cc/metrics/average_lag_tracker.h

// Copyright 2019 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.

#ifndef CC_METRICS_AVERAGE_LAG_TRACKER_H_
#define CC_METRICS_AVERAGE_LAG_TRACKER_H_

#include <deque>
#include <string>

#include "base/time/time.h"
#include "cc/cc_export.h"

namespace cc {

// A class for reporting AverageLag metrics. See
// https://docs.google.com/document/d/1e8NuzPblIv2B9bz01oSj40rmlse7_PHq5oFS3lqz6N4/
class CC_EXPORT AverageLagTracker {
 public:
  enum class EventType { ScrollBegin, ScrollUpdate };

  struct EventInfo {
    EventInfo(float event_scroll_delta,
              float predicted_scroll_delta,
              base::TimeTicks event_timestamp,
              EventType event_type)
        : event_scroll_delta(event_scroll_delta),
          predicted_scroll_delta(predicted_scroll_delta),
          event_timestamp(event_timestamp),
          event_type(event_type) {}
    // Delta reported by the scroll event (begin or update).
    float event_scroll_delta;
    // Delta predicted (when prediction is on, otherwise should be equals to
    // |event_scroll_delta|).
    float predicted_scroll_delta;
    // Timestamp when the scroll event happened.
    base::TimeTicks event_timestamp;
    // Timestamp when the scroll event's frame finished, which is currently
    // when the frame swap completed.
    base::TimeTicks finish_timestamp;
    // Scroll event type (begin or update).
    EventType event_type;
  };

  AverageLagTracker();
  ~AverageLagTracker();

  // Disallow copy and assign.
  AverageLagTracker(const AverageLagTracker&) = delete;
  AverageLagTracker& operator=(AverageLagTracker const&) = delete;

  // Adds a scroll event defined by |event_info|.
  void AddScrollEventInFrame(const EventInfo& event_info);

 protected:
  std::string GetAverageLagMetricName(EventType) const;

 private:
  struct LagAreaInFrame {
    explicit LagAreaInFrame(base::TimeTicks time,
                            float rendered_pos = 0,
                            float rendered_pos_no_prediction = 0)
        : frame_time(time),
          rendered_accumulated_delta(rendered_pos),
          lag_area(0),
          rendered_accumulated_delta_no_prediction(rendered_pos_no_prediction),
          lag_area_no_prediction(0) {}
    base::TimeTicks frame_time;
    // |rendered_accumulated_delta| is the cumulative delta that was swapped for
    // this frame; this is based on the predicted delta, if prediction is
    // enabled.
    float rendered_accumulated_delta;
    // |lag_area| is computed once a future input is processed that occurs after
    // the swap timestamp (so that we can compute how far the rendered delta
    // was from the actual position at the swap time).
    float lag_area;
    // |rendered_accumulated_delta_no_prediction| is the what would have been
    // rendered if prediction was not taken into account, i.e., the actual delta
    // from the input event.
    float rendered_accumulated_delta_no_prediction;
    // |lag_area_no_prediction| is computed the same as |lag_area| but using
    // rendered_accumulated_delta_no_prediction as the rendered delta.
    float lag_area_no_prediction;
  };

  // Processes |event_info| as a ScrollBegin event and add it to the Lag.
  void AddScrollBeginInFrame(const EventInfo& event_info);
  // Processes |event_info| as a ScrollUpdate event and add it to the Lag.
  void AddScrollUpdateInFrame(const EventInfo& event_info);

  // Calculate lag in 1 seconds intervals and report UMA.
  void CalculateAndReportAverageLagUma(bool send_anyway = false);

  // Helper function to calculate lag area between |front_time| to
  // |back_time|.
  float LagBetween(base::TimeTicks front_time,
                   base::TimeTicks back_time,
                   float scroll_delta,
                   base::TimeTicks event_time,
                   float rendered_accumulated_delta);

  float LagForUnfinishedFrame(float rendered_accumulated_delta);

  std::deque<LagAreaInFrame> frame_lag_infos_;

  // Last scroll event's timestamp in the sequence, reset on ScrollBegin.
  base::TimeTicks last_event_timestamp_;
  // Timestamp of the last frame popped from |frame_lag_infos_| queue.
  base::TimeTicks last_finished_frame_time_;

  // Accumulated scroll delta for actual scroll update events. Cumulated from
  // event_scroll_delta. Reset on ScrollBegin.
  float last_event_accumulated_delta_ = 0;
  // Accumulated scroll delta got rendered on gpu swap. Cumulated from
  // predicted_scroll_delta. It always has same value as
  // |last_event_accumulated_delta_| when scroll prediction is disabled.
  float last_rendered_accumulated_delta_ = 0;

  // This keeps track of the last report_time when we report to UMA, so we can
  // calculate the report's duration by current - last. Reset on ScrollBegin.
  base::TimeTicks last_reported_time_;

  // True if the first element of |frame_lag_infos_| is for ScrollBegin.
  // For ScrollBegin, we don't wait for the 1 second interval but record the
  // UMA once the frame is finished.
  bool is_begin_ = false;

  // Accumulated lag area in the 1 second intervals.
  float accumulated_lag_ = 0;
  // Accumulated lag not taking into account the predicted deltas.
  float accumulated_lag_no_prediction_ = 0;
};

}  // namespace cc

#endif  // CC_METRICS_AVERAGE_LAG_TRACKER_H_