6883834: ParNew: assert(!_g->to()->is_in_reserved(obj),"Scanning field twice?") with LargeObjects tests

Summary: Fixed process_chunk_boundaries(), used for parallel card scanning when using ParNew/CMS, so as to prevent double-scanning, or worse, non-scanning of imprecisely marked objects exceeding parallel chunk size. Made some sizing parameters for parallel card scanning diagnostic, disabled ParallelGCRetainPLAB, and elaborated and clarified some comments.
Reviewed-by: stefank, johnc

8 files changed, 295 insertions, 138 deletions

diff --git a/src/share/vm/gc_implementation/parNew/parCardTableModRefBS.cpp b/src/share/vm/gc_implementation/parNew/parCardTableModRefBS.cpp
index afc8797ea..bfb4b5809 100644
--- a/src/share/vm/gc_implementation/parNew/parCardTableModRefBS.cpp
+++ b/src/share/vm/gc_implementation/parNew/parCardTableModRefBS.cpp
@@ -29,13 +29,14 @@
 #include "memory/sharedHeap.hpp"
 #include "memory/space.inline.hpp"
 #include "memory/universe.hpp"
+#include "oops/oop.inline.hpp"
 #include "runtime/java.hpp"
 #include "runtime/mutexLocker.hpp"
 #include "runtime/virtualspace.hpp"
 
 void CardTableModRefBS::non_clean_card_iterate_parallel_work(Space* sp, MemRegion mr,
-                                                             DirtyCardToOopClosure* dcto_cl,
-                                                             ClearNoncleanCardWrapper* cl,
+                                                             OopsInGenClosure* cl,
+                                                             CardTableRS* ct,
                                                              int n_threads) {
   assert(n_threads > 0, "Error: expected n_threads > 0");
   assert((n_threads == 1 && ParallelGCThreads == 0) ||
@@ -49,14 +50,14 @@ void CardTableModRefBS::non_clean_card_iterate_parallel_work(Space* sp, MemRegio
                           lowest_non_clean_base_chunk_index,
                           lowest_non_clean_chunk_size);
 
-  int n_strides = n_threads * StridesPerThread;
+  int n_strides = n_threads * ParGCStridesPerThread;
   SequentialSubTasksDone* pst = sp->par_seq_tasks();
   pst->set_n_threads(n_threads);
   pst->set_n_tasks(n_strides);
 
   int stride = 0;
   while (!pst->is_task_claimed(/* reference */ stride)) {
-    process_stride(sp, mr, stride, n_strides, dcto_cl, cl,
+    process_stride(sp, mr, stride, n_strides, cl, ct,
                    lowest_non_clean,
                    lowest_non_clean_base_chunk_index,
                    lowest_non_clean_chunk_size);
@@ -79,13 +80,13 @@ CardTableModRefBS::
 process_stride(Space* sp,
                MemRegion used,
                jint stride, int n_strides,
-               DirtyCardToOopClosure* dcto_cl,
-               ClearNoncleanCardWrapper* cl,
+               OopsInGenClosure* cl,
+               CardTableRS* ct,
                jbyte** lowest_non_clean,
                uintptr_t lowest_non_clean_base_chunk_index,
                size_t    lowest_non_clean_chunk_size) {
-  // We don't have to go downwards here; it wouldn't help anyway,
-  // because of parallelism.
+  // We go from higher to lower addresses here; it wouldn't help that much
+  // because of the strided parallelism pattern used here.
 
   // Find the first card address of the first chunk in the stride that is
   // at least "bottom" of the used region.
@@ -98,25 +99,35 @@ process_stride(Space* sp,
   if ((uintptr_t)stride >= start_chunk_stride_num) {
     chunk_card_start = (jbyte*)(start_card +
                                 (stride - start_chunk_stride_num) *
-                                CardsPerStrideChunk);
+                                ParGCCardsPerStrideChunk);
   } else {
     // Go ahead to the next chunk group boundary, then to the requested stride.
     chunk_card_start = (jbyte*)(start_card +
                                 (n_strides - start_chunk_stride_num + stride) *
-                                CardsPerStrideChunk);
+                                ParGCCardsPerStrideChunk);
   }
 
   while (chunk_card_start < end_card) {
-    // We don't have to go downwards here; it wouldn't help anyway,
-    // because of parallelism.  (We take care with "min_done"; see below.)
+    // Even though we go from lower to higher addresses below, the
+    // strided parallelism can interleave the actual processing of the
+    // dirty pages in various ways. For a specific chunk within this
+    // stride, we take care to avoid double scanning or missing a card
+    // by suitably initializing the "min_done" field in process_chunk_boundaries()
+    // below, together with the dirty region extension accomplished in
+    // DirtyCardToOopClosure::do_MemRegion().
+    jbyte*    chunk_card_end = chunk_card_start + ParGCCardsPerStrideChunk;
     // Invariant: chunk_mr should be fully contained within the "used" region.
-    jbyte*    chunk_card_end = chunk_card_start + CardsPerStrideChunk;
     MemRegion chunk_mr       = MemRegion(addr_for(chunk_card_start),
                                          chunk_card_end >= end_card ?
                                            used.end() : addr_for(chunk_card_end));
     assert(chunk_mr.word_size() > 0, "[chunk_card_start > used_end)");
     assert(used.contains(chunk_mr), "chunk_mr should be subset of used");
 
