots while active_workers
+  // is being used for parallelism elsewhere.
+  bool is_par = sh->n_par_threads() > 0;
+  assert(!is_par ||
+         (SharedHeap::heap()->n_par_threads() ==
+          SharedHeap::heap()->workers()->active_workers()), "Mismatch");
   int cp = SharedHeap::heap()->strong_roots_parity();
   ALL_JAVA_THREADS(p) {
     if (p->claim_oops_do(is_par, cp)) {
diff --git a/src/share/vm/utilities/workgroup.cpp b/src/share/vm/utilities/workgroup.cpp
index e53d78d49..8b695528e 100644
--- a/src/share/vm/utilities/workgroup.cpp
+++ b/src/share/vm/utilities/workgroup.cpp
@@ -57,7 +57,6 @@ WorkGang::WorkGang(const char* name,
                    bool        are_GC_task_threads,
                    bool        are_ConcurrentGC_threads) :
   AbstractWorkGang(name, are_GC_task_threads, are_ConcurrentGC_threads) {
-  // Save arguments.
   _total_workers = workers;
 }
 
@@ -127,6 +126,12 @@ GangWorker* AbstractWorkGang::gang_worker(int i) const {
 }
 
 void WorkGang::run_task(AbstractGangTask* task) {
+  run_task(task, total_workers());
+}
+
+void WorkGang::run_task(AbstractGangTask* task, uint no_of_parallel_workers) {
+  task->set_for_termination(no_of_parallel_workers);
+
   // This thread is executed by the VM thread which does not block
   // on ordinary MutexLocker's.
   MutexLockerEx ml(monitor(), Mutex::_no_safepoint_check_flag);
@@ -143,22 +148,32 @@ void WorkGang::run_task(AbstractGangTask* task) {
   // Tell the workers to get to work.
   monitor()->notify_all();
   // Wait for them to be finished
-  while (finished_workers() < total_workers()) {
+  while (finished_workers() < (int) no_of_parallel_workers) {
     if (TraceWorkGang) {
       tty->print_cr("Waiting in work gang %s: %d/%d finished sequence %d",
-                    name(), finished_workers(), total_workers(),
+                    name(), finished_workers(), no_of_parallel_workers,
                     _sequence_number);
     }
     monitor()->wait(/* no_safepoint_check */ true);
   }
   _task = NULL;
   if (TraceWorkGang) {
-    tty->print_cr("/nFinished work gang %s: %d/%d sequence %d",
-                  name(), finished_workers(), total_workers(),
+    tty->print_cr("\nFinished work gang %s: %d/%d sequence %d",
+                  name(), finished_workers(), no_of_parallel_workers,
                   _sequence_number);
+    Thread* me = Thread::current();
+    tty->print_cr("  T: 0x%x  VM_thread: %d", me, me->is_VM_thread());
   }
 }
 
+void FlexibleWorkGang::run_task(AbstractGangTask* task) {
+  // If active_workers() is passed, _finished_workers
+  // must only be incremented for workers that find non_null
+  // work (as opposed to all those that just check that the
+  // task is not null).
+  WorkGang::run_task(task, (uint) active_workers());
+}
+
 void AbstractWorkGang::stop() {
   // Tell all workers to terminate, then wait for them to become inactive.
   MutexLockerEx ml(monitor(), Mutex::_no_safepoint_check_flag);
@@ -168,10 +183,10 @@ void AbstractWorkGang::stop() {
   _task = NULL;
   _terminate = true;
   monitor()->notify_all();
-  while (finished_workers() < total_workers()) {
+  while (finished_workers() < active_workers()) {
     if (TraceWorkGang) {
       tty->print_cr("Waiting in work gang %s: %d/%d finished",
-                    name(), finished_workers(), total_workers());
+                    name(), finished_workers(), active_workers());
     }
     monitor()->wait(/* no_safepoint_check */ true);
   }
@@ -275,10 +290,12 @@ void GangWorker::loop() {
         // Check for new work.
         if ((data.task() != NULL) &&
             (data.sequence_number() != previous_sequence_number)) {
-          gang()->internal_note_start();
-          gang_monitor->notify_all();
-          part = gang()->started_workers() - 1;
-          break;
+          if (gang()->needs_more_workers()) {
+            gang()->internal_note_start();
+            gang_monitor->notify_all();
+            part = gang()->started_workers() - 1;
+            break;
+          }
         }
         // Nothing to do.
         gang_monitor->wait(/* no_safepoint_check */ true);
@@ -350,6 +367,9 @@ const char* AbstractGangTask::name() const {
 
