src/runtime/mgcpacer.go

// Copyright 2021 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 (
	"internal/cpu"
	"internal/goexperiment"
	"runtime/internal/atomic"
	"unsafe"
)

const (
	// gcGoalUtilization is the goal CPU utilization for
	// marking as a fraction of GOMAXPROCS.
	//
	// Increasing the goal utilization will shorten GC cycles as the GC
	// has more resources behind it, lessening costs from the write barrier,
	// but comes at the cost of increasing mutator latency.
	gcGoalUtilization = gcBackgroundUtilization

	// gcBackgroundUtilization is the fixed CPU utilization for background
	// marking. It must be <= gcGoalUtilization. The difference between
	// gcGoalUtilization and gcBackgroundUtilization will be made up by
	// mark assists. The scheduler will aim to use within 50% of this
	// goal.
	//
	// As a general rule, there's little reason to set gcBackgroundUtilization
	// < gcGoalUtilization. One reason might be in mostly idle applications,
	// where goroutines are unlikely to assist at all, so the actual
	// utilization will be lower than the goal. But this is moot point
	// because the idle mark workers already soak up idle CPU resources.
	// These two values are still kept separate however because they are
	// distinct conceptually, and in previous iterations of the pacer the
	// distinction was more important.
	gcBackgroundUtilization = 0.25

	// gcCreditSlack is the amount of scan work credit that can
	// accumulate locally before updating gcController.heapScanWork and,
	// optionally, gcController.bgScanCredit. Lower values give a more
	// accurate assist ratio and make it more likely that assists will
	// successfully steal background credit. Higher values reduce memory
	// contention.
	gcCreditSlack = 2000

	// gcAssistTimeSlack is the nanoseconds of mutator assist time that
	// can accumulate on a P before updating gcController.assistTime.
	gcAssistTimeSlack = 5000

	// gcOverAssistWork determines how many extra units of scan work a GC
	// assist does when an assist happens. This amortizes the cost of an
	// assist by pre-paying for this many bytes of future allocations.
	gcOverAssistWork = 64 << 10

	// defaultHeapMinimum is the value of heapMinimum for GOGC==100.
	defaultHeapMinimum = (goexperiment.HeapMinimum512KiBInt)*(512<<10) +
		(1-goexperiment.HeapMinimum512KiBInt)*(4<<20)

	// scannableStackSizeSlack is the bytes of stack space allocated or freed
	// that can accumulate on a P before updating gcController.stackSize.
	scannableStackSizeSlack = 8 << 10
)

func init() {
	if offset := unsafe.Offsetof(gcController.heapLive); offset%8 != 0 {
		println(offset)
		throw("gcController.heapLive not aligned to 8 bytes")
	}
}

// gcController implements the GC pacing controller that determines
// when to trigger concurrent garbage collection and how much marking
// work to do in mutator assists and background marking.
//
// It calculates the ratio between the allocation rate (in terms of CPU
// time) and the GC scan throughput to determine the heap size at which to
// trigger a GC cycle such that no GC assists are required to finish on time.
// This algorithm thus optimizes GC CPU utilization to the dedicated background
// mark utilization of 25% of GOMAXPROCS by minimizing GC assists.
// GOMAXPROCS. The high-level design of this algorithm is documented
// at https://github.com/golang/proposal/blob/master/design/44167-gc-pacer-redesign.md.
// See https://golang.org/s/go15gcpacing for additional historical context.
var gcController gcControllerState

type gcControllerState struct {

	// Initialized from GOGC. GOGC=off means no GC.
	gcPercent atomic.Int32

	_ uint32 // padding so following 64-bit values are 8-byte aligned

	// heapMinimum is the minimum heap size at which to trigger GC.
	// For small heaps, this overrides the usual GOGC*live set rule.
	//
	// When there is a very small live set but a lot of allocation, simply
	// collecting when the heap reaches GOGC*live results in many GC
	// cycles and high total per-GC overhead. This minimum amortizes this
	// per-GC overhead while keeping the heap reasonably small.
	//
	// During initialization this is set to 4MB*GOGC/100. In the case of
	// GOGC==0, this will set heapMinimum to 0, resulting in constant
	// collection even when the heap size is small, which is useful for
	// debugging.
	heapMinimum uint64

	// trigger is the heap size that triggers marking.
	//
	// When heapLive ≥ trigger, the mark phase will start.
	// This is also the heap size by which proportional sweeping
	// must be complete.
	//
	// This is computed from consMark during mark termination for
	// the next cycle's trigger.
	//
	// Protected by mheap_.lock or a STW.
	trigger uint64

