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 scoped_refptr<DelayBasedTimeSourceHighRes> DelayBasedTimeSourceHighRes::Create(
  41     base::TimeDelta interval,
  42     base::SingleThreadTaskRunner* task_runner) {
  43   return make_scoped_refptr(
  44       new DelayBasedTimeSourceHighRes(interval, task_runner));
  45 }
  46
  47 DelayBasedTimeSourceHighRes::DelayBasedTimeSourceHighRes(
  48     base::TimeDelta interval,
  49     base::SingleThreadTaskRunner* task_runner)
  50     : DelayBasedTimeSource(interval, task_runner) {
  51 }
  52
  53 DelayBasedTimeSourceHighRes::~DelayBasedTimeSourceHighRes() {}
  54
  55 base::TimeTicks DelayBasedTimeSourceHighRes::Now() const {
  56   return base::TimeTicks::Now();
  57 }
  58
  59 // The following methods correspond to the DelayBasedTimeSource that uses
  60 // the base::TimeTicks::Now as the timebase.
  61 scoped_refptr<DelayBasedTimeSource> DelayBasedTimeSource::Create(
  62     base::TimeDelta interval,
  63     base::SingleThreadTaskRunner* task_runner) {
  64   return make_scoped_refptr(new DelayBasedTimeSource(interval, task_runner));
  65 }
  66
  67 DelayBasedTimeSource::DelayBasedTimeSource(
  68     base::TimeDelta interval,
  69     base::SingleThreadTaskRunner* task_runner)
  70     : client_(NULL),
  71       last_tick_time_(base::TimeTicks() - interval),
  72       current_parameters_(interval, base::TimeTicks()),
  73       next_parameters_(interval, base::TimeTicks()),
  74       active_(false),
  75       task_runner_(task_runner),
  76       weak_factory_(this) {
  77   DCHECK_GT(interval.ToInternalValue(), 0);
  78 }
  79
  80 DelayBasedTimeSource::~DelayBasedTimeSource() {}
  81
  82 base::TimeTicks DelayBasedTimeSource::SetActive(bool active) {
  83   TRACE_EVENT1("cc", "DelayBasedTimeSource::SetActive", "active", active);
  84   if (active == active_)
  85     return base::TimeTicks();
  86   active_ = active;
  87
  88   if (!active_) {
  89     weak_factory_.InvalidateWeakPtrs();
  90     return base::TimeTicks();
  91   }
  92
  93   PostNextTickTask(Now());
  94
  95   // Determine if there was a tick that was missed while not active.
  96   base::TimeTicks last_tick_time_if_always_active =
  97     current_parameters_.tick_target - current_parameters_.interval;
  98   base::TimeTicks new_tick_time_threshold =
  99     last_tick_time_ + current_parameters_.interval / kDoubleTickDivisor;
 100   if (last_tick_time_if_always_active >  new_tick_time_threshold) {
 101     last_tick_time_ = last_tick_time_if_always_active;
 102     return last_tick_time_;
 103   }
 104
 105   return base::TimeTicks();
 106 }
 107
 108 bool DelayBasedTimeSource::Active() const { return active_; }
 109
 110 base::TimeTicks DelayBasedTimeSource::LastTickTime() const {
 111   return last_tick_time_;
 112 }
 113
 114 base::TimeTicks DelayBasedTimeSource::NextTickTime() const {
 115   return Active() ? current_parameters_.tick_target : base::TimeTicks();
 116 }
 117
 118 void DelayBasedTimeSource::OnTimerFired() {
 119   DCHECK(active_);
 120
 121   last_tick_time_ = current_parameters_.tick_target;
 122
 123   PostNextTickTask(Now());
 124
 125   // Fire the tick.
 126   if (client_)
 127     client_->OnTimerTick();
 128 }
 129
 130 void DelayBasedTimeSource::SetClient(TimeSourceClient* client) {
 131   client_ = client;
 132 }
 133
 134 void DelayBasedTimeSource::SetTimebaseAndInterval(base::TimeTicks timebase,
 135                                                   base::TimeDelta interval) {
 136   DCHECK_GT(interval.ToInternalValue(), 0);
 137   next_parameters_.interval = interval;
 138   next_parameters_.tick_target = timebase;
 139
 140   if (!active_) {
 141     // If we aren't active, there's no need to reset the timer.