+    DirtyCardToOopClosure* dcto_cl = sp->new_dcto_cl(cl, precision(),
+                                                     cl->gen_boundary());
+    ClearNoncleanCardWrapper clear_cl(dcto_cl, ct);
+
+
     // Process the chunk.
     process_chunk_boundaries(sp,
                              dcto_cl,
@@ -126,17 +137,30 @@ process_stride(Space* sp,
                              lowest_non_clean_base_chunk_index,
                              lowest_non_clean_chunk_size);
 
+    // We want the LNC array updates above in process_chunk_boundaries
+    // to be visible before any of the card table value changes as a
+    // result of the dirty card iteration below.
+    OrderAccess::storestore();
+
     // We do not call the non_clean_card_iterate_serial() version because
-    // we want to clear the cards, and the ClearNoncleanCardWrapper closure
-    // itself does the work of finding contiguous dirty ranges of cards to
-    // process (and clear).
-    cl->do_MemRegion(chunk_mr);
+    // we want to clear the cards: clear_cl here does the work of finding
+    // contiguous dirty ranges of cards to process and clear.
+    clear_cl.do_MemRegion(chunk_mr);
 
     // Find the next chunk of the stride.
-    chunk_card_start += CardsPerStrideChunk * n_strides;
+    chunk_card_start += ParGCCardsPerStrideChunk * n_strides;
   }
 }
 
+
+// If you want a talkative process_chunk_boundaries,
+// then #define NOISY(x) x
+#ifdef NOISY
+#error "Encountered a global preprocessor flag, NOISY, which might clash with local definition to follow"
+#else
+#define NOISY(x)
+#endif
+
 void
 CardTableModRefBS::
 process_chunk_boundaries(Space* sp,
@@ -147,127 +171,233 @@ process_chunk_boundaries(Space* sp,
                          uintptr_t lowest_non_clean_base_chunk_index,
                          size_t    lowest_non_clean_chunk_size)
 {
-  // We must worry about the chunk boundaries.
+  // We must worry about non-array objects that cross chunk boundaries,
+  // because such objects are both precisely and imprecisely marked:
+  // .. if the head of such an object is dirty, the entire object
+  //    needs to be scanned, under the interpretation that this
+  //    was an imprecise mark
+  // .. if the head of such an object is not dirty, we can assume
+  //    precise marking and it's efficient to scan just the dirty
+  //    cards.
+  // In either case, each scanned reference must be scanned precisely
+  // once so as to avoid cloning of a young referent. For efficiency,
+  // our closures depend on this property and do not protect against
+  // double scans.
 
-  // First, set our max_to_do:
-  HeapWord* max_to_do = NULL;
   uintptr_t cur_chunk_index = addr_to_chunk_index(chunk_mr.start());
   cur_chunk_index           = cur_chunk_index - lowest_non_clean_base_chunk_index;
 