	// consMark is the estimated per-CPU consMark ratio for the application.
	//
	// It represents the ratio between the application's allocation
	// rate, as bytes allocated per CPU-time, and the GC's scan rate,
	// as bytes scanned per CPU-time.
	// The units of this ratio are (B / cpu-ns) / (B / cpu-ns).
	//
	// At a high level, this value is computed as the bytes of memory
	// allocated (cons) per unit of scan work completed (mark) in a GC
	// cycle, divided by the CPU time spent on each activity.
	//
	// Updated at the end of each GC cycle, in endCycle.
	consMark float64

	// consMarkController holds the state for the mark-cons ratio
	// estimation over time.
	//
	// Its purpose is to smooth out noisiness in the computation of
	// consMark; see consMark for details.
	consMarkController piController

	_ uint32 // Padding for atomics on 32-bit platforms.

	// heapGoal is the goal heapLive for when next GC ends.
	// Set to ^uint64(0) if disabled.
	//
	// Read and written atomically, unless the world is stopped.
	heapGoal uint64

	// lastHeapGoal is the value of heapGoal for the previous GC.
	// Note that this is distinct from the last value heapGoal had,
	// because it could change if e.g. gcPercent changes.
	//
	// Read and written with the world stopped or with mheap_.lock held.
	lastHeapGoal uint64

	// heapLive is the number of bytes considered live by the GC.
	// That is: retained by the most recent GC plus allocated
	// since then. heapLive ≤ memstats.heapAlloc, since heapAlloc includes
	// unmarked objects that have not yet been swept (and hence goes up as we
	// allocate and down as we sweep) while heapLive excludes these
	// objects (and hence only goes up between GCs).
	//
	// This is updated atomically without locking. To reduce
	// contention, this is updated only when obtaining a span from
	// an mcentral and at this point it counts all of the
	// unallocated slots in that span (which will be allocated
	// before that mcache obtains another span from that
	// mcentral). Hence, it slightly overestimates the "true" live
	// heap size. It's better to overestimate than to
	// underestimate because 1) this triggers the GC earlier than
	// necessary rather than potentially too late and 2) this
	// leads to a conservative GC rate rather than a GC rate that
	// is potentially too low.
	//
	// Reads should likewise be atomic (or during STW).
	//
	// Whenever this is updated, call traceHeapAlloc() and
	// this gcControllerState's revise() method.
	heapLive uint64

	// heapScan is the number of bytes of "scannable" heap. This
	// is the live heap (as counted by heapLive), but omitting
	// no-scan objects and no-scan tails of objects.
	//
	// This value is fixed at the start of a GC cycle, so during a
	// GC cycle it is safe to read without atomics, and it represents
	// the maximum scannable heap.
	heapScan uint64

	// lastHeapScan is the number of bytes of heap that were scanned
	// last GC cycle. It is the same as heapMarked, but only
	// includes the "scannable" parts of objects.
	//
	// Updated when the world is stopped.
	lastHeapScan uint64

	// stackScan is a snapshot of scannableStackSize taken at each GC
	// STW pause and is used in pacing decisions.
	//
	// Updated only while the world is stopped.
	stackScan uint64

	// scannableStackSize is the amount of allocated goroutine stack space in
	// use by goroutines.
	//
	// This number tracks allocated goroutine stack space rather than used
	// goroutine stack space (i.e. what is actually scanned) because used
	// goroutine stack space is much harder to measure cheaply. By using
	// allocated space, we make an overestimate; this is OK, it's better
	// to conservatively overcount than undercount.
	//
	// Read and updated atomically.
	scannableStackSize uint64

	// globalsScan is the total amount of global variable space
	// that is scannable.
	//
	// Read and updated atomically.
	globalsScan uint64

	// heapMarked is the number of bytes marked by the previous
	// GC. After mark termination, heapLive == heapMarked, but
	// unlike heapLive, heapMarked does not change until the
	// next mark termination.
	heapMarked uint64

	// heapScanWork is the total heap scan work performed this cycle.
	// stackScanWork is the total stack scan work performed this cycle.
	// globalsScanWork is the total globals scan work performed this cycle.
	//
	// These are updated atomically during the cycle. Updates occur in
	// bounded batches, since they are both written and read
	// throughout the cycle. At the end of the cycle, heapScanWork is how
	// much of the retained heap is scannable.
	//
	// Currently these are measured in bytes. For most uses, this is an
	// opaque unit of work, but for estimation the definition is important.
	//
	// Note that stackScanWork includes all allocated space, not just the
	// size of the stack itself, mirroring stackSize.
	heapScanWork    atomic.Int64
	stackScanWork   atomic.Int64
	globalsScanWork atomic.Int64