 142     return;
 143   }
 144
 145   // If the change in interval is larger than the change threshold,
 146   // request an immediate reset.
 147   double interval_delta =
 148       std::abs((interval - current_parameters_.interval).InSecondsF());
 149   double interval_change = interval_delta / interval.InSecondsF();
 150   if (interval_change > kIntervalChangeThreshold) {
 151     TRACE_EVENT_INSTANT0("cc", "DelayBasedTimeSource::IntervalChanged",
 152                          TRACE_EVENT_SCOPE_THREAD);
 153     SetActive(false);
 154     SetActive(true);
 155     return;
 156   }
 157
 158   // If the change in phase is greater than the change threshold in either
 159   // direction, request an immediate reset. This logic might result in a false
 160   // negative if there is a simultaneous small change in the interval and the
 161   // fmod just happens to return something near zero. Assuming the timebase
 162   // is very recent though, which it should be, we'll still be ok because the
 163   // old clock and new clock just happen to line up.
 164   double target_delta =
 165       std::abs((timebase - current_parameters_.tick_target).InSecondsF());
 166   double phase_change =
 167       fmod(target_delta, interval.InSecondsF()) / interval.InSecondsF();
 168   if (phase_change > kPhaseChangeThreshold &&
 169       phase_change < (1.0 - kPhaseChangeThreshold)) {
 170     TRACE_EVENT_INSTANT0("cc", "DelayBasedTimeSource::PhaseChanged",
 171                          TRACE_EVENT_SCOPE_THREAD);
 172     SetActive(false);
 173     SetActive(true);
 174     return;
 175   }
 176 }
 177
 178 base::TimeTicks DelayBasedTimeSource::Now() const {
 179   return base::TimeTicks::Now();
 180 }
 181
 182 // This code tries to achieve an average tick rate as close to interval_ as
 183 // possible.  To do this, it has to deal with a few basic issues:
 184 //   1. PostDelayedTask can delay only at a millisecond granularity. So, 16.666
 185 //   has to posted as 16 or 17.
 186 //   2. A delayed task may come back a bit late (a few ms), or really late
 187 //   (frames later)
 188 //
 189 // The basic idea with this scheduler here is to keep track of where we *want*
 190 // to run in tick_target_. We update this with the exact interval.
 191 //
 192 // Then, when we post our task, we take the floor of (tick_target_ and Now()).
 193 // If we started at now=0, and 60FPs (all times in milliseconds):
 194 //      now=0    target=16.667   PostDelayedTask(16)
 195 //
 196 // When our callback runs, we figure out how far off we were from that goal.
 197 // Because of the flooring operation, and assuming our timer runs exactly when
 198 // it should, this yields:
 199 //      now=16   target=16.667
 200 //
 201 // Since we can't post a 0.667 ms task to get to now=16, we just treat this as a
 202 // tick. Then, we update target to be 33.333. We now post another task based on
 203 // the difference between our target and now:
 204 //      now=16   tick_target=16.667  new_target=33.333   -->
 205 //          PostDelayedTask(floor(33.333 - 16)) --> PostDelayedTask(17)
 206 //
 207 // Over time, with no late tasks, this leads to us posting tasks like this:
 208 //      now=0    tick_target=0       new_target=16.667   -->
 209 //          tick(), PostDelayedTask(16)
 210 //      now=16   tick_target=16.667  new_target=33.333   -->
 211 //          tick(), PostDelayedTask(17)
 212 //      now=33   tick_target=33.333  new_target=50.000   -->
 213 //          tick(), PostDelayedTask(17)
 214 //      now=50   tick_target=50.000  new_target=66.667   -->
 215 //          tick(), PostDelayedTask(16)
 216 //
 217 // We treat delays in tasks differently depending on the amount of delay we
 218 // encounter. Suppose we posted a task with a target=16.667:
 219 //   Case 1: late but not unrecoverably-so
 220 //      now=18 tick_target=16.667
 221 //
 222 //   Case 2: so late we obviously missed the tick
 223 //      now=25.0 tick_target=16.667
 224 //
 225 // We treat the first case as a tick anyway, and assume the delay was unusual.