+  NOISY(tty->print_cr("===========================================================================");)
+  NOISY(tty->print_cr(" process_chunk_boundary: Called with [" PTR_FORMAT "," PTR_FORMAT ")",
+                      chunk_mr.start(), chunk_mr.end());)
+
+  // First, set "our" lowest_non_clean entry, which would be
+  // used by the thread scanning an adjoining left chunk with
+  // a non-array object straddling the mutual boundary.
+  // Find the object that spans our boundary, if one exists.
+  // first_block is the block possibly straddling our left boundary.
+  HeapWord* first_block = sp->block_start(chunk_mr.start());
+  assert((chunk_mr.start() != used.start()) || (first_block == chunk_mr.start()),
+         "First chunk should always have a co-initial block");
+  // Does the block straddle the chunk's left boundary, and is it
+  // a non-array object?
+  if (first_block < chunk_mr.start()        // first block straddles left bdry
+      && sp->block_is_obj(first_block)      // first block is an object
+      && !(oop(first_block)->is_objArray()  // first block is not an array (arrays are precisely dirtied)
+           || oop(first_block)->is_typeArray())) {
+    // Find our least non-clean card, so that a left neighbour
+    // does not scan an object straddling the mutual boundary
+    // too far to the right, and attempt to scan a portion of
+    // that object twice.
+    jbyte* first_dirty_card = NULL;
+    jbyte* last_card_of_first_obj =
+        byte_for(first_block + sp->block_size(first_block) - 1);
+    jbyte* first_card_of_cur_chunk = byte_for(chunk_mr.start());
+    jbyte* last_card_of_cur_chunk = byte_for(chunk_mr.last());
+    jbyte* last_card_to_check =
+      (jbyte*) MIN2((intptr_t) last_card_of_cur_chunk,
+                    (intptr_t) last_card_of_first_obj);
+    // Note that this does not need to go beyond our last card
+    // if our first object completely straddles this chunk.
+    for (jbyte* cur = first_card_of_cur_chunk;
+         cur <= last_card_to_check; cur++) {
+      jbyte val = *cur;
+      if (card_will_be_scanned(val)) {
+        first_dirty_card = cur; break;
+      } else {
+        assert(!card_may_have_been_dirty(val), "Error");
+      }
+    }
+    if (first_dirty_card != NULL) {
+      NOISY(tty->print_cr(" LNC: Found a dirty card at " PTR_FORMAT " in current chunk",
+                    first_dirty_card);)
+      assert(0 <= cur_chunk_index && cur_chunk_index < lowest_non_clean_chunk_size,
+             "Bounds error.");
+      assert(lowest_non_clean[cur_chunk_index] == NULL,
+             "Write exactly once : value should be stable hereafter for this round");
+      lowest_non_clean[cur_chunk_index] = first_dirty_card;
+    } NOISY(else {
+      tty->print_cr(" LNC: Found no dirty card in current chunk; leaving LNC entry NULL");
+      // In the future, we could have this thread look for a non-NULL value to copy from its
+      // right neighbour (up to the end of the first object).
+      if (last_card_of_cur_chunk < last_card_of_first_obj) {
+        tty->print_cr(" LNC: BEWARE!!! first obj straddles past right end of chunk:\n"
+                      "   might be efficient to get value from right neighbour?");
+      }
+    })
+  } else {
+    // In this case we can help our neighbour by just asking them
+    // to stop at our first card (even though it may not be dirty).
+    NOISY(tty->print_cr(" LNC: first block is not a non-array object; setting LNC to first card of current chunk");)
+    assert(lowest_non_clean[cur_chunk_index] == NULL, "Write once : value should be stable hereafter");
+    jbyte* first_card_of_cur_chunk = byte_for(chunk_mr.start());
+    lowest_non_clean[cur_chunk_index] = first_card_of_cur_chunk;
+  }
+  NOISY(tty->print_cr(" process_chunk_boundary: lowest_non_clean[" INTPTR_FORMAT "] = " PTR_FORMAT
+                "   which corresponds to the heap address " PTR_FORMAT,
+                cur_chunk_index, lowest_non_clean[cur_chunk_index],
+                (lowest_non_clean[cur_chunk_index] != NULL)
+                ? addr_for(lowest_non_clean[cur_chunk_index])
+                : NULL);)
+  NOISY(tty->print_cr("---------------------------------------------------------------------------");)
+
+  // Next, set our own max_to_do, which will strictly/exclusively bound
+  // the highest address that we will scan past the right end of our chunk.
+  HeapWord* max_to_do = NULL;
   if (chunk_mr.end() < used.end()) {
-    // This is not the last chunk in the used region.  What is the last
-    // object?
-    HeapWord* last_block = sp->block_start(chunk_mr.end());
+    // This is not the last chunk in the used region.
+    // What is our last block? We check the first block of
+    // the next (right) chunk rather than strictly check our last block
+    // because it's potentially more efficient to do so.
+    HeapWord* const last_block = sp->block_start(chunk_mr.end());
     assert(last_block <= chunk_mr.end(), "In case this property changes.");
-    if (last_block == chunk_mr.end()
-        || !sp->block_is_obj(last_block)) {
+    if ((last_block == chunk_mr.end())     // our last block does not straddle boundary
+        || !sp->block_is_obj(last_block)   // last_block isn't an object
+        || oop(last_block)->is_objArray()  // last_block is an array (precisely marked)
+        || oop(last_block)->is_typeArray()) {
       max_to_do = chunk_mr.end();
-
+      NOISY(tty->print_cr(" process_chunk_boundary: Last block on this card is not a non-array object;\n"
+                         "   max_to_do left at " PTR_FORMAT, max_to_do);)
     } else {
-      // It is an object and starts before the end of the current chunk.
+      assert(last_block < chunk_mr.end(), "Tautology");
+      // It is a non-array object that straddles the right boundary of this chunk.
       // last_obj_card is the card corresponding to the start of the last object
       // in the chunk.  Note that the last object may not start in
       // the chunk.
-      jbyte* last_obj_card = byte_for(last_block);
-      if (!card_may_have_been_dirty(*last_obj_card)) {
-        // The card containing the head is not dirty.  Any marks in
+      jbyte* const last_obj_card = byte_for(last_block);
+      const jbyte val = *last_obj_card;
+      if (!card_will_be_scanned(val)) {
+        assert(!card_may_have_been_dirty(val), "Error");
+        // The card containing the head is not dirty.  Any marks on
         // subsequent cards still in this chunk must have been made
-        // precisely; we can cap processing at the end.
+        // precisely; we can cap processing at the end of our chunk.
         max_to_do = chunk_mr.end();
+        NOISY(tty->print_cr(" process_chunk_boundary: Head of last object on this card is not dirty;\n"
+                            "   max_to_do left at " PTR_FORMAT,
+                            max_to_do);)
       } else {
         // The last object must be considered dirty, and extends onto the
         // following chunk.  Look for a dirty card in that chunk that will
         // bound our processing.
         jbyte* limit_card = NULL;
-        size_t last_block_size = sp->block_size(last_block);
-        jbyte* last_card_of_last_obj =
+        const size_t last_block_size = sp->block_size(last_block);
+        jbyte* const last_card_of_last_obj =
           byte_for(last_block + last_block_size - 1);
-        jbyte* first_card_of_next_chunk = byte_for(chunk_mr.end());
+        jbyte* const first_card_of_next_chunk = byte_for(chunk_mr.end());
         // This search potentially goes a long distance looking
-        // for the next card that will be scanned.  For example,
-        // an object that is an array of primitives will not
-        // have any cards covering regions interior to the array
