cc: Heuristic for Renderer latency recovery

cc/scheduler/scheduler.cc

Index: cc/scheduler/scheduler.cc
diff --git a/cc/scheduler/scheduler.cc b/cc/scheduler/scheduler.cc
index cd0d37ea2a81c32d1d37d6698b3cf04c296d6e47..647340af077e7b4e3185e5b13175f92dfaa79f19 100644
--- a/cc/scheduler/scheduler.cc
+++ b/cc/scheduler/scheduler.cc
@@ -460,10 +460,15 @@ void Scheduler::BeginImplFrameWithDeadline(const BeginFrameArgs& args) {
   BeginFrameArgs adjusted_args = args;
   adjusted_args.deadline -= compositor_timing_history_->DrawDurationEstimate();
 
-  if (!state_machine_.impl_latency_takes_priority() &&
-      main_thread_is_in_high_latency_mode &&
-      CanCommitAndActivateBeforeDeadline()) {
+  if (ShouldRecoverMainLatency(adjusted_args)) {
+    TRACE_EVENT_INSTANT0("cc", "SkipBeginMainFrameToReduceLatency",
+                         TRACE_EVENT_SCOPE_THREAD);
     state_machine_.SetSkipNextBeginMainFrameToReduceLatency();
+  } else if (ShouldRecoverImplLatency(adjusted_args)) {
+    TRACE_EVENT_INSTANT0("cc", "SkipBeginImplFrameToReduceLatency",
+                         TRACE_EVENT_SCOPE_THREAD);
+    frame_source_->DidFinishFrame(begin_retro_frame_args_.size());
+    return;
   }
 
   BeginImplFrame(adjusted_args);
@@ -515,7 +520,6 @@ void Scheduler::ScheduleBeginImplFrameDeadline() {
 
   begin_impl_frame_deadline_mode_ =
       state_machine_.CurrentBeginImplFrameDeadlineMode();
-
   base::TimeTicks deadline;
   switch (begin_impl_frame_deadline_mode_) {
     case SchedulerStateMachine::BEGIN_IMPL_FRAME_DEADLINE_MODE_NONE:
@@ -697,6 +701,8 @@ scoped_refptr<base::trace_event::ConvertableToTraceFormat> Scheduler::AsValue()
 }
 
 void Scheduler::AsValueInto(base::trace_event::TracedValue* state) const {
+  base::TimeTicks now = Now();
+
   state->BeginDictionary("state_machine");
   state_machine_.AsValueInto(state);
   state->EndDictionary();
@@ -725,9 +731,14 @@ void Scheduler::AsValueInto(base::trace_event::TracedValue* state) const {
                     !begin_impl_frame_deadline_task_.IsCancelled());
   state->SetString("inside_action",
                    SchedulerStateMachine::ActionToString(inside_action_));
+
   state->BeginDictionary("begin_impl_frame_args");
-  begin_impl_frame_tracker_.AsValueInto(Now(), state);
+  begin_impl_frame_tracker_.AsValueInto(now, state);
   state->EndDictionary();
+
+  state->SetString("begin_impl_frame_deadline_mode_",
+                   SchedulerStateMachine::BeginImplFrameDeadlineModeToString(
+                       begin_impl_frame_deadline_mode_));
   state->EndDictionary();
 
   state->BeginDictionary("compositor_timing_history");
@@ -740,10 +751,45 @@ void Scheduler::UpdateCompositorTimingHistoryRecordingEnabled() {
       state_machine_.HasInitializedOutputSurface() && state_machine_.visible());
 }
 
-bool Scheduler::CanCommitAndActivateBeforeDeadline() const {
-  BeginFrameArgs args =
-      begin_impl_frame_tracker_.DangerousMethodCurrentOrLast();
+bool Scheduler::ShouldRecoverMainLatency(const BeginFrameArgs& args) const {
+  if (!state_machine_.MainThreadIsInHighLatencyMode())
+    return false;
+
+  if (state_machine_.impl_latency_takes_priority())

[sunnyps, 2015/07/08 22:37:12]

Might be better to check if deadline mode != LATE.

