src/runtime/mgclimit.go - go - Git at Google

 // Copyright 2022 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.

 package runtime

 import "runtime/internal/atomic"

 // gcCPULimiter is a mechanism to limit GC CPU utilization in situations
 // where it might become excessive and inhibit application progress (e.g.
 // a death spiral).
 //
 // The core of the limiter is a leaky bucket mechanism that fills with GC
 // CPU time and drains with mutator time. Because the bucket fills and
 // drains with time directly (i.e. without any weighting), this effectively
 // sets a very conservative limit of 50%. This limit could be enforced directly,
 // however, but the purpose of the bucket is to accomodate spikes in GC CPU
 // utilization without hurting throughput.
 //
 // Note that the bucket in the leaky bucket mechanism can never go negative,
 // so the GC never gets credit for a lot of CPU time spent without the GC
 // running. This is intentional, as an application that stays idle for, say,
 // an entire day, could build up enough credit to fail to prevent a death
 // spiral the following day. The bucket's capacity is the GC's only leeway.
 //
 // The capacity thus also sets the window the limiter considers. For example,
 // if the capacity of the bucket is 1 cpu-second, then the limiter will not
 // kick in until at least 1 full cpu-second in the last 2 cpu-second window
 // is spent on GC CPU time.
 var gcCPULimiter gcCPULimiterState

 type gcCPULimiterState struct {
 	lock atomic.Uint32

 	enabled atomic.Bool
 	bucket  struct {
 		// Invariants:
 		// - fill >= 0
 		// - capacity >= 0
 		// - fill <= capacity
 		fill, capacity uint64
 	}
 	// TODO(mknyszek): Export this as a runtime/metric to provide an estimate of
 	// how much GC work is being dropped on the floor.
 	overflow uint64

 	// gcEnabled is an internal copy of gcBlackenEnabled that determines
 	// whether the limiter tracks total assist time.
 	//
 	// gcBlackenEnabled isn't used directly so as to keep this structure
 	// unit-testable.
 	gcEnabled bool

 	// transitioning is true when the GC is in a STW and transitioning between
 	// the mark and sweep phases.
 	transitioning bool

 	// lastTotalAssistTime is the last value of a monotonically increasing
 	// count of GC assist time, like gcController.assistTime.
 	lastTotalAssistTime int64

 	_ uint32 // Align lastUpdate on 32-bit platforms.

 	// lastUpdate is the nanotime timestamp of the last time update was called.
 	//
 	// Updated under lock, but may be read concurrently.
 	lastUpdate atomic.Int64

 	// nprocs is an internal copy of gomaxprocs, used to determine total available
 	// CPU time.
 	//
 	// gomaxprocs isn't used directly so as to keep this structure unit-testable.
 	nprocs int32
 }

 // limiting returns true if the CPU limiter is currently enabled, meaning the Go GC
 // should take action to limit CPU utilization.
 //
 // It is safe to call concurrently with other operations.
 func (l *gcCPULimiterState) limiting() bool {
 	return l.enabled.Load()
 }

 // startGCTransition notifies the limiter of a GC transition. totalAssistTime
 // is the same as described for update. now must be the start of the STW pause
 // for the GC transition.
 //
 // This call takes ownership of the limiter and disables all other means of
 // updating the limiter. Release ownership by calling finishGCTransition.
 //
 // It is safe to call concurrently with other operations.
 func (l *gcCPULimiterState) startGCTransition(enableGC bool, totalAssistTime, now int64) {
 	if !l.tryLock() {
 		// This must happen during a STW, so we can't fail to acquire the lock.
 		// If we did, something went wrong. Throw.
 		throw("failed to acquire lock to start a GC transition")
 	}
 	if l.gcEnabled == enableGC {
 		throw("transitioning GC to the same state as before?")
 	}
 	// Flush whatever was left between the last update and now.
 	l.updateLocked(totalAssistTime, now)
 	if enableGC && totalAssistTime != 0 {
 		throw("assist time must be zero on entry to a GC cycle")
 	}
 	l.gcEnabled = enableGC
 	l.transitioning = true
 	// N.B. finishGCTransition releases the lock.
 	//
 	// We don't release here to increase the chance that if there's a failure
 	// to finish the transition, that we throw on failing to acquire the lock.
 }

