gecko-dev/dom/base/CCGCScheduler.h

/* 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 "js/SliceBudget.h"
#include "mozilla/TimeStamp.h"
#include "mozilla/MainThreadIdlePeriod.h"
#include "nsCycleCollector.h"

static const TimeDuration kOneMinute = TimeDuration::FromSeconds(60.0f);

// The amount of time we wait between a request to CC (after GC ran)
// and doing the actual CC.
static const TimeDuration kCCDelay = TimeDuration::FromSeconds(6);

static const TimeDuration kCCSkippableDelay =
    TimeDuration::FromMilliseconds(250);

// In case the cycle collector isn't run at all, we don't want forget skippables
// to run too often. So limit the forget skippable cycle to start at earliest 2
// seconds after the end of the previous cycle.
static const TimeDuration kTimeBetweenForgetSkippableCycles =
    TimeDuration::FromSeconds(2);

// ForgetSkippable is usually fast, so we can use small budgets.
// This isn't a real budget but a hint to IdleTaskRunner whether there
// is enough time to call ForgetSkippable.
static const TimeDuration kForgetSkippableSliceDuration =
    TimeDuration::FromMilliseconds(2);

// Maximum amount of time that should elapse between incremental CC slices
static const TimeDuration kICCIntersliceDelay =
    TimeDuration::FromMilliseconds(64);

// Time budget for an incremental CC slice when using timer to run it.
static const TimeDuration kICCSliceBudget = TimeDuration::FromMilliseconds(3);
// Minimum budget for an incremental CC slice when using idle time to run it.
static const TimeDuration kIdleICCSliceBudget =
    TimeDuration::FromMilliseconds(2);

// Maximum total duration for an ICC
static const TimeDuration kMaxICCDuration = TimeDuration::FromSeconds(2);

// Force a CC after this long if there's more than NS_CC_FORCED_PURPLE_LIMIT
// objects in the purple buffer.
static const TimeDuration kCCForced = kOneMinute * 2;
static const uint32_t kCCForcedPurpleLimit = 10;

// Don't allow an incremental GC to lock out the CC for too long.
static const TimeDuration kMaxCCLockedoutTime = TimeDuration::FromSeconds(30);

// Trigger a CC if the purple buffer exceeds this size when we check it.
static const uint32_t kCCPurpleLimit = 200;

// if you add statics here, add them to the list in StartupJSEnvironment

namespace mozilla {

MOZ_ALWAYS_INLINE
static TimeDuration TimeBetween(TimeStamp aStart, TimeStamp aEnd) {
  MOZ_ASSERT(aEnd >= aStart);
  return aEnd - aStart;
}

static inline js::SliceBudget BudgetFromDuration(TimeDuration aDuration) {
  return js::SliceBudget(js::TimeBudget(aDuration.ToMilliseconds()));
}

class CCGCScheduler {
 public:
  // Parameter setting

  void SetActiveIntersliceGCBudget(TimeDuration aDuration) {
    mActiveIntersliceGCBudget = aDuration;
  }

  // State retrieval

  Maybe<TimeDuration> GetCCBlockedTime(TimeStamp now) const {
    MOZ_ASSERT_IF(mCCBlockStart.IsNull(), !mInIncrementalGC);
    if (mCCBlockStart.IsNull()) {
      return {};
    }
    return Some(now - mCCBlockStart);
  }

  bool InIncrementalGC() const { return mInIncrementalGC; }

  TimeStamp GetLastCCEndTime() const { return mLastCCEndTime; }

  // State modification

  void NoteGCBegin() {
    // Treat all GC as incremental here; non-incremental GC will just appear to
    // be one slice.
    mInIncrementalGC = true;
  }

  void NoteGCEnd() {
    mCCBlockStart = TimeStamp();
    mInIncrementalGC = false;
  }

  // When we decide to do a cycle collection but we're in the middle of an
  // incremental GC, the CC is "locked out" until the GC completes -- unless
  // the wait is too long, and we decide to finish the incremental GC early.
  enum IsStartingCCLockout { StartingLockout = true, AlreadyLockedOut = false };
  IsStartingCCLockout EnsureCCIsBlocked(TimeStamp aNow) {
    MOZ_ASSERT(mInIncrementalGC);

    if (mCCBlockStart) {
      return AlreadyLockedOut;
    }

    mCCBlockStart = aNow;
    return StartingLockout;
  }

  void UnblockCC() { mCCBlockStart = TimeStamp(); }

  void NoteForgetSkippableComplete(TimeStamp now) {
    mLastForgetSkippableEndTime = aNow;
  }

  void NoteCCEnd(TimeStamp when) { mLastCCEndTime = when; }

  void NoteForgetSkippableOnlyCycle() {
    mLastForgetSkippableCycleEndTime = TimeStamp::Now();
  }

  // Scheduling

  TimeDuration ComputeInterSliceGCBudget(TimeStamp aDeadline) const {
    // We use longer budgets when the CC has been locked out but the CC has
    // tried to run since that means we may have a significant amount of
    // garbage to collect and it's better to GC in several longer slices than
    // in a very long one.
    TimeDuration budget = aDeadline.IsNull() ? mActiveIntersliceGCBudget * 2
                                             : aDeadline - TimeStamp::Now();
    if (!mCCBlockStart) {
      return budget;
    }

    TimeDuration blockedTime = TimeStamp::Now() - mCCBlockStart;
    TimeDuration maxSliceGCBudget = mActiveIntersliceGCBudget * 10;
    double percentOfBlockedTime =
        std::min(blockedTime / kMaxCCLockedoutTime, 1.0);
    return std::max(budget, maxSliceGCBudget.MultDouble(percentOfBlockedTime));
  }

