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

incremental marking.

BUG=chromium:671994

Review-Url: https://codereview.chromium.org/2565173004
Cr-Commit-Position: refs/heads/master@{#41666}

src/heap/heap.h

@@ -1827,11 +1827,15 @@ class Heap {
   // 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;
   }

test/cctest/heap/test-heap.cc

@@ -7066,7 +7066,7 @@ HEAP_TEST(Regress670675) {
   }
   size_t array_length = Page::kPageSize / kPointerSize + 100;
   size_t n = heap->OldGenerationSpaceAvailable() / array_length;
-  for (size_t i = 0; i < n + 10; i++) {
+  for (size_t i = 0; i < n + 40; i++) {
     {
       HandleScope inner_scope(isolate);
       isolate->factory()->NewFixedArray(static_cast<int>(array_length));