Add memory pressure notification API

include/v8.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.
 
 /** \mainpage V8 API Reference Guide
  *
  * V8 is Google's open source JavaScript engine.
  *
  * This set of documents provides reference material generated from the
  * V8 header file, include/v8.h.
@@ skipping 5493 matching lines @@
   /**
    * Types of garbage collections that can be requested via
    * RequestGarbageCollectionForTesting.
    */
   enum GarbageCollectionType {
     kFullGarbageCollection,
     kMinorGarbageCollection
   };
 
   /**
+   * Memory pressure level for the MemoryPressureNotification.
+   * kNone hints V8 that there is no memory pressure.
+   * kModerate hints V8 to speed up incremental garbage collection.
+   * kCritical hints V8 to free memory as soon as possible.
+   */
+  enum class MemoryPressureLevel { kNone, kModerate, kCritical };
+
+  /**
    * Features reported via the SetUseCounterCallback callback. Do not change
    * assigned numbers of existing items; add new features to the end of this
    * list.
    */
   enum UseCounterFeature {
     kUseAsm = 0,
     kBreakIterator = 1,
     kLegacyConst = 2,
     kMarkDequeOverflow = 3,
     kStoreBufferOverflow = 4,
@@ skipping 60 matching lines @@
    * then V8 will abort if either:
    * - no custom callback is set.
    * - the custom callback set returns true.
    * Otherwise, the custom callback will not be called and V8 will not abort.
    */
   typedef bool (*AbortOnUncaughtExceptionCallback)(Isolate*);
   void SetAbortOnUncaughtExceptionCallback(
       AbortOnUncaughtExceptionCallback callback);
 
   /**
+   * Optional notification that the system is running low on memory.
+   * V8 uses these notifications to guide heuristics.
+   * It is allowed to call this function from another thread while
+   * the isolate is executing long running JavaScript code.

[jochen (gone - plz use gerrit), 2016/03/17 19:33:39]

do we expect the embedder to set the pressure level to kNone if the memory
pressure ends?

[ulan, 2016/03/18 12:49:40]

That it nice to have, but we do not rely on that. After GC we set the level to
kNone ourselves. If the embedder sets to kNone proactively before GC, then it
will be good for performance.

+   */
+  void MemoryPressureNotification(MemoryPressureLevel level);
+
+  /**
    * Methods below this point require holding a lock (using Locker) in
    * a multi-threaded environment.
    */
 
   /**
    * Sets this isolate as the entered one for the current thread.
    * Saves the previously entered one (if any), so that it can be
    * restored when exiting.  Re-entering an isolate is allowed.
    */
   void Enter();
@@ skipping 3054 matching lines @@
  */
 
 
 }  // namespace v8
 
 
 #undef TYPE_CHECK
 
 
 #endif  // INCLUDE_V8_H_