 // finishGCTransition notifies the limiter that the GC transition is complete
 // and releases ownership of it. It also accumulates STW time in the bucket.
 // now must be the timestamp from the end of the STW pause.
 func (l *gcCPULimiterState) finishGCTransition(now int64) {
 	if !l.transitioning {
 		throw("finishGCTransition called without starting one?")
 	}
 	// Count the full nprocs set of CPU time because the world is stopped
 	// between startGCTransition and finishGCTransition. Even though the GC
 	// isn't running on all CPUs, it is preventing user code from doing so,
 	// so it might as well be.
 	if lastUpdate := l.lastUpdate.Load(); now >= lastUpdate {
 		l.accumulate(0, (now-lastUpdate)*int64(l.nprocs))
 	}
 	l.lastUpdate.Store(now)
 	l.transitioning = false
 	// Reset lastTotalAssistTime for the next GC cycle.
 	l.lastTotalAssistTime = 0
 	l.unlock()
 }

 // gcCPULimiterUpdatePeriod dictates the maximum amount of wall-clock time
 // we can go before updating the limiter.
 const gcCPULimiterUpdatePeriod = 10e6 // 10ms

 // needUpdate returns true if the limiter's maximum update period has been
 // exceeded, and so would benefit from an update.
 func (l *gcCPULimiterState) needUpdate(now int64) bool {
 	return now-l.lastUpdate.Load() > gcCPULimiterUpdatePeriod
 }

 // update updates the bucket given runtime-specific information. totalAssistTime must
 // be a value that increases monotonically throughout the GC cycle, and is reset
 // at the start of a new mark phase. now is the current monotonic time in nanoseconds.
 //
 // This is safe to call concurrently with other operations, except *GCTransition.
 func (l *gcCPULimiterState) update(totalAssistTime int64, now int64) {
 	if !l.tryLock() {
 		// We failed to acquire the lock, which means something else is currently
 		// updating. Just drop our update, the next one to update will include
 		// our total assist time.
 		return
 	}
 	if l.transitioning {
 		throw("update during transition")
 	}
 	l.updateLocked(totalAssistTime, now)
 	l.unlock()
 }

 // updatedLocked is the implementation of update. l.lock must be held.
 func (l *gcCPULimiterState) updateLocked(totalAssistTime int64, now int64) {
 	lastUpdate := l.lastUpdate.Load()
 	if now < lastUpdate || totalAssistTime < l.lastTotalAssistTime {
 		// Defensively avoid overflow. This isn't even the latest update anyway.
 		// This might seem like a lot to back out on, but provided that both
 		// totalAssistTime and now are fresh, updaters must've been closely
 		// racing. It's close enough that it doesn't matter, and in the long
 		// term the result is the same.
 		return
 	}
 	windowTotalTime := (now - lastUpdate) * int64(l.nprocs)
 	l.lastUpdate.Store(now)
 	if !l.gcEnabled {
 		l.accumulate(windowTotalTime, 0)
 		return
 	}
 	windowGCTime := totalAssistTime - l.lastTotalAssistTime
 	windowGCTime += int64(float64(windowTotalTime) * gcBackgroundUtilization)
 	l.accumulate(windowTotalTime-windowGCTime, windowGCTime)
 	l.lastTotalAssistTime = totalAssistTime
 }

 // accumulate adds time to the bucket and signals whether the limiter is enabled.
 //
 // This is an internal function that deals just with the bucket. Prefer update.
 // l.lock must be held.
 func (l *gcCPULimiterState) accumulate(mutatorTime, gcTime int64) {
 	headroom := l.bucket.capacity - l.bucket.fill
 	enabled := headroom == 0

 	// Let's be careful about three things here:
 	// 1. The addition and subtraction, for the invariants.
 	// 2. Overflow.
 	// 3. Excessive mutation of l.enabled, which is accessed
 	//    by all assists, potentially more than once.
 	change := gcTime - mutatorTime

