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 "mozilla/StaticPrefs_idle_period.h"
#include "mozilla/dom/Document.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 + idle_period.min + layout.idle_period.time_limit
// or during page load
//   now + idle_period.during_page_load.min + layout.idle_period.time_limit

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(StaticPrefs::idle_period_min());
  bool busySoon = currentGuess.IsNull() ||
                  (now >= (currentGuess - minIdlePeriod)) ||
                  currentGuess < mLastIdleDeadline;

  // During page load use higher minimum idle period.
  if (!busySoon && XRE_IsContentProcess() &&
      mozilla::dom::Document::HasRecentlyStartedForegroundLoads()) {
    TimeDuration minIdlePeriod = TimeDuration::FromMilliseconds(
        StaticPrefs::idle_period_during_page_load_min());
    busySoon = (now >= (currentGuess - minIdlePeriod));
  }

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

  return NS_OK;
}

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

}  // namespace mozilla