	// bgScanCredit is the scan work credit accumulated by the
	// concurrent background scan. This credit is accumulated by
	// the background scan and stolen by mutator assists. This is
	// updated atomically. Updates occur in bounded batches, since
	// it is both written and read throughout the cycle.
	bgScanCredit int64

	// assistTime is the nanoseconds spent in mutator assists
	// during this cycle. This is updated atomically. Updates
	// occur in bounded batches, since it is both written and read
	// throughout the cycle.
	assistTime int64

	// dedicatedMarkTime is the nanoseconds spent in dedicated
	// mark workers during this cycle. This is updated atomically
	// at the end of the concurrent mark phase.
	dedicatedMarkTime int64

	// fractionalMarkTime is the nanoseconds spent in the
	// fractional mark worker during this cycle. This is updated
	// atomically throughout the cycle and will be up-to-date if
	// the fractional mark worker is not currently running.
	fractionalMarkTime int64

	// idleMarkTime is the nanoseconds spent in idle marking
	// during this cycle. This is updated atomically throughout
	// the cycle.
	idleMarkTime int64

	// markStartTime is the absolute start time in nanoseconds
	// that assists and background mark workers started.
	markStartTime int64

	// dedicatedMarkWorkersNeeded is the number of dedicated mark
	// workers that need to be started. This is computed at the
	// beginning of each cycle and decremented atomically as
	// dedicated mark workers get started.
	dedicatedMarkWorkersNeeded int64

	// idleMarkWorkers is two packed int32 values in a single uint64.
	// These two values are always updated simultaneously.
	//
	// The bottom int32 is the current number of idle mark workers executing.
	//
	// The top int32 is the maximum number of idle mark workers allowed to
	// execute concurrently. Normally, this number is just gomaxprocs. However,
	// during periodic GC cycles it is set to 0 because the system is idle
	// anyway; there's no need to go full blast on all of GOMAXPROCS.
	//
	// The maximum number of idle mark workers is used to prevent new workers
	// from starting, but it is not a hard maximum. It is possible (but
	// exceedingly rare) for the current number of idle mark workers to
	// transiently exceed the maximum. This could happen if the maximum changes
	// just after a GC ends, and an M with no P.
	//
	// Note that if we have no dedicated mark workers, we set this value to
	// 1 in this case we only have fractional GC workers which aren't scheduled
	// strictly enough to ensure GC progress. As a result, idle-priority mark
	// workers are vital to GC progress in these situations.
	//
	// For example, consider a situation in which goroutines block on the GC
	// (such as via runtime.GOMAXPROCS) and only fractional mark workers are
	// scheduled (e.g. GOMAXPROCS=1). Without idle-priority mark workers, the
	// last running M might skip scheduling a fractional mark worker if its
	// utilization goal is met, such that once it goes to sleep (because there's
	// nothing to do), there will be nothing else to spin up a new M for the
	// fractional worker in the future, stalling GC progress and causing a
	// deadlock. However, idle-priority workers will *always* run when there is
	// nothing left to do, ensuring the GC makes progress.
	//
	// See github.com/golang/go/issues/44163 for more details.
	idleMarkWorkers atomic.Uint64

	// assistWorkPerByte is the ratio of scan work to allocated
	// bytes that should be performed by mutator assists. This is
	// computed at the beginning of each cycle and updated every
	// time heapScan is updated.
	assistWorkPerByte atomic.Float64

	// assistBytesPerWork is 1/assistWorkPerByte.
	//
	// Note that because this is read and written independently
	// from assistWorkPerByte users may notice a skew between
	// the two values, and such a state should be safe.
	assistBytesPerWork atomic.Float64

	// fractionalUtilizationGoal is the fraction of wall clock
	// time that should be spent in the fractional mark worker on
	// each P that isn't running a dedicated worker.
	//
	// For example, if the utilization goal is 25% and there are
	// no dedicated workers, this will be 0.25. If the goal is
	// 25%, there is one dedicated worker, and GOMAXPROCS is 5,
	// this will be 0.05 to make up the missing 5%.
	//
	// If this is zero, no fractional workers are needed.
	fractionalUtilizationGoal float64

	// test indicates that this is a test-only copy of gcControllerState.
	test bool

	_ cpu.CacheLinePad
}

func (c *gcControllerState) init(gcPercent int32) {
	c.heapMinimum = defaultHeapMinimum

	c.consMarkController = piController{
		// Tuned first via the Ziegler-Nichols process in simulation,
		// then the integral time was manually tuned against real-world
		// applications to deal with noisiness in the measured cons/mark
		// ratio.
		kp: 0.9,
		ti: 4.0,

