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; }

  // 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(); }

  // 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}));
  }

  // 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;

  // Configuration parameters

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

}  // namespace mozilla