-        // that will need to be scanned. The scan can be terminated
-        // at the last card of the next chunk.  That would leave
-        // limit_card as NULL and would result in "max_to_do"
-        // being set with the LNC value or with the end
-        // of the last block.
-        jbyte* last_card_of_next_chunk = first_card_of_next_chunk +
-          CardsPerStrideChunk;
-        assert(byte_for(chunk_mr.end()) - byte_for(chunk_mr.start())
-          == CardsPerStrideChunk, "last card of next chunk may be wrong");
-        jbyte* last_card_to_check = (jbyte*) MIN2(last_card_of_last_obj,
-                                                  last_card_of_next_chunk);
+        // for the next card that will be scanned, terminating
+        // at the end of the last_block, if no earlier dirty card
+        // is found.
+        assert(byte_for(chunk_mr.end()) - byte_for(chunk_mr.start()) == ParGCCardsPerStrideChunk,
+               "last card of next chunk may be wrong");
         for (jbyte* cur = first_card_of_next_chunk;
-             cur <= last_card_to_check; cur++) {
-          if (card_will_be_scanned(*cur)) {
+             cur <= last_card_of_last_obj; cur++) {
+          const jbyte val = *cur;
+          if (card_will_be_scanned(val)) {
+            NOISY(tty->print_cr(" Found a non-clean card " PTR_FORMAT " with value 0x%x",
+                                cur, (int)val);)
             limit_card = cur; break;
+          } else {
+            assert(!card_may_have_been_dirty(val), "Error: card can't be skipped");
           }
         }
-        assert(0 <= cur_chunk_index+1 &&
-               cur_chunk_index+1 < lowest_non_clean_chunk_size,
-               "Bounds error.");
-        // LNC for the next chunk
-        jbyte* lnc_card = lowest_non_clean[cur_chunk_index+1];
-        if (limit_card == NULL) {
-          limit_card = lnc_card;
-        }
         if (limit_card != NULL) {
-          if (lnc_card != NULL) {
-            limit_card = (jbyte*)MIN2((intptr_t)limit_card,
-                                      (intptr_t)lnc_card);
-          }
           max_to_do = addr_for(limit_card);
+          assert(limit_card != NULL && max_to_do != NULL, "Error");
+          NOISY(tty->print_cr(" process_chunk_boundary: Found a dirty card at " PTR_FORMAT
+                        "   max_to_do set at " PTR_FORMAT " which is before end of last block in chunk: "
+                        PTR_FORMAT " + " PTR_FORMAT " = " PTR_FORMAT,
+                        limit_card, max_to_do, last_block, last_block_size, (last_block+last_block_size));)
         } else {
+          // The following is a pessimistic value, because it's possible
+          // that a dirty card on a subsequent chunk has been cleared by
+          // the time we get to look at it; we'll correct for that further below,
+          // using the LNC array which records the least non-clean card
+          // before cards were cleared in a particular chunk.
+          limit_card = last_card_of_last_obj;
           max_to_do = last_block + last_block_size;
+          assert(limit_card != NULL && max_to_do != NULL, "Error");
+          NOISY(tty->print_cr(" process_chunk_boundary: Found no dirty card before end of last block in chunk\n"
+                              "   Setting limit_card to " PTR_FORMAT
+                              " and max_to_do " PTR_FORMAT " + " PTR_FORMAT " = " PTR_FORMAT,
+                              limit_card, last_block, last_block_size, max_to_do);)
         }
+        assert(0 < cur_chunk_index+1 && cur_chunk_index+1 < lowest_non_clean_chunk_size,
+               "Bounds error.");
+        // It is possible that a dirty card for the last object may have been
+        // cleared before we had a chance to examine it. In that case, the value
+        // will have been logged in the LNC for that chunk.
+        // We need to examine as many chunks to the right as this object
+        // covers.
+        const uintptr_t last_chunk_index_to_check = addr_to_chunk_index(last_block + last_block_size - 1)
+                                                    - lowest_non_clean_base_chunk_index;
+        DEBUG_ONLY(const uintptr_t last_chunk_index = addr_to_chunk_index(used.end())
+                                                      - lowest_non_clean_base_chunk_index;)
+        assert(last_chunk_index_to_check <= last_chunk_index,
+               err_msg("Out of bounds: last_chunk_index_to_check " INTPTR_FORMAT
+                       " exceeds last_chunk_index " INTPTR_FORMAT,
+                       last_chunk_index_to_check, last_chunk_index));
+        for (uintptr_t lnc_index = cur_chunk_index + 1;
+             lnc_index <= last_chunk_index_to_check;
+             lnc_index++) {
+          jbyte* lnc_card = lowest_non_clean[lnc_index];
+          if (lnc_card != NULL) {
+            // we can stop at the first non-NULL entry we find
+            if (lnc_card <= limit_card) {
+              NOISY(tty->print_cr(" process_chunk_boundary: LNC card " PTR_FORMAT " is lower than limit_card " PTR_FORMAT,
+                                  "   max_to_do will be lowered to " PTR_FORMAT " from " PTR_FORMAT,
+                                  lnc_card, limit_card, addr_for(lnc_card), max_to_do);)
+              limit_card = lnc_card;
+              max_to_do = addr_for(limit_card);
+              assert(limit_card != NULL && max_to_do != NULL, "Error");
+            }
+            // In any case, we break now
+            break;
+          }  // else continue to look for a non-NULL entry if any
+        }
+        assert(limit_card != NULL && max_to_do != NULL, "Error");
       }
+      assert(max_to_do != NULL, "OOPS 1 !");
     }
-    assert(max_to_do != NULL, "OOPS!");
+    assert(max_to_do != NULL, "OOPS 2!");
   } else {
     max_to_do = used.end();
+    NOISY(tty->print_cr(" process_chunk_boundary: Last chunk of this space;\n"
+                  "   max_to_do left at " PTR_FORMAT,
+                  max_to_do);)
   }
+  assert(max_to_do != NULL, "OOPS 3!");
   // Now we can set the closure we're using so it doesn't to beyond
   // max_to_do.
   dcto_cl->set_min_done(max_to_do);
 #ifndef PRODUCT
   dcto_cl->set_last_bottom(max_to_do);
 #endif
-
-  // Now we set *our" lowest_non_clean entry.
-  // Find the object that spans our boundary, if one exists.
-  // Nothing to do on the first chunk.
-  if (chunk_mr.start() > used.start()) {
-    // first_block is the block possibly spanning the chunk start
-    HeapWord* first_block = sp->block_start(chunk_mr.start());
-    // Does the block span the start of the chunk and is it
-    // an object?
-    if (first_block < chunk_mr.start() &&
-        sp->block_is_obj(first_block)) {
-      jbyte* first_dirty_card = NULL;
-      jbyte* last_card_of_first_obj =
-          byte_for(first_block + sp->block_size(first_block) - 1);
-      jbyte* first_card_of_cur_chunk = byte_for(chunk_mr.start());
-      jbyte* last_card_of_cur_chunk = byte_for(chunk_mr.last());
-      jbyte* last_card_to_check =
-        (jbyte*) MIN2((intptr_t) last_card_of_cur_chunk,
-                      (intptr_t) last_card_of_first_obj);
-      for (jbyte* cur = first_card_of_cur_chunk;
-           cur <= last_card_to_check; cur++) {
-        if (card_will_be_scanned(*cur)) {
-          first_dirty_card = cur; break;
-        }
-      }
-      if (first_dirty_card != NULL) {
-        assert(0 <= cur_chunk_index &&
-                 cur_chunk_index < lowest_non_clean_chunk_size,
-               "Bounds error.");
-        lowest_non_clean[cur_chunk_index] = first_dirty_card;
-      }
-    }
-  }
+  NOISY(tty->print_cr("===========================================================================\n");)
 }
 