 	// Handle limiting case.
 	if change > 0 && headroom <= uint64(change) {
 		l.overflow += uint64(change) - headroom
 		l.bucket.fill = l.bucket.capacity
 		if !enabled {
 			l.enabled.Store(true)
 		}
 		return
 	}

 	// Handle non-limiting cases.
 	if change < 0 && l.bucket.fill <= uint64(-change) {
 		// Bucket emptied.
 		l.bucket.fill = 0
 	} else {
 		// All other cases.
 		l.bucket.fill -= uint64(-change)
 	}
 	if change != 0 && enabled {
 		l.enabled.Store(false)
 	}
 }

 // tryLock attempts to lock l. Returns true on success.
 func (l *gcCPULimiterState) tryLock() bool {
 	return l.lock.CompareAndSwap(0, 1)
 }

 // unlock releases the lock on l. Must be called if tryLock returns true.
 func (l *gcCPULimiterState) unlock() {
 	old := l.lock.Swap(0)
 	if old != 1 {
 		throw("double unlock")
 	}
 }

 // capacityPerProc is the limiter's bucket capacity for each P in GOMAXPROCS.
 const capacityPerProc = 1e9 // 1 second in nanoseconds

 // resetCapacity updates the capacity based on GOMAXPROCS. Must not be called
 // while the GC is enabled.
 //
 // It is safe to call concurrently with other operations.
 func (l *gcCPULimiterState) resetCapacity(now int64, nprocs int32) {
 	if !l.tryLock() {
 		// This must happen during a STW, so we can't fail to acquire the lock.
 		// If we did, something went wrong. Throw.
 		throw("failed to acquire lock to reset capacity")
 	}
 	// Flush the rest of the time for this period.
 	l.updateLocked(0, now)
 	l.nprocs = nprocs

 	l.bucket.capacity = uint64(nprocs) * capacityPerProc
 	if l.bucket.fill > l.bucket.capacity {
 		l.bucket.fill = l.bucket.capacity
 		l.enabled.Store(true)
 	} else if l.bucket.fill < l.bucket.capacity {
 		l.enabled.Store(false)
 	}
 	l.unlock()
 }
	// Copyright 2022 The Go Authors. All rights reserved.
	// Use of this source code is governed by a BSD-style
	// license that can be found in the LICENSE file.

	package runtime

	import "runtime/internal/atomic"

	// gcCPULimiter is a mechanism to limit GC CPU utilization in situations
	// where it might become excessive and inhibit application progress (e.g.
	// a death spiral).
	//
	// The core of the limiter is a leaky bucket mechanism that fills with GC
	// CPU time and drains with mutator time. Because the bucket fills and
	// drains with time directly (i.e. without any weighting), this effectively
	// sets a very conservative limit of 50%. This limit could be enforced directly,
	// however, but the purpose of the bucket is to accomodate spikes in GC CPU
	// utilization without hurting throughput.
	//
	// Note that the bucket in the leaky bucket mechanism can never go negative,
	// so the GC never gets credit for a lot of CPU time spent without the GC
	// running. This is intentional, as an application that stays idle for, say,
	// an entire day, could build up enough credit to fail to prevent a death
	// spiral the following day. The bucket's capacity is the GC's only leeway.
	//
	// The capacity thus also sets the window the limiter considers. For example,
	// if the capacity of the bucket is 1 cpu-second, then the limiter will not
	// kick in until at least 1 full cpu-second in the last 2 cpu-second window
	// is spent on GC CPU time.
	var gcCPULimiter gcCPULimiterState

	type gcCPULimiterState struct {
	lock atomic.Uint32

	enabled atomic.Bool
	bucket struct {
	// Invariants:
	// - fill >= 0
	// - capacity >= 0
	// - fill <= capacity
	fill, capacity uint64
	}
	// TODO(mknyszek): Export this as a runtime/metric to provide an estimate of
	// how much GC work is being dropped on the floor.
	overflow uint64

	// gcEnabled is an internal copy of gcBlackenEnabled that determines
	// whether the limiter tracks total assist time.
	//
	// gcBlackenEnabled isn't used directly so as to keep this structure
	// unit-testable.
	gcEnabled bool

	// transitioning is true when the GC is in a STW and transitioning between
	// the mark and sweep phases.
	transitioning bool

	// lastTotalAssistTime is the last value of a monotonically increasing
	// count of GC assist time, like gcController.assistTime.
	lastTotalAssistTime int64

	_ uint32 // Align lastUpdate on 32-bit platforms.