		// Set a high reset time in GC cycles.
		// This is inversely proportional to the rate at which we
		// accumulate error from clipping. By making this very high
		// we make the accumulation slow. In general, clipping is
		// OK in our situation, hence the choice.
		//
		// Tune this if we get unintended effects from clipping for
		// a long time.
		tt:  1000,
		min: -1000,
		max: 1000,
	}

	// This will also compute and set the GC trigger and goal.
	c.setGCPercent(gcPercent)
}

// startCycle resets the GC controller's state and computes estimates
// for a new GC cycle. The caller must hold worldsema and the world
// must be stopped.
func (c *gcControllerState) startCycle(markStartTime int64, procs int, trigger gcTrigger) {
	c.heapScanWork.Store(0)
	c.stackScanWork.Store(0)
	c.globalsScanWork.Store(0)
	c.bgScanCredit = 0
	c.assistTime = 0
	c.dedicatedMarkTime = 0
	c.fractionalMarkTime = 0
	c.idleMarkTime = 0
	c.markStartTime = markStartTime
	c.stackScan = atomic.Load64(&c.scannableStackSize)

	// Ensure that the heap goal is at least a little larger than
	// the current live heap size. This may not be the case if GC
	// start is delayed or if the allocation that pushed gcController.heapLive
	// over trigger is large or if the trigger is really close to
	// GOGC. Assist is proportional to this distance, so enforce a
	// minimum distance, even if it means going over the GOGC goal
	// by a tiny bit.
	if c.heapGoal < c.heapLive+64<<10 {
		c.heapGoal = c.heapLive + 64<<10
	}

	// Compute the background mark utilization goal. In general,
	// this may not come out exactly. We round the number of
	// dedicated workers so that the utilization is closest to
	// 25%. For small GOMAXPROCS, this would introduce too much
	// error, so we add fractional workers in that case.
	totalUtilizationGoal := float64(procs) * gcBackgroundUtilization
	c.dedicatedMarkWorkersNeeded = int64(totalUtilizationGoal + 0.5)
	utilError := float64(c.dedicatedMarkWorkersNeeded)/totalUtilizationGoal - 1
	const maxUtilError = 0.3
	if utilError < -maxUtilError || utilError > maxUtilError {
		// Rounding put us more than 30% off our goal. With
		// gcBackgroundUtilization of 25%, this happens for
		// GOMAXPROCS<=3 or GOMAXPROCS=6. Enable fractional
		// workers to compensate.
		if float64(c.dedicatedMarkWorkersNeeded) > totalUtilizationGoal {
			// Too many dedicated workers.
			c.dedicatedMarkWorkersNeeded--
		}
		c.fractionalUtilizationGoal = (totalUtilizationGoal - float64(c.dedicatedMarkWorkersNeeded)) / float64(procs)
	} else {
		c.fractionalUtilizationGoal = 0
	}

	// In STW mode, we just want dedicated workers.
	if debug.gcstoptheworld > 0 {
		c.dedicatedMarkWorkersNeeded = int64(procs)
		c.fractionalUtilizationGoal = 0
	}

	// Clear per-P state
	for _, p := range allp {
		p.gcAssistTime = 0
		p.gcFractionalMarkTime = 0
	}

	if trigger.kind == gcTriggerTime {
		// During a periodic GC cycle, reduce the number of idle mark workers
		// required. However, we need at least one dedicated mark worker or
		// idle GC worker to ensure GC progress in some scenarios (see comment
		// on maxIdleMarkWorkers).
		if c.dedicatedMarkWorkersNeeded > 0 {
			c.setMaxIdleMarkWorkers(0)
		} else {
			// TODO(mknyszek): The fundamental reason why we need this is because
			// we can't count on the fractional mark worker to get scheduled.
			// Fix that by ensuring it gets scheduled according to its quota even
			// if the rest of the application is idle.
			c.setMaxIdleMarkWorkers(1)
		}
	} else {
		// N.B. gomaxprocs and dedicatedMarkWorkersNeeded is guaranteed not to
		// change during a GC cycle.
		c.setMaxIdleMarkWorkers(int32(procs) - int32(c.dedicatedMarkWorkersNeeded))
	}

