cc/trees/frame_rate_counter.cc

// Copyright 2012 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 "cc/trees/frame_rate_counter.h"

#include <stddef.h>

#include <algorithm>
#include <limits>

#include "base/memory/ptr_util.h"
#include "base/metrics/histogram_macros.h"

namespace cc {

// The following constants are measured in seconds.

// Two thresholds (measured in seconds) that describe what is considered to be a
// "no-op frame" that should not be counted.
// - if the frame is too fast, then given our compositor implementation, the
// frame probably was a no-op and did not draw.
// - if the frame is too slow, then there is probably not animating content, so
// we should not pollute the average.
static const double kFrameTooFast = 1.0 / 70.0;
static const double kFrameTooSlow = 1.5;

// If a frame takes longer than this threshold (measured in seconds) then we
// (naively) assume that it missed a screen refresh; that is, we dropped a
// frame.
// TODO(brianderson): Determine this threshold based on monitor refresh rate,
// crbug.com/138642.
static const double kDroppedFrameTime = 1.0 / 50.0;

// static
std::unique_ptr<FrameRateCounter> FrameRateCounter::Create(
    bool has_impl_thread) {
  return base::WrapUnique(new FrameRateCounter(has_impl_thread));
}

base::TimeDelta FrameRateCounter::RecentFrameInterval(size_t n) const {
  DCHECK_GT(n, 0u);
  DCHECK_LT(n, ring_buffer_.BufferSize());
  return ring_buffer_.ReadBuffer(n) - ring_buffer_.ReadBuffer(n - 1);
}

FrameRateCounter::FrameRateCounter(bool has_impl_thread)
    : has_impl_thread_(has_impl_thread), dropped_frame_count_(0) {}

void FrameRateCounter::SaveTimeStamp(base::TimeTicks timestamp, bool software) {
  ring_buffer_.SaveToBuffer(timestamp);

  // Check if frame interval can be computed.
  if (ring_buffer_.CurrentIndex() < 2)
    return;

  base::TimeDelta frame_interval_seconds =
      RecentFrameInterval(ring_buffer_.BufferSize() - 1);

  if (!IsBadFrameInterval(frame_interval_seconds) &&
      frame_interval_seconds.InSecondsF() > kDroppedFrameTime)
    dropped_frame_count_ +=
        frame_interval_seconds.InSecondsF() / kDroppedFrameTime;
}

bool FrameRateCounter::IsBadFrameInterval(
    base::TimeDelta interval_between_consecutive_frames) const {
  double delta = interval_between_consecutive_frames.InSecondsF();
  bool scheduler_allows_double_frames = !has_impl_thread_;
  bool interval_too_fast =
      scheduler_allows_double_frames ? delta < kFrameTooFast : delta <= 0.0;
  bool interval_too_slow = delta > kFrameTooSlow;
  return interval_too_fast || interval_too_slow;
}

void FrameRateCounter::GetMinAndMaxFPS(double* min_fps, double* max_fps) const {
  *min_fps = std::numeric_limits<double>::max();
  *max_fps = 0.0;

  for (RingBufferType::Iterator it = --ring_buffer_.End(); it; --it) {
    base::TimeDelta delta = RecentFrameInterval(it.index() + 1);

    if (IsBadFrameInterval(delta))
      continue;

    DCHECK_GT(delta.InSecondsF(), 0.f);
    double fps = 1.0 / delta.InSecondsF();

    *min_fps = std::min(fps, *min_fps);
    *max_fps = std::max(fps, *max_fps);
  }

  if (*min_fps > *max_fps)
    *min_fps = *max_fps;
}

double FrameRateCounter::GetAverageFPS() const {
  int frame_count = 0;
  double frame_times_total = 0.0;
  double average_fps = 0.0;

  // Walk backwards through the samples looking for a run of good frame
  // timings from which to compute the mean.
  //
  // Slow frames occur just because the user is inactive, and should be
  // ignored. Fast frames are ignored if the scheduler is in single-thread
  // mode in order to represent the true frame rate in spite of the fact that
  // the first few swapbuffers happen instantly which skews the statistics
  // too much for short lived animations.
  //
  // IsBadFrameInterval encapsulates the frame too slow/frame too fast logic.

  for (RingBufferType::Iterator it = --ring_buffer_.End();
       it && frame_times_total < 1.0; --it) {
    base::TimeDelta delta = RecentFrameInterval(it.index() + 1);

    if (!IsBadFrameInterval(delta)) {
      frame_count++;
      frame_times_total += delta.InSecondsF();
    } else if (frame_count) {
      break;
    }
  }

  if (frame_count) {
    DCHECK_GT(frame_times_total, 0.0);
    average_fps = frame_count / frame_times_total;
  }

  return average_fps;
}

}  // namespace cc