Tune the memory pressure handler to perform a second GC immediately

src/heap/heap.cc

Index: src/heap/heap.cc
diff --git a/src/heap/heap.cc b/src/heap/heap.cc
index dbb219a51d6b9395e91d10e9cbc044e140879b75..38a98e33616213e3ba86b424aa4514dc7c059f6e 100644
--- a/src/heap/heap.cc
+++ b/src/heap/heap.cc
@@ -4466,8 +4466,36 @@ void Heap::CheckMemoryPressure() {
 }
 
 void Heap::CollectGarbageOnMemoryPressure(const char* source) {
+  const int kGarbageThresholdInBytes = 8 * MB;
+  const double kGarbageThresholdAsFractionOfTotalMemory = 0.1;
+  const double kMaxMemoryPressurePauseMs = 100;

[Michael Lippautz, 2016/06/10 11:20:34]

nit: If you can, please document why 100ms. (I was not part of all discussions
when this API landed.)

[ulan, 2016/06/10 11:36:01]

Done.

+
+  double start = MonotonicallyIncreasingTimeInMs();
   CollectAllGarbage(kReduceMemoryFootprintMask | kAbortIncrementalMarkingMask,
-                    source);
+                    source, kGCCallbackFlagCollectAllAvailableGarbage);
+  new_space_.Shrink();

[Michael Lippautz, 2016/06/10 11:20:34]

Why is the shrinking needed? This should be done through
GarbageCollectionEpilogue as ShouldReduceMemory() is true?

[ulan, 2016/06/10 11:36:01]

Yes, thanks. Done.

+  double end = MonotonicallyIncreasingTimeInMs();
+
+  // Estimate how much memory we can free.
+  int64_t potential_garbage = (CommittedMemory() - SizeOfObjects()) +
+                              amount_of_external_allocated_memory_;
+  // If we can potentially free large amount of memory, then start GC right
+  // away instead of waiting for memory reducer.
+  if (potential_garbage >= kGarbageThresholdInBytes &&
+      potential_garbage >=
+          CommittedMemory() * kGarbageThresholdAsFractionOfTotalMemory) {
+    // If we spent less than half of the time budget, then perform full GC
+    // Otherwise, start incremental marking.
+    if (end - start < kMaxMemoryPressurePauseMs / 2) {
+      CollectAllGarbage(
+          kReduceMemoryFootprintMask | kAbortIncrementalMarkingMask, source,
+          kGCCallbackFlagCollectAllAvailableGarbage);
+    } else {
+      if (FLAG_incremental_marking && incremental_marking()->IsStopped()) {

[Michael Lippautz, 2016/06/10 11:20:34]

nit (not for now): At some point I think it would be good if maybe
StartIncrementalMarking and its variants would return a bool that returns
whether the start was successful. It's hard to remember that you have to check
the FLAG_ and IsStopped() when we want to start it.

[ulan, 2016/06/10 11:36:01]

Acknowledged.

+        StartIdleIncrementalMarking();
+      }
+    }
+  }
 }
 
 void Heap::MemoryPressureNotification(MemoryPressureLevel level,