cc/scheduler/delay_based_time_source.cc

   1 // Copyright 2011 The Chromium Authors. All rights reserved.
   2 // Use of this source code is governed by a BSD-style license that can be
   3 // found in the LICENSE file.
   4
   5 #include "cc/scheduler/delay_based_time_source.h"
   6
   7 #include <algorithm>
   8 #include <cmath>
   9
  10 #include "base/bind.h"
  11 #include "base/debug/trace_event.h"
  12 #include "base/location.h"
  13 #include "base/logging.h"
  14 #include "base/single_thread_task_runner.h"
  15
  16 namespace cc {
  17
  18 namespace {
  19
  20 // kDoubleTickDivisor prevents ticks from running within the specified
  21 // fraction of an interval.  This helps account for jitter in the timebase as
  22 // well as quick timer reactivation.
  23 static const int kDoubleTickDivisor = 2;
  24
  25 // kIntervalChangeThreshold is the fraction of the interval that will trigger an
  26 // immediate interval change.  kPhaseChangeThreshold is the fraction of the
  27 // interval that will trigger an immediate phase change.  If the changes are
  28 // within the thresholds, the change will take place on the next tick.  If
  29 // either change is outside the thresholds, the next tick will be canceled and
  30 // reissued immediately.
  31 static const double kIntervalChangeThreshold = 0.25;
  32 static const double kPhaseChangeThreshold = 0.25;
  33
  34 }  // namespace
  35
  36 // The following methods correspond to the DelayBasedTimeSource that uses
  37 // the base::TimeTicks::HighResNow as the timebase.
  38 scoped_refptr<DelayBasedTimeSourceHighRes> DelayBasedTimeSourceHighRes::Create(
  39     base::TimeDelta interval,
  40     base::SingleThreadTaskRunner* task_runner) {
  41   return make_scoped_refptr(
  42       new DelayBasedTimeSourceHighRes(interval, task_runner));
  43 }
  44
  45 DelayBasedTimeSourceHighRes::DelayBasedTimeSourceHighRes(
  46     base::TimeDelta interval, base::SingleThreadTaskRunner* task_runner)
  47     : DelayBasedTimeSource(interval, task_runner) {}
  48
  49 DelayBasedTimeSourceHighRes::~DelayBasedTimeSourceHighRes() {}
  50
  51 base::TimeTicks DelayBasedTimeSourceHighRes::Now() const {
  52   return base::TimeTicks::HighResNow();
  53 }
  54
  55 // The following methods correspond to the DelayBasedTimeSource that uses
  56 // the base::TimeTicks::Now as the timebase.
  57 scoped_refptr<DelayBasedTimeSource> DelayBasedTimeSource::Create(
  58     base::TimeDelta interval,
  59     base::SingleThreadTaskRunner* task_runner) {
  60   return make_scoped_refptr(new DelayBasedTimeSource(interval, task_runner));
  61 }
  62
  63 DelayBasedTimeSource::DelayBasedTimeSource(
  64     base::TimeDelta interval, base::SingleThreadTaskRunner* task_runner)
  65     : client_(NULL),
  66       last_tick_time_(base::TimeTicks() - interval),
  67       current_parameters_(interval, base::TimeTicks()),
  68       next_parameters_(interval, base::TimeTicks()),
  69       active_(false),
  70       task_runner_(task_runner),
  71       weak_factory_(this) {}
  72
  73 DelayBasedTimeSource::~DelayBasedTimeSource() {}
  74
  75 base::TimeTicks DelayBasedTimeSource::SetActive(bool active) {
  76   TRACE_EVENT1("cc", "DelayBasedTimeSource::SetActive", "active", active);
  77   if (active == active_)
  78     return base::TimeTicks();
  79   active_ = active;
  80
  81   if (!active_) {
  82     weak_factory_.InvalidateWeakPtrs();
  83     return base::TimeTicks();
  84   }
  85
  86   PostNextTickTask(Now());
  87
  88   // Determine if there was a tick that was missed while not active.
