Report expected task queueing time via UMA

components/scheduler/renderer/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/renderer/queueing_time_estimator.h"
+
+#include "base/time/default_tick_clock.h"
+
+namespace scheduler {
+
+namespace {
+
+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::TickClock* time_source,
+    base::TimeDelta window_duration)
+    : client_(client),
+      time_source_(time_source),
+      outstanding_task_count_(0),
+      window_duration_(window_duration),
+      window_start_time_(time_source->NowTicks()) {}
+
+QueueingTimeEstimator::~QueueingTimeEstimator() {}
+
+void QueueingTimeEstimator::WillProcessTask(
+    const base::PendingTask& pending_task) {
+  // Avoid measuring the duration in nested run loops.
+  if (++outstanding_task_count_ != 1)
+    return;
+
+  task_start_time_ = time_source_->NowTicks();
+}
+
+void QueueingTimeEstimator::DidProcessTask(
+    const base::PendingTask& pending_task) {
+  if (--outstanding_task_count_ != 0)
+    return;
+
+  base::TimeTicks task_end_time = time_source_->NowTicks();
+
+  bool start_past_window =
+      task_start_time_ > window_start_time_ + window_duration_;
+  bool end_past_window =
+      task_end_time > window_start_time_ + window_duration_;
+
+  while (end_past_window) {
+    if (!start_past_window) {
+      // 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();
+
+    start_past_window =
+        task_start_time_ > window_start_time_ + window_duration_;
+    end_past_window =
+        task_end_time > window_start_time_ + window_duration_;
+  }
+
+  current_expected_queueing_time_ += ExpectedQueueingTimeFromTask(
+      task_start_time_, task_end_time, window_start_time_,
+      window_start_time_ + window_duration_);
+}
+
+}  // namespace scheduler