We mustn't finalize sweeping when V8 started collecting garbage.

third_party/WebKit/Source/platform/heap/ThreadState.cpp

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
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@@ skipping 614 matching lines @@
 #if PRINT_HEAP_STATS
         dataLogF("Scheduled IdleGC\n");
 #endif
         scheduleIdleGC();
         return;
     }
 }
 
 void ThreadState::willStartV8GC()
 {
-    // Finish Oilpan's complete sweeping before running a V8 GC.
-    // This will let the GC collect more V8 objects.
-    //
-    // TODO(haraken): It's a bit too late for a major GC to schedule
-    // completeSweep() here, because gcPrologue for a major GC is called
-    // not at the point where the major GC started but at the point where
-    // the major GC requests object grouping.
-    completeSweep();
-
     // The fact that the PageNavigation GC is scheduled means that there is

[sof, 2015/11/20 14:39:37]

This means we cannot do page navigation GCs either.

[jochen (gone - plz use gerrit), 2015/11/20 14:46:18]

hum, right :-/

     // a dead frame. In common cases, a sequence of Oilpan's GC => V8 GC =>
     // Oilpan's GC is needed to collect the dead frame. So we force the
     // PageNavigation GC before running the V8 GC.
     if (gcState() == PageNavigationGCScheduled) {
 #if PRINT_HEAP_STATS
         dataLogF("Scheduled PageNavigationGC\n");
 #endif
         Heap::collectGarbage(BlinkGC::HeapPointersOnStack, BlinkGC::GCWithSweep, BlinkGC::PageNavigationGC);
     }
 }
@@ skipping 828 matching lines @@
     threadDump->addScalar("dead_count", "objects", totalDeadCount);
     threadDump->addScalar("live_size", "bytes", totalLiveSize);
     threadDump->addScalar("dead_size", "bytes", totalDeadSize);
 
     WebMemoryAllocatorDump* heapsDump = BlinkGCMemoryDumpProvider::instance()->createMemoryAllocatorDumpForCurrentGC(heapsDumpName);
     WebMemoryAllocatorDump* classesDump = BlinkGCMemoryDumpProvider::instance()->createMemoryAllocatorDumpForCurrentGC(classesDumpName);
     BlinkGCMemoryDumpProvider::instance()->currentProcessMemoryDump()->addOwnershipEdge(classesDump->guid(), heapsDump->guid());
 }
 
 } // namespace blink