  89   base::TimeTicks last_tick_time_if_always_active =
  90     current_parameters_.tick_target - current_parameters_.interval;
  91   base::TimeTicks new_tick_time_threshold =
  92     last_tick_time_ + current_parameters_.interval / kDoubleTickDivisor;
  93   if (last_tick_time_if_always_active >  new_tick_time_threshold) {
  94     last_tick_time_ = last_tick_time_if_always_active;
  95     return last_tick_time_;
  96   }
  97
  98   return base::TimeTicks();
  99 }
 100
 101 bool DelayBasedTimeSource::Active() const { return active_; }
 102
 103 base::TimeTicks DelayBasedTimeSource::LastTickTime() { return last_tick_time_; }
 104
 105 base::TimeTicks DelayBasedTimeSource::NextTickTime() {
 106   return Active() ? current_parameters_.tick_target : base::TimeTicks();
 107 }
 108
 109 void DelayBasedTimeSource::OnTimerFired() {
 110   DCHECK(active_);
 111
 112   last_tick_time_ = current_parameters_.tick_target;
 113
 114   PostNextTickTask(Now());
 115
 116   // Fire the tick.
 117   if (client_)
 118     client_->OnTimerTick();
 119 }
 120
 121 void DelayBasedTimeSource::SetClient(TimeSourceClient* client) {
 122   client_ = client;
 123 }
 124
 125 void DelayBasedTimeSource::SetTimebaseAndInterval(base::TimeTicks timebase,
 126                                                   base::TimeDelta interval) {
 127   next_parameters_.interval = interval;
 128   next_parameters_.tick_target = timebase;
 129
 130   if (!active_) {
 131     // If we aren't active, there's no need to reset the timer.
 132     return;
 133   }
 134
 135   // If the change in interval is larger than the change threshold,
 136   // request an immediate reset.
 137   double interval_delta =
 138       std::abs((interval - current_parameters_.interval).InSecondsF());
 139   double interval_change = interval_delta / interval.InSecondsF();
 140   if (interval_change > kIntervalChangeThreshold) {
 141     TRACE_EVENT_INSTANT0("cc", "DelayBasedTimeSource::IntervalChanged",
 142                          TRACE_EVENT_SCOPE_THREAD);
 143     SetActive(false);
 144     SetActive(true);
 145     return;
 146   }
 147
 148   // If the change in phase is greater than the change threshold in either
 149   // direction, request an immediate reset. This logic might result in a false
 150   // negative if there is a simultaneous small change in the interval and the
 151   // fmod just happens to return something near zero. Assuming the timebase
 152   // is very recent though, which it should be, we'll still be ok because the
 153   // old clock and new clock just happen to line up.
 154   double target_delta =
 155       std::abs((timebase - current_parameters_.tick_target).InSecondsF());
 156   double phase_change =
 157       fmod(target_delta, interval.InSecondsF()) / interval.InSecondsF();
 158   if (phase_change > kPhaseChangeThreshold &&
 159       phase_change < (1.0 - kPhaseChangeThreshold)) {
 160     TRACE_EVENT_INSTANT0("cc", "DelayBasedTimeSource::PhaseChanged",
 161                          TRACE_EVENT_SCOPE_THREAD);
 162     SetActive(false);
 163     SetActive(true);
 164     return;
 165   }
 166 }
 167
 168 base::TimeTicks DelayBasedTimeSource::Now() const {
 169   return base::TimeTicks::Now();
 170 }
 171
 172 // This code tries to achieve an average tick rate as close to interval_ as
 173 // possible.  To do this, it has to deal with a few basic issues:
 174 //   1. PostDelayedTask can delay only at a millisecond granularity. So, 16.666
 175 //   has to posted as 16 or 17.
 176 //   2. A delayed task may come back a bit late (a few ms), or really late
 177 //   (frames later)
 178 //
 179 // The basic idea with this scheduler here is to keep track of where we *want*
 180 // to run in tick_target_. We update this with the exact interval.
 181 //
 182 // Then, when we post our task, we take the floor of (tick_target_ and Now()).
 183 // If we started at now=0, and 60FPs (all times in milliseconds):
 184 //      now=0    target=16.667   PostDelayedTask(16)
 185 //
 186 // When our callback runs, we figure out how far off we were from that goal.