+#undef NOISY
+
 void
 CardTableModRefBS::
 get_LNC_array_for_space(Space* sp,
@@ -283,8 +413,8 @@ get_LNC_array_for_space(Space* sp,
   // LNC array for the covered region.  Any later expansion can't affect
   // the used_at_save_marks region.
   // (I observed a bug in which the first thread to execute this would
-  // resize, and then it would cause "expand_and_allocates" that would
-  // Increase the number of chunks in the covered region.  Then a second
+  // resize, and then it would cause "expand_and_allocate" that would
+  // increase the number of chunks in the covered region.  Then a second
   // thread would come and execute this, see that the size didn't match,
   // and free and allocate again.  So the first thread would be using a
   // freed "_lowest_non_clean" array.)
diff --git a/src/share/vm/gc_implementation/parNew/parOopClosures.inline.hpp b/src/share/vm/gc_implementation/parNew/parOopClosures.inline.hpp
index c9c50b3f6..00177bb75 100644
--- a/src/share/vm/gc_implementation/parNew/parOopClosures.inline.hpp
+++ b/src/share/vm/gc_implementation/parNew/parOopClosures.inline.hpp
@@ -77,7 +77,23 @@ inline void ParScanClosure::do_oop_work(T* p,
   if (!oopDesc::is_null(heap_oop)) {
     oop obj = oopDesc::decode_heap_oop_not_null(heap_oop);
     if ((HeapWord*)obj < _boundary) {
-      assert(!_g->to()->is_in_reserved(obj), "Scanning field twice?");
+#ifndef PRODUCT
+      if (_g->to()->is_in_reserved(obj)) {
+        tty->print_cr("Scanning field (" PTR_FORMAT ") twice?", p);
+        GenCollectedHeap* gch =  (GenCollectedHeap*)Universe::heap();
+        Space* sp = gch->space_containing(p);
+        oop obj = oop(sp->block_start(p));
+        assert((HeapWord*)obj < (HeapWord*)p, "Error");
+        tty->print_cr("Object: " PTR_FORMAT, obj);
+        tty->print_cr("-------");
+        obj->print();
+        tty->print_cr("-----");
+        tty->print_cr("Heap:");
+        tty->print_cr("-----");
+        gch->print();
+        ShouldNotReachHere();
+      }
+#endif
       // OK, we need to ensure that it is copied.
       // We read the klass and mark in this order, so that we can reliably
       // get the size of the object: if the mark we read is not a
diff --git a/src/share/vm/memory/cardTableModRefBS.cpp b/src/share/vm/memory/cardTableModRefBS.cpp
index 356a77ade..c6645015d 100644
--- a/src/share/vm/memory/cardTableModRefBS.cpp
+++ b/src/share/vm/memory/cardTableModRefBS.cpp
@@ -455,25 +455,29 @@ bool CardTableModRefBS::mark_card_deferred(size_t card_index) {
   return true;
 }
 