 #endif /* PRODUCT */
 
+// FlexibleWorkGang
+
+
 // *** WorkGangBarrierSync
 
 WorkGangBarrierSync::WorkGangBarrierSync()
@@ -411,10 +431,8 @@ bool SubTasksDone::valid() {
 }
 
 void SubTasksDone::set_n_threads(int t) {
-#ifdef ASSERT
   assert(_claimed == 0 || _threads_completed == _n_threads,
          "should not be called while tasks are being processed!");
-#endif
   _n_threads = (t == 0 ? 1 : t);
 }
 
diff --git a/src/share/vm/utilities/workgroup.hpp b/src/share/vm/utilities/workgroup.hpp
index 8e9effebd..911616347 100644
--- a/src/share/vm/utilities/workgroup.hpp
+++ b/src/share/vm/utilities/workgroup.hpp
@@ -96,11 +96,14 @@ private:
 
 protected:
   // Constructor and desctructor: only construct subclasses.
-  AbstractGangTask(const char* name) {
+  AbstractGangTask(const char* name)
+  {
     NOT_PRODUCT(_name = name);
     _counter = 0;
   }
   virtual ~AbstractGangTask() { }
+
+public:
 };
 
 class AbstractGangTaskWOopQueues : public AbstractGangTask {
@@ -116,6 +119,7 @@ class AbstractGangTaskWOopQueues : public AbstractGangTask {
   OopTaskQueueSet* queues() { return _queues; }
 };
 
+
 // Class AbstractWorkGang:
 // An abstract class representing a gang of workers.
 // You subclass this to supply an implementation of run_task().
@@ -130,6 +134,8 @@ public:
   virtual void run_task(AbstractGangTask* task) = 0;
   // Stop and terminate all workers.
   virtual void stop();
+  // Return true if more workers should be applied to the task.
+  virtual bool needs_more_workers() const { return true; }
 public:
   // Debugging.
   const char* name() const;
@@ -287,20 +293,62 @@ public:
   AbstractWorkGang* gang() const { return _gang; }
 };
 
+// Dynamic number of worker threads
+//
+// This type of work gang is used to run different numbers of
+// worker threads at different times.  The
+// number of workers run for a task is "_active_workers"
+// instead of "_total_workers" in a WorkGang.  The method
+// "needs_more_workers()" returns true until "_active_workers"
+// have been started and returns false afterwards.  The
+// implementation of "needs_more_workers()" in WorkGang always
+// returns true so that all workers are started.  The method
+// "loop()" in GangWorker was modified to ask "needs_more_workers()"
+// in its loop to decide if it should start working on a task.
+// A worker in "loop()" waits for notification on the WorkGang
+// monitor and execution of each worker as it checks for work
+// is serialized via the same monitor.  The "needs_more_workers()"
+// call is serialized and additionally the calculation for the
+// "part" (effectively the worker id for executing the task) is
+// serialized to give each worker a unique "part".  Workers that
+// are not needed for this tasks (i.e., "_active_workers" have
+// been started before it, continue to wait for work.
+
 class FlexibleWorkGang: public WorkGang {
+  // The currently active workers in this gang.
+  // This is a number that is dynamically adjusted
+  // and checked in the run_task() method at each invocation.
+  // As described above _active_workers determines the number
+  // of threads started on a task.  It must also be used to
+  // determine completion.
+
  protected:
   int _active_workers;
  public:
   // Constructor and destructor.
+  // Initialize active_workers to a minimum value.  Setting it to
+  // the parameter "workers" will initialize it to a maximum
+  // value which is not desirable.
   FlexibleWorkGang(const char* name, int workers,
                    bool are_GC_task_threads,
                    bool  are_ConcurrentGC_threads) :
-    WorkGang(name, workers, are_GC_task_threads, are_ConcurrentGC_threads) {
-    _active_workers = ParallelGCThreads;
-  };
+    WorkGang(name, workers, are_GC_task_threads, are_ConcurrentGC_threads),
+    _active_workers(UseDynamicNumberOfGCThreads ? 1 : ParallelGCThreads) {};
   // Accessors for fields
   virtual int active_workers() const { return _active_workers; }
-  void set_active_workers(int v) { _active_workers = v; }
+  void set_active_workers(int v) {
+    assert(v <= _total_workers,
+           "Trying to set more workers active than there are");
+    _active_workers = MIN2(v, _total_workers);
+    assert(v != 0, "Trying to set active workers to 0");
+    _active_workers = MAX2(1, _active_workers);
+    assert(UseDynamicNumberOfGCThreads || _active_workers == _total_workers,
+           "Unless dynamic should use total workers");
+  }
+  virtual void run_task(AbstractGangTask* task);
+  virtual bool needs_more_workers() const {
+    return _started_workers < _active_workers;
+  }
 };
 