 226 // Thus, we compute the new_target based on the old timebase:
 227 //      now=18   tick_target=16.667  new_target=33.333   -->
 228 //          tick(), PostDelayedTask(floor(33.333-18)) --> PostDelayedTask(15)
 229 // This brings us back to 18+15 = 33, which was where we would have been if the
 230 // task hadn't been late.
 231 //
 232 // For the really late delay, we we move to the next logical tick. The timebase
 233 // is not reset.
 234 //      now=37   tick_target=16.667  new_target=50.000  -->
 235 //          tick(), PostDelayedTask(floor(50.000-37)) --> PostDelayedTask(13)
 236 base::TimeTicks DelayBasedTimeSource::NextTickTarget(base::TimeTicks now) {
 237   base::TimeTicks new_tick_target = now.SnappedToNextTick(
 238       next_parameters_.tick_target, next_parameters_.interval);
 239   DCHECK(now <= new_tick_target)
 240       << "now = " << now.ToInternalValue()
 241       << "; new_tick_target = " << new_tick_target.ToInternalValue()
 242       << "; new_interval = " << next_parameters_.interval.InMicroseconds()
 243       << "; tick_target = " << next_parameters_.tick_target.ToInternalValue();
 244
 245   // Avoid double ticks when:
 246   // 1) Turning off the timer and turning it right back on.
 247   // 2) Jittery data is passed to SetTimebaseAndInterval().
 248   if (new_tick_target - last_tick_time_ <=
 249       next_parameters_.interval / kDoubleTickDivisor)
 250     new_tick_target += next_parameters_.interval;
 251
 252   return new_tick_target;
 253 }
 254
 255 void DelayBasedTimeSource::PostNextTickTask(base::TimeTicks now) {
 256   base::TimeTicks new_tick_target = NextTickTarget(now);
 257
 258   // Post another task *before* the tick and update state
 259   base::TimeDelta delay;
 260   if (now <= new_tick_target)
 261     delay = new_tick_target - now;
 262   task_runner_->PostDelayedTask(FROM_HERE,
 263                                 base::Bind(&DelayBasedTimeSource::OnTimerFired,
 264                                            weak_factory_.GetWeakPtr()),
 265                                 delay);
 266
 267   next_parameters_.tick_target = new_tick_target;
 268   current_parameters_ = next_parameters_;
 269 }
 270
 271 std::string DelayBasedTimeSource::TypeString() const {
 272   return "DelayBasedTimeSource";
 273 }
 274
 275 std::string DelayBasedTimeSourceHighRes::TypeString() const {
 276   return "DelayBasedTimeSourceHighRes";
 277 }
 278
 279 void DelayBasedTimeSource::AsValueInto(
 280     base::trace_event::TracedValue* state) const {
 281   state->SetString("type", TypeString());
 282   state->SetDouble("last_tick_time_us", LastTickTime().ToInternalValue());
 283   state->SetDouble("next_tick_time_us", NextTickTime().ToInternalValue());
 284
 285   state->BeginDictionary("current_parameters");
 286   state->SetDouble("interval_us",
 287                    current_parameters_.interval.InMicroseconds());
 288   state->SetDouble("tick_target_us",
 289                    current_parameters_.tick_target.ToInternalValue());
 290   state->EndDictionary();
 291
 292   state->BeginDictionary("next_parameters");
 293   state->SetDouble("interval_us", next_parameters_.interval.InMicroseconds());
 294   state->SetDouble("tick_target_us",
 295                    next_parameters_.tick_target.ToInternalValue());
 296   state->EndDictionary();
 297
 298   state->SetBoolean("active", active_);
 299 }
 300
 301 }  // namespace cc