Report expected task queueing time via UMA

components/scheduler/base/queueing_time_estimator.cc

+// Copyright 2016 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 "components/scheduler/base/queueing_time_estimator.h"
+
+#include "base/time/default_tick_clock.h"
+
+namespace scheduler {
+
+namespace {
+
+// This method computes the expected queueing time of a randomly distributed
+// task R within a window containing a single task T. Let T' be the time range
+// for which T overlaps the window. We first compute the probability that R will
+// start within T'. We then compute the expected queueing duration if R does
+// start within this range. Since the start time of R is uniformly distributed
+// within the window, this is equal to the average of the queueing times if R
+// started at the beginning or end of T'. The expected queueing time of T is the
+// probability that R will start within T', multiplied by the expected queueing
+// duration if R does fall in this range.
+base::TimeDelta ExpectedQueueingTimeFromTask(base::TimeTicks task_start,
+                                             base::TimeTicks task_end,
+                                             base::TimeTicks window_start,
+                                             base::TimeTicks window_end) {
+  DCHECK(task_start <= task_end);
+  DCHECK(task_start <= window_end);
+  DCHECK(window_start < window_end);
+  DCHECK(task_end >= window_start);
+  base::TimeTicks task_in_window_start_time =
+      std::max(task_start, window_start);
+  base::TimeTicks task_in_window_end_time =
+      std::min(task_end, window_end);
+  DCHECK(task_in_window_end_time <= task_in_window_end_time);
+
+  double probability_of_this_task =
+      static_cast<double>((task_in_window_end_time - task_in_window_start_time)
+                              .InMicroseconds()) /
+      (window_end - window_start).InMicroseconds();
+
+  base::TimeDelta expected_queueing_duration_within_task =
+      ((task_end - task_in_window_start_time) +
+       (task_end - task_in_window_end_time)) /
+      2;
+
+  return base::TimeDelta::FromMillisecondsD(
+      probability_of_this_task *
+      expected_queueing_duration_within_task.InMillisecondsF());
+}
+
+}  // namespace
+
+QueueingTimeEstimator::QueueingTimeEstimator(
+    QueueingTimeEstimator::Client* client,
+    base::TimeDelta window_duration)
+    : client_(client),
+      window_duration_(window_duration),
+      window_start_time_() {}
+
+void QueueingTimeEstimator::OnToplevelTaskCompleted(
+    base::TimeTicks task_start_time,
+    base::TimeTicks task_end_time) {
+  if (window_start_time_.is_null())
+    window_start_time_ = task_start_time;
+
+  while (TimePastWindowEnd(task_end_time)) {
+    if (!TimePastWindowEnd(task_start_time)) {
+      // Include the current task in this window.
+      current_expected_queueing_time_ += ExpectedQueueingTimeFromTask(
+          task_start_time, task_end_time, window_start_time_,
+          window_start_time_ + window_duration_);
+    }
+    client_->OnQueueingTimeForWindowEstimated(current_expected_queueing_time_);
+    window_start_time_ += window_duration_;
+    current_expected_queueing_time_ = base::TimeDelta();
+  }
+
+  current_expected_queueing_time_ += ExpectedQueueingTimeFromTask(
+      task_start_time, task_end_time, window_start_time_,
+      window_start_time_ + window_duration_);
+}
+
+bool QueueingTimeEstimator::TimePastWindowEnd(base::TimeTicks time) {
+  return time > window_start_time_ + window_duration_;
+}
+
+}  // namespace scheduler