scheduler: Only prioritize rAF if it is fast

components/scheduler/renderer/renderer_scheduler_impl.cc

Index: components/scheduler/renderer/renderer_scheduler_impl.cc
diff --git a/components/scheduler/renderer/renderer_scheduler_impl.cc b/components/scheduler/renderer/renderer_scheduler_impl.cc
index f241d7bb755b5c2ca1f66aeb42c9d38581ed7d3f..4873fd1d9761a126655ce98e4991aabc61954d10 100644
--- a/components/scheduler/renderer/renderer_scheduler_impl.cc
+++ b/components/scheduler/renderer/renderer_scheduler_impl.cc
@@ -679,6 +679,13 @@ void RendererSchedulerImpl::UpdatePolicyLocked(UpdateType update_type) {
     MainThreadOnly().current_policy_expiration_time = base::TimeTicks();
   }
 
+  // Avoid prioritizing main thread compositing (e.g., rAF) if it is extremely
+  // slow, because that can cause starvation in other task sources.
+  bool main_thread_compositing_is_fast =
+      MainThreadOnly().idle_time_estimator.GetExpectedIdleDuration(
+          MainThreadOnly().compositor_frame_interval) >
+      base::TimeDelta::FromSeconds(0);

[alex clarke (OOO till 29th), 2016/05/03 09:47:01]

As discussed off line I'm wondering if this might be a fairer computation:

double min_idle_ratio = 0.2;  // needs a better name
bool main_thread_compositing_is_fast =
    MainThreadOnly().idle_time_estimator.GetExpectedIdleDuration(
         MainThreadOnly().compositor_frame_interval) >
    MainThreadOnly().compositor_frame_interval * min_idle_ratio;

[Sami, 2016/05/03 12:50:00]

Yep, that's probably safer. Done.

+
   Policy new_policy;
   ExpensiveTaskPolicy expensive_task_policy = ExpensiveTaskPolicy::RUN;
   switch (use_case) {
@@ -697,7 +704,9 @@ void RendererSchedulerImpl::UpdatePolicyLocked(UpdateType update_type) {
       break;
 
     case UseCase::SYNCHRONIZED_GESTURE:
-      new_policy.compositor_queue_policy.priority = TaskQueue::HIGH_PRIORITY;
+      new_policy.compositor_queue_policy.priority =
+          main_thread_compositing_is_fast ? TaskQueue::HIGH_PRIORITY
+                                          : TaskQueue::NORMAL_PRIORITY;
       if (touchstart_expected_soon) {
         expensive_task_policy = ExpensiveTaskPolicy::BLOCK;
       } else {
@@ -710,7 +719,9 @@ void RendererSchedulerImpl::UpdatePolicyLocked(UpdateType update_type) {
       // things we should be prioritizing, so we don't attempt to block
       // expensive tasks because we don't know whether they were integral to the
       // page's functionality or not.
-      new_policy.compositor_queue_policy.priority = TaskQueue::HIGH_PRIORITY;
+      new_policy.compositor_queue_policy.priority =
+          main_thread_compositing_is_fast ? TaskQueue::HIGH_PRIORITY
+                                          : TaskQueue::NORMAL_PRIORITY;
       break;
 
     case UseCase::TOUCHSTART: