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// 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 "config.h"
#include "CCDelayBasedTimeSource.h"
#include "base/debug/trace_event.h"
#include "base/logging.h"
#include <algorithm>
#include <wtf/CurrentTime.h>
#include <wtf/MathExtras.h>
namespace cc {
namespace {
// doubleTickThreshold 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.
const double doubleTickThreshold = 0.25;
// intervalChangeThreshold is the fraction of the interval that will trigger an immediate interval change.
// phaseChangeThreshold 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.
const double intervalChangeThreshold = 0.25;
const double phaseChangeThreshold = 0.25;
} // namespace
scoped_refptr<CCDelayBasedTimeSource> CCDelayBasedTimeSource::create(base::TimeDelta interval, CCThread* thread)
{
return make_scoped_refptr(new CCDelayBasedTimeSource(interval, thread));
}
CCDelayBasedTimeSource::CCDelayBasedTimeSource(base::TimeDelta interval, CCThread* thread)
: m_client(0)
, m_hasTickTarget(false)
, m_currentParameters(interval, base::TimeTicks())
, m_nextParameters(interval, base::TimeTicks())
, m_state(STATE_INACTIVE)
, m_timer(thread, this)
{
}
CCDelayBasedTimeSource::~CCDelayBasedTimeSource()
{
}
void CCDelayBasedTimeSource::setActive(bool active)
{
TRACE_EVENT1("cc", "CCDelayBasedTimeSource::setActive", "active", active);
if (!active) {
m_state = STATE_INACTIVE;
m_timer.stop();
return;
}
if (m_state == STATE_STARTING || m_state == STATE_ACTIVE)
return;
if (!m_hasTickTarget) {
// Becoming active the first time is deferred: we post a 0-delay task. When
// it runs, we use that to establish the timebase, become truly active, and
// fire the first tick.
m_state = STATE_STARTING;
m_timer.startOneShot(0);
return;
}
m_state = STATE_ACTIVE;
postNextTickTask(now());
}
bool CCDelayBasedTimeSource::active() const
{
return m_state != STATE_INACTIVE;
}
base::TimeTicks CCDelayBasedTimeSource::lastTickTime()
{
return m_lastTickTime;
}
base::TimeTicks CCDelayBasedTimeSource::nextTickTime()
{
return active() ? m_currentParameters.tickTarget : base::TimeTicks();
}
void CCDelayBasedTimeSource::onTimerFired()
{
DCHECK(m_state != STATE_INACTIVE);
base::TimeTicks now = this->now();
m_lastTickTime = now;
if (m_state == STATE_STARTING) {
setTimebaseAndInterval(now, m_currentParameters.interval);
m_state = STATE_ACTIVE;
}
postNextTickTask(now);
// Fire the tick
if (m_client)
m_client->onTimerTick();
}
void CCDelayBasedTimeSource::setClient(CCTimeSourceClient* client)
{
m_client = client;
}
void CCDelayBasedTimeSource::setTimebaseAndInterval(base::TimeTicks timebase, base::TimeDelta interval)
{
m_nextParameters.interval = interval;
m_nextParameters.tickTarget = timebase;
m_hasTickTarget = true;
if (m_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 intervalDelta = std::abs((interval - m_currentParameters.interval).InSecondsF());
double intervalChange = intervalDelta / interval.InSecondsF();
if (intervalChange > intervalChangeThreshold) {
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 targetDelta = std::abs((timebase - m_currentParameters.tickTarget).InSecondsF());
double phaseChange = fmod(targetDelta, interval.InSecondsF()) / interval.InSecondsF();
if (phaseChange > phaseChangeThreshold && phaseChange < (1.0 - phaseChangeThreshold)) {
setActive(false);
setActive(true);
return;
}
}
base::TimeTicks CCDelayBasedTimeSource::now() const
{
return base::TimeTicks::Now();
}
// This code tries to achieve an average tick rate as close to m_interval as possible.
// To do this, it has to deal with a few basic issues:
// 1. postDelayedTask can delay only at a millisecond granularity. So, 16.666 has to
// posted as 16 or 17.
// 2. A delayed task may come back a bit late (a few ms), or really late (frames later)
//
// The basic idea with this scheduler here is to keep track of where we *want* to run in
// m_tickTarget. We update this with the exact interval.
//
// Then, when we post our task, we take the floor of (m_tickTarget and now()). If we
// started at now=0, and 60FPs (all times in milliseconds):
// now=0 target=16.667 postDelayedTask(16)
//
// When our callback runs, we figure out how far off we were from that goal. Because of the flooring
// operation, and assuming our timer runs exactly when it should, this yields:
// now=16 target=16.667
//
// Since we can't post a 0.667 ms task to get to now=16, we just treat this as a tick. Then,
// we update target to be 33.333. We now post another task based on the difference between our target
// and now:
// now=16 tickTarget=16.667 newTarget=33.333 --> postDelayedTask(floor(33.333 - 16)) --> postDelayedTask(17)
//
// Over time, with no late tasks, this leads to us posting tasks like this:
// now=0 tickTarget=0 newTarget=16.667 --> tick(), postDelayedTask(16)
// now=16 tickTarget=16.667 newTarget=33.333 --> tick(), postDelayedTask(17)
// now=33 tickTarget=33.333 newTarget=50.000 --> tick(), postDelayedTask(17)
// now=50 tickTarget=50.000 newTarget=66.667 --> tick(), postDelayedTask(16)
//
// We treat delays in tasks differently depending on the amount of delay we encounter. Suppose we
// posted a task with a target=16.667:
// Case 1: late but not unrecoverably-so
// now=18 tickTarget=16.667
//
// Case 2: so late we obviously missed the tick
// now=25.0 tickTarget=16.667
//
// We treat the first case as a tick anyway, and assume the delay was
// unusual. Thus, we compute the newTarget based on the old timebase:
// now=18 tickTarget=16.667 newTarget=33.333 --> 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 tickTarget=16.667 newTarget=50.000 --> tick(), postDelayedTask(floor(50.000-37)) --> postDelayedTask(13)
base::TimeTicks CCDelayBasedTimeSource::nextTickTarget(base::TimeTicks now)
{
base::TimeDelta newInterval = m_nextParameters.interval;
int intervalsElapsed = static_cast<int>(floor((now - m_nextParameters.tickTarget).InSecondsF() / newInterval.InSecondsF()));
base::TimeTicks lastEffectiveTick = m_nextParameters.tickTarget + newInterval * intervalsElapsed;
base::TimeTicks newTickTarget = lastEffectiveTick + newInterval;
DCHECK(newTickTarget > now);
// Avoid double ticks when:
// 1) Turning off the timer and turning it right back on.
// 2) Jittery data is passed to setTimebaseAndInterval().
if (newTickTarget - m_lastTickTime <= newInterval / static_cast<int>(1.0 / doubleTickThreshold))
newTickTarget += newInterval;
return newTickTarget;
}
void CCDelayBasedTimeSource::postNextTickTask(base::TimeTicks now)
{
base::TimeTicks newTickTarget = nextTickTarget(now);
// Post another task *before* the tick and update state
base::TimeDelta delay = newTickTarget - now;
DCHECK(delay.InMillisecondsF() <=
m_nextParameters.interval.InMillisecondsF() * (1.0 + doubleTickThreshold));
m_timer.startOneShot(delay.InSecondsF());
m_nextParameters.tickTarget = newTickTarget;
m_currentParameters = m_nextParameters;
}
} // namespace cc
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