-
 void CardTableModRefBS::non_clean_card_iterate_possibly_parallel(Space* sp,
                                                                  MemRegion mr,
-                                                                 DirtyCardToOopClosure* dcto_cl,
-                                                                 ClearNoncleanCardWrapper* cl) {
+                                                                 OopsInGenClosure* cl,
+                                                                 CardTableRS* ct) {
   if (!mr.is_empty()) {
     int n_threads = SharedHeap::heap()->n_par_threads();
     if (n_threads > 0) {
 #ifndef SERIALGC
-      non_clean_card_iterate_parallel_work(sp, mr, dcto_cl, cl, n_threads);
+      non_clean_card_iterate_parallel_work(sp, mr, cl, ct, n_threads);
 #else  // SERIALGC
       fatal("Parallel gc not supported here.");
 #endif // SERIALGC
     } else {
       // We do not call the non_clean_card_iterate_serial() version below because
       // we want to clear the cards (which non_clean_card_iterate_serial() does not
-      // do for us), and the ClearNoncleanCardWrapper closure itself does the work
-      // of finding contiguous dirty ranges of cards to process (and clear).
-      cl->do_MemRegion(mr);
+      // do for us): clear_cl here does the work of finding contiguous dirty ranges
+      // of cards to process and clear.
+
+      DirtyCardToOopClosure* dcto_cl = sp->new_dcto_cl(cl, precision(),
+                                                       cl->gen_boundary());
+      ClearNoncleanCardWrapper clear_cl(dcto_cl, ct);
+
+      clear_cl.do_MemRegion(mr);
     }
   }
 }
diff --git a/src/share/vm/memory/cardTableModRefBS.hpp b/src/share/vm/memory/cardTableModRefBS.hpp
index a8d48515c..6dc449707 100644
--- a/src/share/vm/memory/cardTableModRefBS.hpp
+++ b/src/share/vm/memory/cardTableModRefBS.hpp
@@ -173,18 +173,17 @@ class CardTableModRefBS: public ModRefBarrierSet {
   // A variant of the above that will operate in a parallel mode if
   // worker threads are available, and clear the dirty cards as it
   // processes them.
-  // ClearNoncleanCardWrapper cl must wrap the DirtyCardToOopClosure dcto_cl,
-  // which may itself be modified by the method.
+  // XXX ??? MemRegionClosure above vs OopsInGenClosure below XXX
+  // XXX some new_dcto_cl's take OopClosure's, plus as above there are
+  // some MemRegionClosures. Clean this up everywhere. XXX
   void non_clean_card_iterate_possibly_parallel(Space* sp, MemRegion mr,
-                                                DirtyCardToOopClosure* dcto_cl,
-                                                ClearNoncleanCardWrapper* cl);
+                                                OopsInGenClosure* cl, CardTableRS* ct);
 