	// Compute initial values for controls that are updated
	// throughout the cycle.
	c.revise()

	if debug.gcpacertrace > 0 {
		assistRatio := c.assistWorkPerByte.Load()
		print("pacer: assist ratio=", assistRatio,
			" (scan ", gcController.heapScan>>20, " MB in ",
			work.initialHeapLive>>20, "->",
			c.heapGoal>>20, " MB)",
			" workers=", c.dedicatedMarkWorkersNeeded,
			"+", c.fractionalUtilizationGoal, "\n")
	}
}

// revise updates the assist ratio during the GC cycle to account for
// improved estimates. This should be called whenever gcController.heapScan,
// gcController.heapLive, or gcController.heapGoal is updated. It is safe to
// call concurrently, but it may race with other calls to revise.
//
// The result of this race is that the two assist ratio values may not line
// up or may be stale. In practice this is OK because the assist ratio
// moves slowly throughout a GC cycle, and the assist ratio is a best-effort
// heuristic anyway. Furthermore, no part of the heuristic depends on
// the two assist ratio values being exact reciprocals of one another, since
// the two values are used to convert values from different sources.
//
// The worst case result of this raciness is that we may miss a larger shift
// in the ratio (say, if we decide to pace more aggressively against the
// hard heap goal) but even this "hard goal" is best-effort (see #40460).
// The dedicated GC should ensure we don't exceed the hard goal by too much
// in the rare case we do exceed it.
//
// It should only be called when gcBlackenEnabled != 0 (because this
// is when assists are enabled and the necessary statistics are
// available).
func (c *gcControllerState) revise() {
	gcPercent := c.gcPercent.Load()
	if gcPercent < 0 {
		// If GC is disabled but we're running a forced GC,
		// act like GOGC is huge for the below calculations.
		gcPercent = 100000
	}
	live := atomic.Load64(&c.heapLive)
	scan := atomic.Load64(&c.heapScan)
	work := c.heapScanWork.Load() + c.stackScanWork.Load() + c.globalsScanWork.Load()

	// Assume we're under the soft goal. Pace GC to complete at
	// heapGoal assuming the heap is in steady-state.
	heapGoal := int64(atomic.Load64(&c.heapGoal))

	// The expected scan work is computed as the amount of bytes scanned last
	// GC cycle, plus our estimate of stacks and globals work for this cycle.
	scanWorkExpected := int64(c.lastHeapScan + c.stackScan + c.globalsScan)

	// maxScanWork is a worst-case estimate of the amount of scan work that
	// needs to be performed in this GC cycle. Specifically, it represents
	// the case where *all* scannable memory turns out to be live.
	maxScanWork := int64(scan + c.stackScan + c.globalsScan)
	if work > scanWorkExpected {
		// We've already done more scan work than expected. Because our expectation
		// is based on a steady-state scannable heap size, we assume this means our
		// heap is growing. Compute a new heap goal that takes our existing runway
		// computed for scanWorkExpected and extrapolates it to maxScanWork, the worst-case
		// scan work. This keeps our assist ratio stable if the heap continues to grow.
		//
		// The effect of this mechanism is that assists stay flat in the face of heap
		// growths. It's OK to use more memory this cycle to scan all the live heap,
		// because the next GC cycle is inevitably going to use *at least* that much
		// memory anyway.
		extHeapGoal := int64(float64(heapGoal-int64(c.trigger))/float64(scanWorkExpected)*float64(maxScanWork)) + int64(c.trigger)
		scanWorkExpected = maxScanWork

		// hardGoal is a hard limit on the amount that we're willing to push back the
		// heap goal, and that's twice the heap goal (i.e. if GOGC=100 and the heap and/or
		// stacks and/or globals grow to twice their size, this limits the current GC cycle's
		// growth to 4x the original live heap's size).
		//
		// This maintains the invariant that we use no more memory than the next GC cycle
		// will anyway.
		hardGoal := int64((1.0 + float64(gcPercent)/100.0) * float64(heapGoal))
		if extHeapGoal > hardGoal {
			extHeapGoal = hardGoal
		}
		heapGoal = extHeapGoal
	}
	if int64(live) > heapGoal {
		// We're already past our heap goal, even the extrapolated one.
		// Leave ourselves some extra runway, so in the worst case we
		// finish by that point.
		const maxOvershoot = 1.1
		heapGoal = int64(float64(heapGoal) * maxOvershoot)

		// Compute the upper bound on the scan work remaining.
		scanWorkExpected = maxScanWork
	}

