[heap] Special handling for small heaps in eager finalization of

src/heap/heap.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef V8_HEAP_HEAP_H_
 #define V8_HEAP_HEAP_H_
 
 #include <cmath>
 #include <map>
 
@@ skipping 1809 matching lines @@
   inline size_t OldGenerationSpaceAvailable() {
     if (old_generation_allocation_limit_ <= PromotedTotalSize()) return 0;
     return old_generation_allocation_limit_ -
            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() {
+    // This guards against too eager finalization in small heaps.
+    // The number is chosen based on v8.browsing_mobile on Nexus 7v2.
+    size_t kMarginForSmallHeaps = 32u * MB;
     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.
+    // Overshoot margin is 50% of allocation limit or half-way to the max heap
+    // with special handling of small heaps.
     uint64_t margin =
-        Min(old_generation_allocation_limit_ / 2,
+        Min(Max(old_generation_allocation_limit_ / 2, kMarginForSmallHeaps),
             (max_old_generation_size_ - old_generation_allocation_limit_) / 2);
     return overshoot >= margin;
   }
 
   void UpdateTotalGCTime(double duration);
 
   bool MaximumSizeScavenge() { return maximum_size_scavenges_ > 0; }
 
   // ===========================================================================
   // Growing strategy. =========================================================
@@ skipping 817 matching lines @@
   friend class LargeObjectSpace;
   friend class NewSpace;
   friend class PagedSpace;
   DISALLOW_COPY_AND_ASSIGN(AllocationObserver);
 };
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_HEAP_HEAP_H_