Presweep one page before parallel/concurrent sweeping.

src/mark-compact.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
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 // modification, are permitted provided that the following conditions are
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@@ skipping 3756 matching lines @@
                                       sweeper == PARALLEL_CONSERVATIVE ||
                                       sweeper == CONCURRENT_CONSERVATIVE);
   space->ClearStats();
 
   PageIterator it(space);
 
   intptr_t freed_bytes = 0;
   int pages_swept = 0;
   bool lazy_sweeping_active = false;
   bool unused_page_present = false;
+  bool presweep = true;
 
   while (it.has_next()) {
     Page* p = it.next();
 
     ASSERT(p->parallel_sweeping() == 0);
     // Clear sweeping flags indicating that marking bits are still intact.
     p->ClearSweptPrecisely();
     p->ClearSweptConservatively();
 
     if (p->IsEvacuationCandidate()) {
@@ skipping 51 matching lines @@
         space->SetPagesToSweep(p->next_page());
         lazy_sweeping_active = true;
         break;
       }
       case CONCURRENT_CONSERVATIVE:
       case PARALLEL_CONSERVATIVE: {
         if (FLAG_gc_verbose) {
           PrintF("Sweeping 0x%" V8PRIxPTR " conservatively in parallel.\n",
                  reinterpret_cast<intptr_t>(p));
         }
-        p->set_parallel_sweeping(1);
-        space->IncreaseUnsweptFreeBytes(p);
+        if (presweep) {

[Michael Starzinger, 2013/03/05 15:35:20]

Can we use a "parallel_sweeping_flag" instead, to be in sync with lazy sweeping?
Also can you change to pre-sweeping during lazy-sweeping to have the same
pattern (i.e. move block into the switch)?

[Hannes Payer (out of office), 2013/03/05 17:22:48]

Done.

+          SweepConservatively<SWEEP_SEQUENTIALLY>(space, NULL, p);
+          pages_swept++;
+          presweep = false;
+        } else {
+          p->set_parallel_sweeping(1);
+          space->IncreaseUnsweptFreeBytes(p);
+        }
         break;
       }
       case PRECISE: {
         if (FLAG_gc_verbose) {
           PrintF("Sweeping 0x%" V8PRIxPTR " precisely.\n",
                  reinterpret_cast<intptr_t>(p));
         }
         if (space->identity() == CODE_SPACE) {
           SweepPrecisely<SWEEP_ONLY, REBUILD_SKIP_LIST>(space, p, NULL);
         } else {
@@ skipping 260 matching lines @@
   while (buffer != NULL) {
     SlotsBuffer* next_buffer = buffer->next();
     DeallocateBuffer(buffer);
     buffer = next_buffer;
   }
   *buffer_address = NULL;
 }
 
 
 } }  // namespace v8::internal