 187 // Because of the flooring operation, and assuming our timer runs exactly when
 188 // it should, this yields:
 189 //      now=16   target=16.667
 190 //
 191 // Since we can't post a 0.667 ms task to get to now=16, we just treat this as a
 192 // tick. Then, we update target to be 33.333. We now post another task based on
 193 // the difference between our target and now:
 194 //      now=16   tick_target=16.667  new_target=33.333   -->
 195 //          PostDelayedTask(floor(33.333 - 16)) --> PostDelayedTask(17)
 196 //
 197 // Over time, with no late tasks, this leads to us posting tasks like this:
 198 //      now=0    tick_target=0       new_target=16.667   -->
 199 //          tick(), PostDelayedTask(16)
 200 //      now=16   tick_target=16.667  new_target=33.333   -->
 201 //          tick(), PostDelayedTask(17)
 202 //      now=33   tick_target=33.333  new_target=50.000   -->
 203 //          tick(), PostDelayedTask(17)
 204 //      now=50   tick_target=50.000  new_target=66.667   -->
 205 //          tick(), PostDelayedTask(16)
 206 //
 207 // We treat delays in tasks differently depending on the amount of delay we
 208 // encounter. Suppose we posted a task with a target=16.667:
 209 //   Case 1: late but not unrecoverably-so
 210 //      now=18 tick_target=16.667
 211 //
 212 //   Case 2: so late we obviously missed the tick
 213 //      now=25.0 tick_target=16.667
 214 //
 215 // We treat the first case as a tick anyway, and assume the delay was unusual.
 216 // Thus, we compute the new_target based on the old timebase:
 217 //      now=18   tick_target=16.667  new_target=33.333   -->
 218 //          tick(), PostDelayedTask(floor(33.333-18)) --> PostDelayedTask(15)
 219 // This brings us back to 18+15 = 33, which was where we would have been if the
 220 // task hadn't been late.
 221 //
 222 // For the really late delay, we we move to the next logical tick. The timebase
 223 // is not reset.
 224 //      now=37   tick_target=16.667  new_target=50.000  -->
 225 //          tick(), PostDelayedTask(floor(50.000-37)) --> PostDelayedTask(13)
 226 base::TimeTicks DelayBasedTimeSource::NextTickTarget(base::TimeTicks now) {
 227   base::TimeDelta new_interval = next_parameters_.interval;
 228
 229   // |interval_offset| is the offset from |now| to the next multiple of
 230   // |interval| after |tick_target|, possibly negative if in the past.
 231   base::TimeDelta interval_offset = base::TimeDelta::FromInternalValue(
 232       (next_parameters_.tick_target - now).ToInternalValue() %
 233       new_interval.ToInternalValue());
 234   // If |now| is exactly on the interval (i.e. offset==0), don't adjust.
 235   // Otherwise, if |tick_target| was in the past, adjust forward to the next
 236   // tick after |now|.
 237   if (interval_offset.ToInternalValue() != 0 &&
 238       next_parameters_.tick_target < now) {
 239     interval_offset += new_interval;
 240   }
 241
 242   base::TimeTicks new_tick_target = now + interval_offset;
 243   DCHECK(now <= new_tick_target)
 244       << "now = " << now.ToInternalValue()
 245       << "; new_tick_target = " << new_tick_target.ToInternalValue()
 246       << "; new_interval = " << new_interval.InMicroseconds()
 247       << "; tick_target = " << next_parameters_.tick_target.ToInternalValue()
 248       << "; interval_offset = " << interval_offset.ToInternalValue();
 249
 250   // Avoid double ticks when:
 251   // 1) Turning off the timer and turning it right back on.
 252   // 2) Jittery data is passed to SetTimebaseAndInterval().
 253   if (new_tick_target - last_tick_time_ <= new_interval / kDoubleTickDivisor)
 254     new_tick_target += new_interval;
 255
 256   return new_tick_target;
 257 }
 258
 259 void DelayBasedTimeSource::PostNextTickTask(base::TimeTicks now) {
 260   base::TimeTicks new_tick_target = NextTickTarget(now);
 261
 262   // Post another task *before* the tick and update state
 263   base::TimeDelta delay;
 264   if (now <= new_tick_target)
 265     delay = new_tick_target - now;
 266   task_runner_->PostDelayedTask(FROM_HERE,
 267                                 base::Bind(&DelayBasedTimeSource::OnTimerFired,
 268                                            weak_factory_.GetWeakPtr()),
 269                                 delay);
 270
 271   next_parameters_.tick_target = new_tick_target;
 272   current_parameters_ = next_parameters_;
 273 }
 274
 275 }  // namespace cc