gecko-dev/xpcom/threads/MainThreadIdlePeriod.cpp

/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* vim: set ts=8 sts=2 et sw=2 tw=80: */
/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

#include "MainThreadIdlePeriod.h"

#include "mozilla/Maybe.h"
#include "mozilla/Preferences.h"
#include "nsRefreshDriver.h"
#include "nsThreadUtils.h"

// The amount of idle time (milliseconds) reserved for a long idle period.
static const double kLongIdlePeriodMS = 50.0;

// The minimum amount of time (milliseconds) required for an idle period to be
// scheduled on the main thread. N.B. layout.idle_period.time_limit adds
// padding at the end of the idle period, which makes the point in time that we
// expect to become busy again be:
//   now + kMinIdlePeriodMS + layout.idle_period.time_limit
static const double kMinIdlePeriodMS = 3.0;

static const uint32_t kMaxTimerThreadBound = 5;        // milliseconds
static const uint32_t kMaxTimerThreadBoundClamp = 15;  // milliseconds

namespace mozilla {

NS_IMETHODIMP
MainThreadIdlePeriod::GetIdlePeriodHint(TimeStamp* aIdleDeadline) {
  MOZ_ASSERT(NS_IsMainThread());
  MOZ_ASSERT(aIdleDeadline);

  TimeStamp now = TimeStamp::Now();
  TimeStamp currentGuess =
      now + TimeDuration::FromMilliseconds(kLongIdlePeriodMS);

  currentGuess = nsRefreshDriver::GetIdleDeadlineHint(currentGuess);
  currentGuess = NS_GetTimerDeadlineHintOnCurrentThread(currentGuess,
                                                        kMaxTimerThreadBound);

  // If the idle period is too small, then just return a null time
  // to indicate we are busy. Otherwise return the actual deadline.
  TimeDuration minIdlePeriod = TimeDuration::FromMilliseconds(kMinIdlePeriodMS);
  bool busySoon = currentGuess.IsNull() ||
                  (now >= (currentGuess - minIdlePeriod)) ||
                  currentGuess < mLastIdleDeadline;

  if (!busySoon) {
    *aIdleDeadline = mLastIdleDeadline = currentGuess;
  }

  return NS_OK;
}

/* static */ float MainThreadIdlePeriod::GetLongIdlePeriod() {
  return kLongIdlePeriodMS;
}

}  // namespace mozilla