	// lastUpdate is the nanotime timestamp of the last time update was called.
	//
	// Updated under lock, but may be read concurrently.
	lastUpdate atomic.Int64

	// nprocs is an internal copy of gomaxprocs, used to determine total available
	// CPU time.
	//
	// gomaxprocs isn't used directly so as to keep this structure unit-testable.
	nprocs int32
	}

	// limiting returns true if the CPU limiter is currently enabled, meaning the Go GC
	// should take action to limit CPU utilization.
	//
	// It is safe to call concurrently with other operations.
	func (l *gcCPULimiterState) limiting() bool {
	return l.enabled.Load()
	}

	// startGCTransition notifies the limiter of a GC transition. totalAssistTime
	// is the same as described for update. now must be the start of the STW pause
	// for the GC transition.
	//
	// This call takes ownership of the limiter and disables all other means of
	// updating the limiter. Release ownership by calling finishGCTransition.
	//
	// It is safe to call concurrently with other operations.
	func (l *gcCPULimiterState) startGCTransition(enableGC bool, totalAssistTime, now int64) {
	if !l.tryLock() {
	// This must happen during a STW, so we can't fail to acquire the lock.
	// If we did, something went wrong. Throw.
	throw("failed to acquire lock to start a GC transition")
	}
	if l.gcEnabled == enableGC {
	throw("transitioning GC to the same state as before?")
	}
	// Flush whatever was left between the last update and now.
	l.updateLocked(totalAssistTime, now)
	if enableGC && totalAssistTime != 0 {
	throw("assist time must be zero on entry to a GC cycle")
	}
	l.gcEnabled = enableGC
	l.transitioning = true
	// N.B. finishGCTransition releases the lock.
	//
	// We don't release here to increase the chance that if there's a failure
	// to finish the transition, that we throw on failing to acquire the lock.
	}

	// finishGCTransition notifies the limiter that the GC transition is complete
	// and releases ownership of it. It also accumulates STW time in the bucket.
	// now must be the timestamp from the end of the STW pause.
	func (l *gcCPULimiterState) finishGCTransition(now int64) {
	if !l.transitioning {
	throw("finishGCTransition called without starting one?")
	}
	// Count the full nprocs set of CPU time because the world is stopped
	// between startGCTransition and finishGCTransition. Even though the GC
	// isn't running on all CPUs, it is preventing user code from doing so,
	// so it might as well be.
	if lastUpdate := l.lastUpdate.Load(); now >= lastUpdate {
	l.accumulate(0, (now-lastUpdate)*int64(l.nprocs))
	}
	l.lastUpdate.Store(now)
	l.transitioning = false
	// Reset lastTotalAssistTime for the next GC cycle.
	l.lastTotalAssistTime = 0
	l.unlock()
	}

	// gcCPULimiterUpdatePeriod dictates the maximum amount of wall-clock time
	// we can go before updating the limiter.
	const gcCPULimiterUpdatePeriod = 10e6 // 10ms

	// needUpdate returns true if the limiter's maximum update period has been
	// exceeded, and so would benefit from an update.
	func (l *gcCPULimiterState) needUpdate(now int64) bool {
	return now-l.lastUpdate.Load() > gcCPULimiterUpdatePeriod
	}

	// update updates the bucket given runtime-specific information. totalAssistTime must
	// be a value that increases monotonically throughout the GC cycle, and is reset
	// at the start of a new mark phase. now is the current monotonic time in nanoseconds.
	//
	// This is safe to call concurrently with other operations, except *GCTransition.
	func (l *gcCPULimiterState) update(totalAssistTime int64, now int64) {
	if !l.tryLock() {
	// We failed to acquire the lock, which means something else is currently
	// updating. Just drop our update, the next one to update will include
	// our total assist time.
	return
	}
	if l.transitioning {
	throw("update during transition")
	}
	l.updateLocked(totalAssistTime, now)
	l.unlock()
	}

