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

cc/scheduler/scheduler_state_machine.h

 // Copyright 2011 The Chromium 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 CC_SCHEDULER_SCHEDULER_STATE_MACHINE_H_
 #define CC_SCHEDULER_SCHEDULER_STATE_MACHINE_H_
 
 #include <string>
 
 #include "base/basictypes.h"
@@ skipping 128 matching lines @@
   // callback nor send more than one BeginMainFrame message.
   void OnBeginImplFrame(const BeginFrameArgs& args);
   void OnBeginImplFrameDeadlinePending();
   void OnBeginImplFrameDeadline();
   void OnBeginImplFrameIdle();
   bool ShouldTriggerBeginImplFrameDeadlineEarly() const;
   BeginImplFrameState begin_impl_frame_state() const {
     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.
   void DidEnterPollForAnticipatedDrawTriggers();
   void DidLeavePollForAnticipatedDrawTriggers();
   bool inside_poll_for_anticipated_draw_triggers() const {
     return inside_poll_for_anticipated_draw_triggers_;
   }
 
   // Indicates whether the LayerTreeHostImpl is visible.
@@ skipping 43 matching lines @@
   // Request exclusive access to the textures that back single buffered
   // layers on behalf of the main thread. Upon acquisition,
   // ACTION_DRAW_AND_SWAP_IF_POSSIBLE will not draw until the main thread
   // releases the
   // textures to the impl thread by committing the layers.
   void SetMainThreadNeedsLayerTextures();
 
   // 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.
   void SetCanDraw(bool can);
 
   // Indicates that the pending tree is ready for activation.
   void NotifyReadyToActivate();
 
   bool has_pending_tree() const { return has_pending_tree_; }
 
@@ skipping 59 matching lines @@
   bool inside_poll_for_anticipated_draw_triggers_;
   bool visible_;
   bool can_start_;
   bool can_draw_;
   bool has_pending_tree_;
   bool pending_tree_is_ready_for_activation_;
   bool active_tree_needs_first_draw_;
   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);
 };
 
 }  // namespace cc
 
 #endif  // CC_SCHEDULER_SCHEDULER_STATE_MACHINE_H_