  // Return a budget along with a boolean saying whether to prefer to run short
  // slices and stop rather than continuing to the next phase of cycle
  // collection.
  js::SliceBudget ComputeCCSliceBudget(TimeStamp aDeadline,
                                       TimeStamp aCCBeginTime,
                                       TimeStamp aPrevSliceEndTime,
                                       bool* aPreferShorterSlices) const {
    TimeStamp now = TimeStamp::Now();

    *aPreferShorterSlices =
        aDeadline.IsNull() || (aDeadline - now) < kICCSliceBudget;

    TimeDuration baseBudget =
        aDeadline.IsNull() ? kICCSliceBudget : aDeadline - now;

    if (aCCBeginTime.IsNull()) {
      // If no CC is in progress, use the standard slice time.
      return BudgetFromDuration(baseBudget);
    }

    // Only run a limited slice if we're within the max running time.
    TimeDuration runningTime = TimeBetween(aCCBeginTime, now);
    if (runningTime >= kMaxICCDuration) {
      return js::SliceBudget::unlimited();
    }

    const TimeDuration maxSlice = TimeDuration::FromMilliseconds(
        MainThreadIdlePeriod::GetLongIdlePeriod());

    // Try to make up for a delay in running this slice.
    double sliceDelayMultiplier =
        TimeBetween(aPrevSliceEndTime, now) / kICCIntersliceDelay;
    TimeDuration delaySliceBudget =
        std::min(baseBudget.MultDouble(sliceDelayMultiplier), maxSlice);

    // Increase slice budgets up to |maxSlice| as we approach
    // half way through the ICC, to avoid large sync CCs.
    double percentToHalfDone =
        std::min(2.0 * (runningTime / kMaxICCDuration), 1.0);
    TimeDuration laterSliceBudget = maxSlice.MultDouble(percentToHalfDone);

    // Note: We may have already overshot the deadline, in which case
    // baseBudget will be negative and we will end up returning
    // laterSliceBudget.
    return BudgetFromDuration(
        std::max({delaySliceBudget, laterSliceBudget, baseBudget}));
  }

  bool ShouldScheduleCC(uint32_t aCleanupsSinceLastGC) const {
    TimeStamp now;

    // Don't run consecutive CCs too often.
    if (aCleanupsSinceLastGC && !mLastCCEndTime.IsNull()) {
      now = TimeStamp::Now();
      if (now - mLastCCEndTime < kCCDelay) {
        return false;
      }
    }

    // If GC hasn't run recently and forget skippable only cycle was run,
    // don't start a new cycle too soon.
    if ((aCleanupsSinceLastGC > kMajorForgetSkippableCalls) &&
        !mLastForgetSkippableCycleEndTime.IsNull()) {
      if (now.IsNull()) {
        now = TimeStamp::Now();
      }
      if (now - mLastForgetSkippableCycleEndTime <
          kTimeBetweenForgetSkippableCycles) {
        return false;
      }
    }

    return true;
  }

  // aStartTimeStamp : when the ForgetSkippable timer fired. This may be some
  // time ago, if an incremental GC needed to be finished.
  js::SliceBudget ComputeForgetSkippableBudget(TimeStamp aStartTimeStamp,
                                               TimeStamp aDeadline) {
    if (mForgetSkippableFrequencyStartTime.IsNull()) {
      mForgetSkippableFrequencyStartTime = aStartTimeStamp;
    } else if (aStartTimeStamp - mForgetSkippableFrequencyStartTime >
               kOneMinute) {
      TimeStamp startPlusMinute =
          mForgetSkippableFrequencyStartTime + kOneMinute;

      // If we had forget skippables only at the beginning of the interval, we
      // still want to use the whole time, minute or more, for frequency
      // calculation. mLastForgetSkippableEndTime is needed if forget skippable
      // takes enough time to push the interval to be over a minute.
      TimeStamp endPoint =
          std::max(startPlusMinute, mLastForgetSkippableEndTime);

      // Duration in minutes.
      double duration =
          (endPoint - mForgetSkippableFrequencyStartTime).ToSeconds() / 60;
      uint32_t frequencyPerMinute =
          uint32_t(mForgetSkippableCounter / duration);
      Telemetry::Accumulate(Telemetry::FORGET_SKIPPABLE_FREQUENCY,
                            frequencyPerMinute);
      mForgetSkippableCounter = 0;
      mForgetSkippableFrequencyStartTime = aStartTimeStamp;
    }
    ++mForgetSkippableCounter;

    TimeDuration budgetTime = aDeadline ? (aDeadline - aStartTimeStamp)
                                        : kForgetSkippableSliceDuration;
    return BudgetFromDuration(budgetTime);
  }

  // State

  // An incremental GC is in progress, which blocks the CC from running for its
  // duration (or until it goes too long and is finished synchronously.)
  bool mInIncrementalGC = false;

  // When the CC started actually waiting for the GC to finish. This will be
  // set to non-null at a later time than mCCLockedOut.
  TimeStamp mCCBlockStart;
  TimeStamp mLastForgetSkippableEndTime;
  uint32_t mForgetSkippableCounter = 0;
  TimeStamp mForgetSkippableFrequencyStartTime;
  TimeStamp mLastCCEndTime;
  TimeStamp mLastForgetSkippableCycleEndTime;

  // Configuration parameters

  TimeDuration mActiveIntersliceGCBudget = TimeDuration::FromMilliseconds(5);
};

}  // namespace mozilla