Reduce ephemeron stack size reservation.

The ephemeron stack is used by the Oilpan GC to handle key-value pairs
over weak references as wanted (i.e., the value is strongly referenced
until the key becomes unreachable). The marking process will push
hash tables containing such onto a stack processing for later processing.

Blink only has a handful of hash tables requiring ephemeron processing,
hence tune down the initial size of the stack accordingly.

As a data point, browsing around on various popular sites resulted in
ephemeron stacks no deeper than in the mid-20s.

R=haraken
BUG=

Review URL: https://codereview.chromium.org/1741833002

Cr-Commit-Position: refs/heads/master@{#378131}

4 files changed, 8 insertions, 2 deletions

diff --git a/third_party/WebKit/Source/platform/heap/BlinkGC.h b/third_party/WebKit/Source/platform/heap/BlinkGC.h
index 4f1447a..688525a 100644
--- a/third_party/WebKit/Source/platform/heap/BlinkGC.h
+++ b/third_party/WebKit/Source/platform/heap/BlinkGC.h
@@ -7,6 +7,7 @@
 
 // BlinkGC.h is a file that defines common things used by Blink GC.
 
+#include "platform/PlatformExport.h"
 #include "wtf/Allocator.h"
 
 namespace blink {
diff --git a/third_party/WebKit/Source/platform/heap/CallbackStack.cpp b/third_party/WebKit/Source/platform/heap/CallbackStack.cpp
index 4cea26a..17e46a1 100644
--- a/third_party/WebKit/Source/platform/heap/CallbackStack.cpp
+++ b/third_party/WebKit/Source/platform/heap/CallbackStack.cpp
@@ -7,6 +7,8 @@
 
 namespace blink {
 
+size_t const CallbackStack::kMinimalBlockSize = WTF::kPageAllocationGranularity / sizeof(CallbackStack::Item);
+
 CallbackStack::Block::Block(Block* next, size_t blockSize)
     : m_blockSize(blockSize)
 {
diff --git a/third_party/WebKit/Source/platform/heap/CallbackStack.h b/third_party/WebKit/Source/platform/heap/CallbackStack.h
index 753941c..3bc2800 100644
--- a/third_party/WebKit/Source/platform/heap/CallbackStack.h
+++ b/third_party/WebKit/Source/platform/heap/CallbackStack.h
@@ -5,7 +5,7 @@
 #ifndef CallbackStack_h
 #define CallbackStack_h
 
-#include "platform/heap/ThreadState.h"
+#include "platform/heap/BlinkGC.h"
 #include "wtf/Allocator.h"
 #include "wtf/Assertions.h"
 
@@ -54,6 +54,8 @@ public:
     bool hasCallbackForObject(const void*);
 #endif
 
+    static const size_t kMinimalBlockSize;
+
 private:
     static const size_t defaultBlockSize = (1 << 13);
 
diff --git a/third_party/WebKit/Source/platform/heap/Heap.cpp b/third_party/WebKit/Source/platform/heap/Heap.cpp
index 11ba827..62e4f91 100644
--- a/third_party/WebKit/Source/platform/heap/Heap.cpp
+++ b/third_party/WebKit/Source/platform/heap/Heap.cpp
@@ -110,7 +110,8 @@ void Heap::init()
     s_markingStack = new CallbackStack();
     s_postMarkingCallbackStack = new CallbackStack();
     s_globalWeakCallbackStack = new CallbackStack();
-    s_ephemeronStack = new CallbackStack();
+    // Use smallest supported block size for ephemerons.
+    s_ephemeronStack = new CallbackStack(CallbackStack::kMinimalBlockSize);
     s_heapDoesNotContainCache = new HeapDoesNotContainCache();
     s_freePagePool = new FreePagePool();
     s_orphanedPagePool = new OrphanedPagePool();