	// Compute the remaining scan work estimate.
	//
	// Note that we currently count allocations during GC as both
	// scannable heap (heapScan) and scan work completed
	// (scanWork), so allocation will change this difference
	// slowly in the soft regime and not at all in the hard
	// regime.
	scanWorkRemaining := scanWorkExpected - work
	if scanWorkRemaining < 1000 {
		// We set a somewhat arbitrary lower bound on
		// remaining scan work since if we aim a little high,
		// we can miss by a little.
		//
		// We *do* need to enforce that this is at least 1,
		// since marking is racy and double-scanning objects
		// may legitimately make the remaining scan work
		// negative, even in the hard goal regime.
		scanWorkRemaining = 1000
	}

	// Compute the heap distance remaining.
	heapRemaining := heapGoal - int64(live)
	if heapRemaining <= 0 {
		// This shouldn't happen, but if it does, avoid
		// dividing by zero or setting the assist negative.
		heapRemaining = 1
	}

	// Compute the mutator assist ratio so by the time the mutator
	// allocates the remaining heap bytes up to heapGoal, it will
	// have done (or stolen) the remaining amount of scan work.
	// Note that the assist ratio values are updated atomically
	// but not together. This means there may be some degree of
	// skew between the two values. This is generally OK as the
	// values shift relatively slowly over the course of a GC
	// cycle.
	assistWorkPerByte := float64(scanWorkRemaining) / float64(heapRemaining)
	assistBytesPerWork := float64(heapRemaining) / float64(scanWorkRemaining)
	c.assistWorkPerByte.Store(assistWorkPerByte)
	c.assistBytesPerWork.Store(assistBytesPerWork)
}

// endCycle computes the consMark estimate for the next cycle.
// userForced indicates whether the current GC cycle was forced
// by the application.
func (c *gcControllerState) endCycle(now int64, procs int, userForced bool) {
	// Record last heap goal for the scavenger.
	// We'll be updating the heap goal soon.
	gcController.lastHeapGoal = gcController.heapGoal

	// Compute the duration of time for which assists were turned on.
	assistDuration := now - c.markStartTime

	// Assume background mark hit its utilization goal.
	utilization := gcBackgroundUtilization
	// Add assist utilization; avoid divide by zero.
	if assistDuration > 0 {
		utilization += float64(c.assistTime) / float64(assistDuration*int64(procs))
	}

	if c.heapLive <= c.trigger {
		// Shouldn't happen, but let's be very safe about this in case the
		// GC is somehow extremely short.
		//
		// In this case though, the only reasonable value for c.heapLive-c.trigger
		// would be 0, which isn't really all that useful, i.e. the GC was so short
		// that it didn't matter.
		//
		// Ignore this case and don't update anything.
		return
	}
	idleUtilization := 0.0
	if assistDuration > 0 {
		idleUtilization = float64(c.idleMarkTime) / float64(assistDuration*int64(procs))
	}
	// Determine the cons/mark ratio.
	//
	// The units we want for the numerator and denominator are both B / cpu-ns.
	// We get this by taking the bytes allocated or scanned, and divide by the amount of
	// CPU time it took for those operations. For allocations, that CPU time is
	//
	//    assistDuration * procs * (1 - utilization)
	//
	// Where utilization includes just background GC workers and assists. It does *not*
	// include idle GC work time, because in theory the mutator is free to take that at
	// any point.
	//
	// For scanning, that CPU time is
	//
	//    assistDuration * procs * (utilization + idleUtilization)
	//
	// In this case, we *include* idle utilization, because that is additional CPU time that the
	// the GC had available to it.
	//
	// In effect, idle GC time is sort of double-counted here, but it's very weird compared
	// to other kinds of GC work, because of how fluid it is. Namely, because the mutator is
	// *always* free to take it.
	//
	// So this calculation is really:
	//     (heapLive-trigger) / (assistDuration * procs * (1-utilization)) /
	//         (scanWork) / (assistDuration * procs * (utilization+idleUtilization)
	//
	// Note that because we only care about the ratio, assistDuration and procs cancel out.
	scanWork := c.heapScanWork.Load() + c.stackScanWork.Load() + c.globalsScanWork.Load()
	currentConsMark := (float64(c.heapLive-c.trigger) * (utilization + idleUtilization)) /
		(float64(scanWork) * (1 - utilization))

