[platform] Add NumberOfAvailableBackgroundThreads

include/v8-platform.h

 // Copyright 2013 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_V8_PLATFORM_H_
 #define V8_V8_PLATFORM_H_
 
 #include <stdint.h>
 
 namespace v8 {
@@ skipping 38 matching lines @@
    * whether to execute the task on a dedicated thread.
    */
   enum ExpectedRuntime {
     kShortRunningTask,
     kLongRunningTask
   };
 
   virtual ~Platform() {}
 
   /**
+   * Gets the number of threads that are used to execute background tasks. Can
+   * be used to limit parallelism in V8.

[jochen (gone - plz use gerrit), 2016/02/05 08:27:07]

I'd remove the "Can be used to limit..." sentence (because it's not true, e.g.,
if the method returns 0, we still post tasks).

You could say that this is used to estimate into how many work packages certain
tasks will be split up.

[jochen (gone - plz use gerrit), 2016/02/05 08:27:53]

and maybe explain what 0 means. I'd guess it means ignore this value, and just
post what you feel is right. The platform always has to be able to execute
background tasks.

[Michael Lippautz, 2016/02/05 09:15:34]

Done.

[Michael Lippautz, 2016/02/05 09:15:34]

Done.

+   */
+  virtual int NumberOfAvailableBackgroundThreads() { return 0; }

[jochen (gone - plz use gerrit), 2016/02/05 08:27:07]

size_t

[Michael Lippautz, 2016/02/05 09:15:34]

Done.

+
+  /**
    * Schedules a task to be invoked on a background thread. |expected_runtime|
    * indicates that the task will run a long time. The Platform implementation
    * takes ownership of |task|. There is no guarantee about order of execution
    * of tasks wrt order of scheduling, nor is there a guarantee about the
    * thread the task will be run on.
    */
   virtual void CallOnBackgroundThread(Task* task,
                                       ExpectedRuntime expected_runtime) = 0;
 
   /**
@@ skipping 83 matching lines @@
    * Sets the duration field of a COMPLETE trace event. It must be called with
    * the handle returned from AddTraceEvent().
    **/
   virtual void UpdateTraceEventDuration(const uint8_t* category_enabled_flag,
                                         const char* name, uint64_t handle) {}
 };
 
 }  // namespace v8
 
 #endif  // V8_V8_PLATFORM_H_