Scheduler: Switch from high to low latency mode if possible.

cc/scheduler/scheduler_state_machine.h

Index: cc/scheduler/scheduler_state_machine.h
diff --git a/cc/scheduler/scheduler_state_machine.h b/cc/scheduler/scheduler_state_machine.h
index 9b6d4fb41ed622c5f2fe766c813e7479ed0b0ec7..6f29503841ab8d5a73b4fc1c8bcdbbf906fab963 100644
--- a/cc/scheduler/scheduler_state_machine.h
+++ b/cc/scheduler/scheduler_state_machine.h
@@ -146,6 +146,10 @@ class CC_EXPORT SchedulerStateMachine {
     return begin_impl_frame_state_;
   }
 
+  // If there's something to draw on BeginImplFrame the impl thread is lagging
+  // behind the main thread. We're thus in high latency mode.

[brianderson, 2013/11/01 00:22:59]

"the main thread is lagging behind the impl thread"

[Dominik Grewe, 2013/11/01 17:09:14]

But the main thread is one frame ahead, isn't it?

[brianderson, 2013/11/01 21:18:30]

The main thread gets into a high latency mode by being too slow. In the high
latency mode, the current commit would have been initiated by the previous
BeginImplFrame, so the main thread is lagging.

+  bool InHighLatencyMode() const;
+
   // PollForAnticipatedDrawTriggers is used by the synchronous compositor to
   // avoid requesting BeginImplFrames when we won't actually draw but still
   // need to advance our state at vsync intervals.
@@ -209,6 +213,8 @@ class CC_EXPORT SchedulerStateMachine {
   // Set that we can create the first OutputSurface and start the scheduler.
   void SetCanStart() { can_start_ = true; }
 
+  void SetSkipBeginMainFrame(bool skip);

[brianderson, 2013/11/01 00:22:59]

SkipNextBeginMainFrame()?

Actually, nevermind; this will allow us to change our mind immediately after a
long commit or activate.

+
   // Indicates whether drawing would, at this time, make sense.
   // CanDraw can be used to supress flashes or checkerboarding
   // when such behavior would be undesirable.
@@ -288,6 +294,7 @@ class CC_EXPORT SchedulerStateMachine {
   bool draw_if_possible_failed_;
   bool did_create_and_initialize_first_output_surface_;
   bool smoothness_takes_priority_;
+  bool skip_begin_main_frame_;

[brianderson, 2013/11/01 00:22:59]

skip_begin_main_frame_to_reduce_latency_

[Dominik Grewe, 2013/11/01 17:09:14]

Done. Also changed the name of the corresponding setter method.

 
  private:
   DISALLOW_COPY_AND_ASSIGN(SchedulerStateMachine);