cc: Fix and simplify DelayBasedTimeSource

cc/scheduler/delay_based_time_source.cc

 // 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.
 
 #include "cc/scheduler/delay_based_time_source.h"
 
 #include <algorithm>
 #include <cmath>
 
 #include "base/debug/trace_event.h"
 #include "base/logging.h"
 #include "base/single_thread_task_runner.h"
 
 namespace cc {
 
 namespace {
 
-// kDoubleTickThreshold prevents ticks from running within the specified
+// kDoubleTickDivisor prevents ticks from running within the specified
 // fraction of an interval.  This helps account for jitter in the timebase as
 // well as quick timer reactivation.
-static const double kDoubleTickThreshold = 0.25;
+static const int kDoubleTickDivisor = 4;

[brianderson, 2013/07/03 00:01:54]

We can divide TimeTicks so I switched this to a divisor to avoid floating point
math.

 
 // kIntervalChangeThreshold is the fraction of the interval that will trigger an
 // immediate interval change.  kPhaseChangeThreshold is the fraction of the
 // interval that will trigger an immediate phase change.  If the changes are
 // within the thresholds, the change will take place on the next tick.  If
 // either change is outside the thresholds, the next tick will be canceled and
 // reissued immediately.
 static const double kIntervalChangeThreshold = 0.25;
 static const double kPhaseChangeThreshold = 0.25;
 
@@ skipping 48 matching lines @@
 
 base::TimeTicks DelayBasedTimeSource::LastTickTime() { return last_tick_time_; }
 
 base::TimeTicks DelayBasedTimeSource::NextTickTime() {
   return Active() ? current_parameters_.tick_target : base::TimeTicks();
 }
 
 void DelayBasedTimeSource::OnTimerFired() {
   DCHECK(state_ != STATE_INACTIVE);
 
+  last_tick_time_ = current_parameters_.tick_target;

[brianderson, 2013/07/03 00:01:54]

Instead of setting the last_tick_time_ to the time at which this task happened
to be scheduled, this will return the ideal tick time giving us a less jittery
timestamp.

+
   base::TimeTicks now = this->Now();
-  last_tick_time_ = now;
-
-  if (state_ == STATE_STARTING) {
-    SetTimebaseAndInterval(now, current_parameters_.interval);
+  if (state_ == STATE_STARTING)
     state_ = STATE_ACTIVE;
-  }
 
   PostNextTickTask(now);
 
   // Fire the tick.
   if (client_)
     client_->OnTimerTick();
 }
 
 void DelayBasedTimeSource::SetClient(TimeSourceClient* client) {
   client_ = client;
 }
 
 void DelayBasedTimeSource::SetTimebaseAndInterval(base::TimeTicks timebase,
                                                   base::TimeDelta interval) {
   next_parameters_.interval = interval;
   next_parameters_.tick_target = timebase;
   has_tick_target_ = true;
 
   if (state_ != STATE_ACTIVE) {
     // If we aren't active, there's no need to reset the timer.
     return;
   }
 
   // If the change in interval is larger than the change threshold,
   // request an immediate reset.
   double interval_delta =
       std::abs((interval - current_parameters_.interval).InSecondsF());
   double interval_change = interval_delta / interval.InSecondsF();
   if (interval_change > kIntervalChangeThreshold) {
+    TRACE_EVENT0("cc", "DelayBasedTimeSource::IntervalChanged");
     SetActive(false);
     SetActive(true);
     return;
   }
 
   // If the change in phase is greater than the change threshold in either
   // direction, request an immediate reset. This logic might result in a false
   // negative if there is a simultaneous small change in the interval and the
   // fmod just happens to return something near zero. Assuming the timebase
   // is very recent though, which it should be, we'll still be ok because the
   // old clock and new clock just happen to line up.
   double target_delta =
       std::abs((timebase - current_parameters_.tick_target).InSecondsF());
   double phase_change =
       fmod(target_delta, interval.InSecondsF()) / interval.InSecondsF();
   if (phase_change > kPhaseChangeThreshold &&
       phase_change < (1.0 - kPhaseChangeThreshold)) {
+    TRACE_EVENT0("cc", "DelayBasedTimeSource::PhaseChanged");

[brianderson, 2013/07/03 00:01:54]

IntervalChanged and PhaseChanged show up way too often on some platforms,
causing unnecessary jank. See
https://code.google.com/p/chromium/issues/detail?id=252087. This will make it
easier to notice and fix.

     SetActive(false);
     SetActive(true);
     return;
   }
 }
 
 base::TimeTicks DelayBasedTimeSource::Now() const {
   return base::TimeTicks::Now();
 }
 
@@ skipping 46 matching lines @@
 //          tick(), PostDelayedTask(floor(33.333-18)) --> PostDelayedTask(15)
 // This brings us back to 18+15 = 33, which was where we would have been if the
 // task hadn't been late.
 //
 // For the really late delay, we we move to the next logical tick. The timebase
 // is not reset.
 //      now=37   tick_target=16.667  new_target=50.000  -->
 //          tick(), PostDelayedTask(floor(50.000-37)) --> PostDelayedTask(13)
 base::TimeTicks DelayBasedTimeSource::NextTickTarget(base::TimeTicks now) {
   base::TimeDelta new_interval = next_parameters_.interval;
-  int intervals_elapsed =
-      static_cast<int>(floor((now - next_parameters_.tick_target).InSecondsF() /
-                             new_interval.InSecondsF()));
+  int intervals_elapsed = (now - next_parameters_.tick_target) / new_interval;

[brianderson, 2013/07/03 00:01:54]

Goodbye floating point math.

   base::TimeTicks last_effective_tick =
       next_parameters_.tick_target + new_interval * intervals_elapsed;
   base::TimeTicks new_tick_target = last_effective_tick + new_interval;
   DCHECK(now < new_tick_target)
       << "now = " << now.ToInternalValue()
       << "; new_tick_target = " << new_tick_target.ToInternalValue()
       << "; new_interval = " << new_interval.InMicroseconds()
       << "; tick_target = " << next_parameters_.tick_target.ToInternalValue()
       << "; intervals_elapsed = " << intervals_elapsed
       << "; last_effective_tick = " << last_effective_tick.ToInternalValue();
 
   // Avoid double ticks when:
   // 1) Turning off the timer and turning it right back on.
   // 2) Jittery data is passed to SetTimebaseAndInterval().
-  if (new_tick_target - last_tick_time_ <=
-      new_interval / static_cast<int>(1.0 / kDoubleTickThreshold))
+  if (new_tick_target - last_tick_time_ <= new_interval / kDoubleTickDivisor)

[brianderson, 2013/07/03 00:01:54]

Goodbye more floating point math.

     new_tick_target += new_interval;
 
   return new_tick_target;
 }
 
 void DelayBasedTimeSource::PostNextTickTask(base::TimeTicks now) {
   base::TimeTicks new_tick_target = NextTickTarget(now);
 
   // Post another task *before* the tick and update state
-  base::TimeDelta delay = new_tick_target - now;
-  DCHECK(delay.InMillisecondsF() <=
-         next_parameters_.interval.InMillisecondsF() *
-         (1.0 + kDoubleTickThreshold));
+  base::TimeDelta delay;
+  if (new_tick_target > now)
+    delay = new_tick_target - now;
   task_runner_->PostDelayedTask(FROM_HERE,
                                 base::Bind(&DelayBasedTimeSource::OnTimerFired,
                                            weak_factory_.GetWeakPtr()),
                                 delay);
 
   next_parameters_.tick_target = new_tick_target;
   current_parameters_ = next_parameters_;
 }
 
 }  // namespace cc