	// updatedLocked is the implementation of update. l.lock must be held.
	func (l *gcCPULimiterState) updateLocked(totalAssistTime int64, now int64) {
	lastUpdate := l.lastUpdate.Load()
	if now < lastUpdate \|\| totalAssistTime < l.lastTotalAssistTime {
	// Defensively avoid overflow. This isn't even the latest update anyway.
	// This might seem like a lot to back out on, but provided that both
	// totalAssistTime and now are fresh, updaters must've been closely
	// racing. It's close enough that it doesn't matter, and in the long
	// term the result is the same.
	return
	}
	windowTotalTime := (now - lastUpdate) * int64(l.nprocs)
	l.lastUpdate.Store(now)
	if !l.gcEnabled {
	l.accumulate(windowTotalTime, 0)
	return
	}
	windowGCTime := totalAssistTime - l.lastTotalAssistTime
	windowGCTime += int64(float64(windowTotalTime) * gcBackgroundUtilization)
	l.accumulate(windowTotalTime-windowGCTime, windowGCTime)
	l.lastTotalAssistTime = totalAssistTime
	}

	// accumulate adds time to the bucket and signals whether the limiter is enabled.
	//
	// This is an internal function that deals just with the bucket. Prefer update.
	// l.lock must be held.
	func (l *gcCPULimiterState) accumulate(mutatorTime, gcTime int64) {
	headroom := l.bucket.capacity - l.bucket.fill
	enabled := headroom == 0

	// Let's be careful about three things here:
	// 1. The addition and subtraction, for the invariants.
	// 2. Overflow.
	// 3. Excessive mutation of l.enabled, which is accessed
	// by all assists, potentially more than once.
	change := gcTime - mutatorTime

	// Handle limiting case.
	if change > 0 && headroom <= uint64(change) {
	l.overflow += uint64(change) - headroom
	l.bucket.fill = l.bucket.capacity
	if !enabled {
	l.enabled.Store(true)
	}
	return
	}

	// Handle non-limiting cases.
	if change < 0 && l.bucket.fill <= uint64(-change) {
	// Bucket emptied.
	l.bucket.fill = 0
	} else {
	// All other cases.
	l.bucket.fill -= uint64(-change)
	}
	if change != 0 && enabled {
	l.enabled.Store(false)
	}
	}

	// tryLock attempts to lock l. Returns true on success.
	func (l *gcCPULimiterState) tryLock() bool {
	return l.lock.CompareAndSwap(0, 1)
	}

	// unlock releases the lock on l. Must be called if tryLock returns true.
	func (l *gcCPULimiterState) unlock() {
	old := l.lock.Swap(0)
	if old != 1 {
	throw("double unlock")
	}
	}

	// capacityPerProc is the limiter's bucket capacity for each P in GOMAXPROCS.
	const capacityPerProc = 1e9 // 1 second in nanoseconds

	// resetCapacity updates the capacity based on GOMAXPROCS. Must not be called
	// while the GC is enabled.
	//
	// It is safe to call concurrently with other operations.
	func (l *gcCPULimiterState) resetCapacity(now int64, nprocs int32) {
	if !l.tryLock() {
	// This must happen during a STW, so we can't fail to acquire the lock.
	// If we did, something went wrong. Throw.
	throw("failed to acquire lock to reset capacity")
	}
	// Flush the rest of the time for this period.
	l.updateLocked(0, now)
	l.nprocs = nprocs

	l.bucket.capacity = uint64(nprocs) * capacityPerProc
	if l.bucket.fill > l.bucket.capacity {
	l.bucket.fill = l.bucket.capacity
	l.enabled.Store(true)
	} else if l.bucket.fill < l.bucket.capacity {
	l.enabled.Store(false)
	}
	l.unlock()
	}