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