 // Work gangs in garbage collectors: 2009-06-10
@@ -357,6 +405,11 @@ public:
 class SubTasksDone: public CHeapObj {
   jint* _tasks;
   int _n_tasks;
+  // _n_threads is used to determine when a sub task is done.
+  // It does not control how many threads will execute the subtask
+  // but must be initialized to the number that do execute the task
+  // in order to correctly decide when the subtask is done (all the
+  // threads working on the task have finished).
   int _n_threads;
   jint _threads_completed;
 #ifdef ASSERT
diff --git a/src/share/vm/utilities/yieldingWorkgroup.cpp b/src/share/vm/utilities/yieldingWorkgroup.cpp
index 075b4cba7..d8daf7a5d 100644
--- a/src/share/vm/utilities/yieldingWorkgroup.cpp
+++ b/src/share/vm/utilities/yieldingWorkgroup.cpp
@@ -125,7 +125,7 @@ void YieldingFlexibleWorkGang::start_task(YieldingFlexibleGangTask* new_task) {
   if (requested_size != 0) {
     _active_workers = MIN2(requested_size, total_workers());
   } else {
-    _active_workers = total_workers();
+    _active_workers = active_workers();
   }
   new_task->set_actual_size(_active_workers);
   new_task->set_for_termination(_active_workers);
@@ -148,22 +148,22 @@ void YieldingFlexibleWorkGang::wait_for_gang() {
   for (Status status = yielding_task()->status();
        status != COMPLETED && status != YIELDED && status != ABORTED;
        status = yielding_task()->status()) {
-    assert(started_workers() <= total_workers(), "invariant");
-    assert(finished_workers() <= total_workers(), "invariant");
-    assert(yielded_workers() <= total_workers(), "invariant");
+    assert(started_workers() <= active_workers(), "invariant");
+    assert(finished_workers() <= active_workers(), "invariant");
+    assert(yielded_workers() <= active_workers(), "invariant");
     monitor()->wait(Mutex::_no_safepoint_check_flag);
   }
   switch (yielding_task()->status()) {
     case COMPLETED:
     case ABORTED: {
-      assert(finished_workers() == total_workers(), "Inconsistent status");
+      assert(finished_workers() == active_workers(), "Inconsistent status");
       assert(yielded_workers() == 0, "Invariant");
       reset();   // for next task; gang<->task binding released
       break;
     }
     case YIELDED: {
       assert(yielded_workers() > 0, "Invariant");
-      assert(yielded_workers() + finished_workers() == total_workers(),
+      assert(yielded_workers() + finished_workers() == active_workers(),
              "Inconsistent counts");
       break;
     }
@@ -182,7 +182,6 @@ void YieldingFlexibleWorkGang::continue_task(
 
   MutexLockerEx ml(monitor(), Mutex::_no_safepoint_check_flag);
   assert(task() != NULL && task() == gang_task, "Incorrect usage");
-  // assert(_active_workers == total_workers(), "For now");
   assert(_started_workers == _active_workers, "Precondition");
   assert(_yielded_workers > 0 && yielding_task()->status() == YIELDED,
          "Else why are we calling continue_task()");
@@ -202,7 +201,7 @@ void YieldingFlexibleWorkGang::reset() {
 void YieldingFlexibleWorkGang::yield() {
   assert(task() != NULL, "Inconsistency; should have task binding");
   MutexLockerEx ml(monitor(), Mutex::_no_safepoint_check_flag);
-  assert(yielded_workers() < total_workers(), "Consistency check");
+  assert(yielded_workers() < active_workers(), "Consistency check");
   if (yielding_task()->status() == ABORTING) {
     // Do not yield; we need to abort as soon as possible
     // XXX NOTE: This can cause a performance pathology in the
@@ -213,7 +212,7 @@ void YieldingFlexibleWorkGang::yield() {
     // us to return at each potential yield point.
     return;
   }
-  if (++_yielded_workers + finished_workers() == total_workers()) {
+  if (++_yielded_workers + finished_workers() == active_workers()) {
     yielding_task()->set_status(YIELDED);
     monitor()->notify_all();
   } else {
diff --git a/src/share/vm/utilities/yieldingWorkgroup.hpp b/src/share/vm/utilities/yieldingWorkgroup.hpp
index a3171c8ca..6d8c6bf66 100644
--- a/src/share/vm/utilities/yieldingWorkgroup.hpp
+++ b/src/share/vm/utilities/yieldingWorkgroup.hpp
@@ -199,17 +199,11 @@ public:
   void abort();
 
 private:
-  int _active_workers;
   int _yielded_workers;
   void wait_for_gang();
 
 public:
   // Accessors for fields
-  int active_workers() const {
-    return _active_workers;
-  }
-
-  // Accessors for fields
   int yielded_workers() const {
     return _yielded_workers;
   }