	// Update cons/mark controller. The time period for this is 1 GC cycle.
	//
	// This use of a PI controller might seem strange. So, here's an explanation:
	//
	// currentConsMark represents the consMark we *should've* had to be perfectly
	// on-target for this cycle. Given that we assume the next GC will be like this
	// one in the steady-state, it stands to reason that we should just pick that
	// as our next consMark. In practice, however, currentConsMark is too noisy:
	// we're going to be wildly off-target in each GC cycle if we do that.
	//
	// What we do instead is make a long-term assumption: there is some steady-state
	// consMark value, but it's obscured by noise. By constantly shooting for this
	// noisy-but-perfect consMark value, the controller will bounce around a bit,
	// but its average behavior, in aggregate, should be less noisy and closer to
	// the true long-term consMark value, provided its tuned to be slightly overdamped.
	var ok bool
	oldConsMark := c.consMark
	c.consMark, ok = c.consMarkController.next(c.consMark, currentConsMark, 1.0)
	if !ok {
		// The error spiraled out of control. This is incredibly unlikely seeing
		// as this controller is essentially just a smoothing function, but it might
		// mean that something went very wrong with how currentConsMark was calculated.
		// Just reset consMark and keep going.
		c.consMark = 0
	}

	if debug.gcpacertrace > 0 {
		printlock()
		goal := gcGoalUtilization * 100
		print("pacer: ", int(utilization*100), "% CPU (", int(goal), " exp.) for ")
		print(c.heapScanWork.Load(), "+", c.stackScanWork.Load(), "+", c.globalsScanWork.Load(), " B work (", c.lastHeapScan+c.stackScan+c.globalsScan, " B exp.) ")
		print("in ", c.trigger, " B -> ", c.heapLive, " B (∆goal ", int64(c.heapLive)-int64(c.heapGoal), ", cons/mark ", oldConsMark, ")")
		if !ok {
			print("[controller reset]")
		}
		println()
		printunlock()
	}
}

// enlistWorker encourages another dedicated mark worker to start on
// another P if there are spare worker slots. It is used by putfull
// when more work is made available.
//
//go:nowritebarrier
func (c *gcControllerState) enlistWorker() {
	// If there are idle Ps, wake one so it will run an idle worker.
	// NOTE: This is suspected of causing deadlocks. See golang.org/issue/19112.
	//
	//	if atomic.Load(&sched.npidle) != 0 && atomic.Load(&sched.nmspinning) == 0 {
	//		wakep()
	//		return
	//	}

	// There are no idle Ps. If we need more dedicated workers,
	// try to preempt a running P so it will switch to a worker.
	if c.dedicatedMarkWorkersNeeded <= 0 {
		return
	}
	// Pick a random other P to preempt.
	if gomaxprocs <= 1 {
		return
	}
	gp := getg()
	if gp == nil || gp.m == nil || gp.m.p == 0 {
		return
	}
	myID := gp.m.p.ptr().id
	for tries := 0; tries < 5; tries++ {
		id := int32(fastrandn(uint32(gomaxprocs - 1)))
		if id >= myID {
			id++
		}
		p := allp[id]
		if p.status != _Prunning {
			continue
		}
		if preemptone(p) {
			return
		}
	}
}

// findRunnableGCWorker returns a background mark worker for _p_ if it
// should be run. This must only be called when gcBlackenEnabled != 0.
func (c *gcControllerState) findRunnableGCWorker(_p_ *p) *g {
	if gcBlackenEnabled == 0 {
		throw("gcControllerState.findRunnable: blackening not enabled")
	}

	if !gcMarkWorkAvailable(_p_) {
		// No work to be done right now. This can happen at
		// the end of the mark phase when there are still
		// assists tapering off. Don't bother running a worker
		// now because it'll just return immediately.
		return nil
	}

	// Grab a worker before we commit to running below.
	node := (*gcBgMarkWorkerNode)(gcBgMarkWorkerPool.pop())
	if node == nil {
		// There is at least one worker per P, so normally there are
		// enough workers to run on all Ps, if necessary. However, once
		// a worker enters gcMarkDone it may park without rejoining the
		// pool, thus freeing a P with no corresponding worker.
		// gcMarkDone never depends on another worker doing work, so it
		// is safe to simply do nothing here.
		//
		// If gcMarkDone bails out without completing the mark phase,
		// it will always do so with queued global work. Thus, that P
		// will be immediately eligible to re-run the worker G it was
		// just using, ensuring work can complete.
		return nil
	}

	decIfPositive := func(ptr *int64) bool {
		for {
			v := atomic.Loadint64(ptr)
			if v <= 0 {
				return false
			}

			if atomic.Casint64(ptr, v, v-1) {
				return true
			}
		}
	}

	if decIfPositive(&c.dedicatedMarkWorkersNeeded) {
		// This P is now dedicated to marking until the end of
		// the concurrent mark phase.
		_p_.gcMarkWorkerMode = gcMarkWorkerDedicatedMode
	} else if c.fractionalUtilizationGoal == 0 {
		// No need for fractional workers.
		gcBgMarkWorkerPool.push(&node.node)
		return nil
	} else {
		// Is this P behind on the fractional utilization
		// goal?
		//
		// This should be kept in sync with pollFractionalWorkerExit.
		delta := nanotime() - c.markStartTime
		if delta > 0 && float64(_p_.gcFractionalMarkTime)/float64(delta) > c.fractionalUtilizationGoal {
			// Nope. No need to run a fractional worker.
			gcBgMarkWorkerPool.push(&node.node)
			return nil
		}
		// Run a fractional worker.
		_p_.gcMarkWorkerMode = gcMarkWorkerFractionalMode
	}

