Presweep one page before parallel/concurrent sweeping.

src/mark-compact.cc

Index: src/mark-compact.cc
diff --git a/src/mark-compact.cc b/src/mark-compact.cc
index ea797a5866a63fb26dd46b76baeadb85effeccc6..3a7018bfb1fca5bd31a7205b93d41f540cdcc156 100644
--- a/src/mark-compact.cc
+++ b/src/mark-compact.cc
@@ -3774,6 +3774,7 @@ void MarkCompactCollector::SweepSpace(PagedSpace* space, SweeperType sweeper) {
   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();
@@ -3845,8 +3846,14 @@ void MarkCompactCollector::SweepSpace(PagedSpace* space, SweeperType sweeper) {
           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: {