  private:
   // Work method used to implement non_clean_card_iterate_possibly_parallel()
   // above in the parallel case.
   void non_clean_card_iterate_parallel_work(Space* sp, MemRegion mr,
-                                            DirtyCardToOopClosure* dcto_cl,
-                                            ClearNoncleanCardWrapper* cl,
+                                            OopsInGenClosure* cl, CardTableRS* ct,
                                             int n_threads);
 
  protected:
@@ -198,11 +197,6 @@ class CardTableModRefBS: public ModRefBarrierSet {
 
   // *** Support for parallel card scanning.
 
-  enum SomeConstantsForParallelism {
-    StridesPerThread    = 2,
-    CardsPerStrideChunk = 256
-  };
-
   // This is an array, one element per covered region of the card table.
   // Each entry is itself an array, with one element per chunk in the
   // covered region.  Each entry of these arrays is the lowest non-clean
@@ -235,7 +229,7 @@ class CardTableModRefBS: public ModRefBarrierSet {
   // covers the given address.
   uintptr_t addr_to_chunk_index(const void* addr) {
     uintptr_t card = (uintptr_t) byte_for(addr);
-    return card / CardsPerStrideChunk;
+    return card / ParGCCardsPerStrideChunk;
   }
 
   // Apply cl, which must either itself apply dcto_cl or be dcto_cl,
@@ -243,8 +237,8 @@ class CardTableModRefBS: public ModRefBarrierSet {
   void process_stride(Space* sp,
                       MemRegion used,
                       jint stride, int n_strides,
-                      DirtyCardToOopClosure* dcto_cl,
-                      ClearNoncleanCardWrapper* cl,
+                      OopsInGenClosure* cl,
+                      CardTableRS* ct,
                       jbyte** lowest_non_clean,
                       uintptr_t lowest_non_clean_base_chunk_index,
                       size_t lowest_non_clean_chunk_size);
@@ -482,7 +476,7 @@ public:
   void verify_dirty_region(MemRegion mr) PRODUCT_RETURN;
 
   static size_t par_chunk_heapword_alignment() {
-    return CardsPerStrideChunk * card_size_in_words;
+    return ParGCCardsPerStrideChunk * card_size_in_words;
   }
 
 };
diff --git a/src/share/vm/memory/cardTableRS.cpp b/src/share/vm/memory/cardTableRS.cpp
index 551ba7eb8..4b47da783 100644
--- a/src/share/vm/memory/cardTableRS.cpp
+++ b/src/share/vm/memory/cardTableRS.cpp
@@ -162,7 +162,7 @@ inline bool ClearNoncleanCardWrapper::clear_card_serial(jbyte* entry) {
 }
 
 ClearNoncleanCardWrapper::ClearNoncleanCardWrapper(
-  MemRegionClosure* dirty_card_closure, CardTableRS* ct) :
+  DirtyCardToOopClosure* dirty_card_closure, CardTableRS* ct) :
     _dirty_card_closure(dirty_card_closure), _ct(ct) {
     _is_par = (SharedHeap::heap()->n_par_threads() > 0);
 }
@@ -246,10 +246,6 @@ void CardTableRS::write_ref_field_gc_par(void* field, oop new_val) {
 
 void CardTableRS::younger_refs_in_space_iterate(Space* sp,
                                                 OopsInGenClosure* cl) {
-  DirtyCardToOopClosure* dcto_cl = sp->new_dcto_cl(cl, _ct_bs->precision(),
-                                                   cl->gen_boundary());
-  ClearNoncleanCardWrapper clear_cl(dcto_cl, this);
-
   const MemRegion urasm = sp->used_region_at_save_marks();
 #ifdef ASSERT
   // Convert the assertion check to a warning if we are running
@@ -275,10 +271,10 @@ void CardTableRS::younger_refs_in_space_iterate(Space* sp,
     if (!urasm.equals(urasm2)) {
       warning("CMS+ParNew: Flickering used_region_at_save_marks()!!");
     }
+    ShouldNotReachHere();
   }
 #endif
-  _ct_bs->non_clean_card_iterate_possibly_parallel(sp, urasm,
-                                                   dcto_cl, &clear_cl);
+  _ct_bs->non_clean_card_iterate_possibly_parallel(sp, urasm, cl, this);
 }
 
 void CardTableRS::clear_into_younger(Generation* gen, bool clear_perm) {
diff --git a/src/share/vm/memory/cardTableRS.hpp b/src/share/vm/memory/cardTableRS.hpp
index 72c5dacf8..a15b85f74 100644
--- a/src/share/vm/memory/cardTableRS.hpp
+++ b/src/share/vm/memory/cardTableRS.hpp
@@ -31,7 +31,6 @@
 
 class Space;
 class OopsInGenClosure;
-class DirtyCardToOopClosure;
 
