[heap] Relax condition for forced finalization of incremental marking.

Forcing finalization after reaching allocation limit regresses gc pause
time in benchmarks as we have to do a lot of non-incremental marking work.

This patch allows overshoot of the limit by some margin.

BUG=chromium:670675,chromium:671994
TBR=mlippautz@chromium.org

Review-Url: https://codereview.chromium.org/2554423005
Cr-Commit-Position: refs/heads/master@{#41625}

src/heap/heap.cc

@@ -265,7 +265,7 @@ GarbageCollector Heap::SelectGarbageCollector(AllocationSpace space,
   }
 
   if (incremental_marking()->NeedsFinalization() &&
-      OldGenerationSpaceAvailable() == 0) {
+      AllocationLimitOvershotByLargeMargin()) {
     *reason = "Incremental marking needs finalization";
     return MARK_COMPACTOR;
   }
@@ -5320,7 +5320,7 @@ bool Heap::ShouldExpandOldGenerationOnSlowAllocation() {
   if (ShouldOptimizeForMemoryUsage()) return false;
 
   if (incremental_marking()->NeedsFinalization()) {
-    return false;
+    return !AllocationLimitOvershotByLargeMargin();
   }
 
   if (incremental_marking()->IsStopped() &&

src/heap/heap.h

@@ -1823,6 +1823,19 @@ class Heap {
            static_cast<size_t>(PromotedTotalSize());
   }
 
+  // We allow incremental marking to overshoot the allocation limit for
+  // performace reasons. If the overshoot is too large then we are more
+  // eager to finalize incremental marking.
+  inline bool AllocationLimitOvershotByLargeMargin() {
+    if (old_generation_allocation_limit_ >= PromotedTotalSize()) return false;
+    uint64_t overshoot = PromotedTotalSize() - old_generation_allocation_limit_;
+    // Overshoot margin is 50% of allocation limit or half-way to the max heap.
+    uint64_t margin =
+        Min(old_generation_allocation_limit_ / 2,
+            (max_old_generation_size_ - old_generation_allocation_limit_) / 2);
+    return overshoot >= margin;
+  }
+
   void UpdateTotalGCTime(double duration);
 
   bool MaximumSizeScavenge() { return maximum_size_scavenges_ > 0; }