[sunnyps, 2015/07/08 22:40:07]

I should clarify what I mean here: we should check if we're going to wait for
commit before drawing - if we are then it makes sense to skip begin main frame
but if we're not then we shouldn't skip begin main frame.

Browser impl-side painting makes this more complicated - we might be waiting for
READY_TO_DRAW - so do we skip begin main frame in that case? or not?

[brianderson, 2015/07/09 01:28:11]

We might not wait for the commit because active_tree_needs_first_draw_ is true.
However, in that case, we still want to be able to recover the main thread
latency.

We can make this more general if needed, but I think impl_latency_takes_priority
is the only case where we never want to wait for the main thread.
The Browser runs in a forced low-latency mode (where we never send the next
BeginMainFrame before the deadline/draw) so this line should never hit.

(Note: this is for the main thread latency recovery.)

+    return false;
+
+  return CanCommitAndActivateBeforeDeadline(args);
+}
+
+bool Scheduler::ShouldRecoverImplLatency(const BeginFrameArgs& args) const {
+  if (!state_machine_.SwapThrottled())
+    return false;
 
+  // The deadline may be in the past if our draw time is too long.
+  bool frame_time_is_before_deadline = args.frame_time < args.deadline;
+
+  SchedulerStateMachine::BeginImplFrameDeadlineMode
+      next_begin_impl_frame_deadline_mode =
+          state_machine_.CurrentBeginImplFrameDeadlineMode();
+  switch (next_begin_impl_frame_deadline_mode) {
+    case SchedulerStateMachine::BEGIN_IMPL_FRAME_DEADLINE_MODE_NONE:
+      return false;
+    case SchedulerStateMachine::BEGIN_IMPL_FRAME_DEADLINE_MODE_IMMEDIATE:
+      return frame_time_is_before_deadline;
+    case SchedulerStateMachine::BEGIN_IMPL_FRAME_DEADLINE_MODE_REGULAR:
+      return frame_time_is_before_deadline;
+    case SchedulerStateMachine::BEGIN_IMPL_FRAME_DEADLINE_MODE_LATE:
+      return CanCommitAndActivateBeforeDeadline(args);
+    case SchedulerStateMachine::
+        BEGIN_IMPL_FRAME_DEADLINE_MODE_BLOCKED_ON_READY_TO_DRAW:

[sunnyps, 2015/07/08 22:37:12]

Would be great if we had comments explaining each case.

[brianderson, 2015/07/09 01:28:11]

In writing the comments, I realized I can't rely directly on the deadline mode
since being swap throttled (and whether we are currently between a
BeginImplFrame and deadline) affects what deadline mode we select.

I'll fix up this code and add comments to that.

[sunnyps, 2015/07/09 23:44:30]

Oh yeah! Nice catch.

+      return false;
+  }
+  NOTREACHED();
+  return false;
+}
+
+bool Scheduler::CanCommitAndActivateBeforeDeadline(
+    const BeginFrameArgs& args) const {
   // Check if the main thread computation and commit can be finished before the
   // impl thread's deadline.
   base::TimeTicks estimated_draw_time =
@@ -752,12 +798,6 @@ bool Scheduler::CanCommitAndActivateBeforeDeadline() const {
       compositor_timing_history_->CommitToReadyToActivateDurationEstimate() +
       compositor_timing_history_->ActivateDurationEstimate();
 
-  TRACE_EVENT2(TRACE_DISABLED_BY_DEFAULT("cc.debug.scheduler"),
-               "CanCommitAndActivateBeforeDeadline",
-               "time_left_after_drawing_ms",
-               (args.deadline - estimated_draw_time).InMillisecondsF(), "state",
-               AsValue());
-
   return estimated_draw_time < args.deadline;
 }