 // This kind of "GenRemSet" uses a card table both as shared data structure
 // for a mod ref barrier set and for the rem set information.
@@ -167,7 +166,7 @@ public:
 };
 
 class ClearNoncleanCardWrapper: public MemRegionClosure {
-  MemRegionClosure* _dirty_card_closure;
+  DirtyCardToOopClosure* _dirty_card_closure;
   CardTableRS* _ct;
   bool _is_par;
 private:
@@ -179,7 +178,7 @@ private:
   inline bool clear_card_parallel(jbyte* entry);
 
 public:
-  ClearNoncleanCardWrapper(MemRegionClosure* dirty_card_closure, CardTableRS* ct);
+  ClearNoncleanCardWrapper(DirtyCardToOopClosure* dirty_card_closure, CardTableRS* ct);
   void do_MemRegion(MemRegion mr);
 };
 
diff --git a/src/share/vm/memory/space.cpp b/src/share/vm/memory/space.cpp
index 1971332cd..ff91ba30c 100644
--- a/src/share/vm/memory/space.cpp
+++ b/src/share/vm/memory/space.cpp
@@ -97,6 +97,14 @@ void DirtyCardToOopClosure::walk_mem_region(MemRegion mr,
   }
 }
 
+// We get called with "mr" representing the dirty region
+// that we want to process. Because of imprecise marking,
+// we may need to extend the incoming "mr" to the right,
+// and scan more. However, because we may already have
+// scanned some of that extended region, we may need to
+// trim its right-end back some so we do not scan what
+// we (or another worker thread) may already have scanned
+// or planning to scan.
 void DirtyCardToOopClosure::do_MemRegion(MemRegion mr) {
 
   // Some collectors need to do special things whenever their dirty
@@ -148,7 +156,7 @@ void DirtyCardToOopClosure::do_MemRegion(MemRegion mr) {
   // e.g. the dirty card region is entirely in a now free object
   // -- something that could happen with a concurrent sweeper.
   bottom = MIN2(bottom, top);
-  mr     = MemRegion(bottom, top);
+  MemRegion extended_mr = MemRegion(bottom, top);
   assert(bottom <= top &&
          (_precision != CardTableModRefBS::ObjHeadPreciseArray ||
           _min_done == NULL ||
@@ -156,8 +164,8 @@ void DirtyCardToOopClosure::do_MemRegion(MemRegion mr) {
          "overlap!");
 
   // Walk the region if it is not empty; otherwise there is nothing to do.
-  if (!mr.is_empty()) {
-    walk_mem_region(mr, bottom_obj, top);
+  if (!extended_mr.is_empty()) {
+    walk_mem_region(extended_mr, bottom_obj, top);
   }
 
   // An idempotent closure might be applied in any order, so we don't
diff --git a/src/share/vm/runtime/globals.hpp b/src/share/vm/runtime/globals.hpp
index 555c2273a..dc90959aa 100644
--- a/src/share/vm/runtime/globals.hpp
+++ b/src/share/vm/runtime/globals.hpp
@@ -1460,8 +1460,10 @@ class CommandLineFlags {
   product(intx, ParallelGCBufferWastePct, 10,                               \
           "wasted fraction of parallel allocation buffer.")                 \
                                                                             \
-  product(bool, ParallelGCRetainPLAB, true,                                 \
-          "Retain parallel allocation buffers across scavenges.")           \
+  diagnostic(bool, ParallelGCRetainPLAB, false,                             \
+             "Retain parallel allocation buffers across scavenges; "        \
+             " -- disabled because this currently conflicts with "          \
+             " parallel card scanning under certain conditions ")           \
                                                                             \
   product(intx, TargetPLABWastePct, 10,                                     \
           "target wasted space in last buffer as pct of overall allocation")\
@@ -1495,7 +1497,15 @@ class CommandLineFlags {
   product(uintx, ParGCDesiredObjsFromOverflowList, 20,                      \
           "The desired number of objects to claim from the overflow list")  \
                                                                             \
-  product(uintx, CMSParPromoteBlocksToClaim, 16,                             \
+  diagnostic(intx, ParGCStridesPerThread, 2,                                \
+          "The number of strides per worker thread that we divide up the "  \
+          "card table scanning work into")                                  \
+                                                                            \
+  diagnostic(intx, ParGCCardsPerStrideChunk, 256,                           \
+          "The number of cards in each chunk of the parallel chunks used "  \
+          "during card table scanning")                                     \
+                                                                            \
+  product(uintx, CMSParPromoteBlocksToClaim, 16,                            \
           "Number of blocks to attempt to claim when refilling CMS LAB for "\
           "parallel GC.")                                                   \
                                                                             \