media/capture/smooth_event_sampler.cc

   1 // Copyright (c) 2015 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 "media/capture/smooth_event_sampler.h"
   6
   7 #include <algorithm>
   8
   9 #include "base/trace_event/trace_event.h"
  10
  11 namespace media {
  12
  13 namespace {
  14
  15 // The maximum amount of time that can elapse before considering unchanged
  16 // content as dirty for the purposes of timer-based overdue sampling.  This is
  17 // the same value found in cc::FrameRateCounter.
  18 const int kOverdueDirtyThresholdMillis = 250;  // 4 FPS
  19
  20 }  // anonymous namespace
  21
  22 SmoothEventSampler::SmoothEventSampler(base::TimeDelta min_capture_period,
  23                                        int redundant_capture_goal)
  24     :  redundant_capture_goal_(redundant_capture_goal),
  25        overdue_sample_count_(0),
  26        token_bucket_(base::TimeDelta::Max()) {
  27   SetMinCapturePeriod(min_capture_period);
  28 }
  29
  30 void SmoothEventSampler::SetMinCapturePeriod(base::TimeDelta period) {
  31   DCHECK_GT(period, base::TimeDelta());
  32   min_capture_period_ = period;
  33   token_bucket_capacity_ = period + period / 2;
  34   token_bucket_ = std::min(token_bucket_capacity_, token_bucket_);
  35 }
  36
  37 void SmoothEventSampler::ConsiderPresentationEvent(base::TimeTicks event_time) {
  38   DCHECK(!event_time.is_null());
  39
  40   // Add tokens to the bucket based on advancement in time.  Then, re-bound the
  41   // number of tokens in the bucket.  Overflow occurs when there is too much
  42   // time between events (a common case), or when RecordSample() is not being
  43   // called often enough (a bug).  On the other hand, if RecordSample() is being
  44   // called too often (e.g., as a reaction to IsOverdueForSamplingAt()), the
  45   // bucket will underflow.
  46   if (!current_event_.is_null()) {
  47     if (current_event_ < event_time) {
  48       token_bucket_ += event_time - current_event_;
  49       if (token_bucket_ > token_bucket_capacity_)
  50         token_bucket_ = token_bucket_capacity_;
  51     }
  52     TRACE_COUNTER1("gpu.capture",
  53                    "MirroringTokenBucketUsec",
  54                    std::max<int64>(0, token_bucket_.InMicroseconds()));
  55   }
  56   current_event_ = event_time;
  57 }
  58
  59 bool SmoothEventSampler::ShouldSample() const {
  60   return token_bucket_ >= min_capture_period_;
  61 }
  62
  63 void SmoothEventSampler::RecordSample() {
  64   token_bucket_ -= min_capture_period_;
  65   if (token_bucket_ < base::TimeDelta())
  66     token_bucket_ = base::TimeDelta();
  67   TRACE_COUNTER1("gpu.capture",
  68                  "MirroringTokenBucketUsec",
  69                  std::max<int64>(0, token_bucket_.InMicroseconds()));
  70
  71   if (HasUnrecordedEvent()) {
  72     last_sample_ = current_event_;
  73     overdue_sample_count_ = 0;
  74   } else {
  75     ++overdue_sample_count_;
  76   }
  77 }
  78
  79 bool SmoothEventSampler::IsOverdueForSamplingAt(base::TimeTicks event_time)
  80     const {
  81   DCHECK(!event_time.is_null());
  82
  83   if (!HasUnrecordedEvent() && overdue_sample_count_ >= redundant_capture_goal_)
  84     return false;  // Not dirty.
  85
  86   if (last_sample_.is_null())
  87     return true;
  88
  89   // If we're dirty but not yet old, then we've recently gotten updates, so we
  90   // won't request a sample just yet.
  91   base::TimeDelta dirty_interval = event_time - last_sample_;
  92   return dirty_interval >=
  93       base::TimeDelta::FromMilliseconds(kOverdueDirtyThresholdMillis);
  94 }
  95
  96 bool SmoothEventSampler::HasUnrecordedEvent() const {
  97   return !current_event_.is_null() && current_event_ != last_sample_;
  98 }
  99
 100 }  // namespace media