Allow main thread to contribute to the sweeping phase.

src/spaces.cc

Index: src/spaces.cc
diff --git a/src/spaces.cc b/src/spaces.cc
index 12613c7cb0fc4898d9a5ca172deda517d56a2922..d431407d84a9c69e0c690de9465c1a029a5be0f6 100644
--- a/src/spaces.cc
+++ b/src/spaces.cc
@@ -2585,8 +2585,20 @@ HeapObject* PagedSpace::WaitForSweeperThreadsAndRetryAllocation(
     int size_in_bytes) {
   MarkCompactCollector* collector = heap()->mark_compact_collector();
 
-  // If sweeper threads are still running, wait for them.
   if (collector->IsConcurrentSweepingInProgress(this)) {
+    // If sweeping is still in progress try to sweep pages on the main thread.
+    int free_chunk =
+        collector->SweepInParallel<MarkCompactCollector::SWEEP_ON_MAIN_THREAD>(
+            this, size_in_bytes);
+    if (free_chunk >= size_in_bytes) {
+      HeapObject* object = free_list_.Allocate(size_in_bytes);
+      // We should be able to allocate an object here since we just freed that
+      // much memory.
+      ASSERT(object != NULL);
+      if (object != NULL) return object;
+    }
+
+    // Wait for the sweeper threads here and complete the sweeping phase.
     collector->WaitUntilSweepingCompleted();
 
     // After waiting for the sweeper threads, there may be new free-list