	// Run the background mark worker.
	gp := node.gp.ptr()
	casgstatus(gp, _Gwaiting, _Grunnable)
	if trace.enabled {
		traceGoUnpark(gp, 0)
	}
	return gp
}

// resetLive sets up the controller state for the next mark phase after the end
// of the previous one. Must be called after endCycle and before commit, before
// the world is started.
//
// The world must be stopped.
func (c *gcControllerState) resetLive(bytesMarked uint64) {
	c.heapMarked = bytesMarked
	c.heapLive = bytesMarked
	c.heapScan = uint64(c.heapScanWork.Load())
	c.lastHeapScan = uint64(c.heapScanWork.Load())

	// heapLive was updated, so emit a trace event.
	if trace.enabled {
		traceHeapAlloc()
	}
}

// markWorkerStop must be called whenever a mark worker stops executing.
//
// It updates mark work accounting in the controller by a duration of
// work in nanoseconds and other bookkeeping.
//
// Safe to execute at any time.
func (c *gcControllerState) markWorkerStop(mode gcMarkWorkerMode, duration int64) {
	switch mode {
	case gcMarkWorkerDedicatedMode:
		atomic.Xaddint64(&c.dedicatedMarkTime, duration)
		atomic.Xaddint64(&c.dedicatedMarkWorkersNeeded, 1)
	case gcMarkWorkerFractionalMode:
		atomic.Xaddint64(&c.fractionalMarkTime, duration)
	case gcMarkWorkerIdleMode:
		atomic.Xaddint64(&c.idleMarkTime, duration)
		c.removeIdleMarkWorker()
	default:
		throw("markWorkerStop: unknown mark worker mode")
	}
}

func (c *gcControllerState) update(dHeapLive, dHeapScan int64) {
	if dHeapLive != 0 {
		atomic.Xadd64(&gcController.heapLive, dHeapLive)
		if trace.enabled {
			// gcController.heapLive changed.
			traceHeapAlloc()
		}
	}
	if gcBlackenEnabled == 0 {
		// Update heapScan when we're not in a current GC. It is fixed
		// at the beginning of a cycle.
		if dHeapScan != 0 {
			atomic.Xadd64(&gcController.heapScan, dHeapScan)
		}
	} else {
		// gcController.heapLive changed.
		c.revise()
	}
}

func (c *gcControllerState) addScannableStack(pp *p, amount int64) {
	if pp == nil {
		atomic.Xadd64(&c.scannableStackSize, amount)
		return
	}
	pp.scannableStackSizeDelta += amount
	if pp.scannableStackSizeDelta >= scannableStackSizeSlack || pp.scannableStackSizeDelta <= -scannableStackSizeSlack {
		atomic.Xadd64(&c.scannableStackSize, pp.scannableStackSizeDelta)
		pp.scannableStackSizeDelta = 0
	}
}

func (c *gcControllerState) addGlobals(amount int64) {
	atomic.Xadd64(&c.globalsScan, amount)
}

// commit recomputes all pacing parameters from scratch, namely
// absolute trigger, the heap goal, mark pacing, and sweep pacing.
//
// This can be called any time. If GC is the in the middle of a
// concurrent phase, it will adjust the pacing of that phase.
//
// This depends on gcPercent, gcController.heapMarked, and
// gcController.heapLive. These must be up to date.
//
// mheap_.lock must be held or the world must be stopped.
func (c *gcControllerState) commit() {
	if !c.test {
		assertWorldStoppedOrLockHeld(&mheap_.lock)
	}

	// Compute the next GC goal, which is when the allocated heap
	// has grown by GOGC/100 over where it started the last cycle,
	// plus additional runway for non-heap sources of GC work.
	goal := ^uint64(0)
	if gcPercent := c.gcPercent.Load(); gcPercent >= 0 {
		goal = c.heapMarked + (c.heapMarked+atomic.Load64(&c.stackScan)+atomic.Load64(&c.globalsScan))*uint64(gcPercent)/100
	}

	// Don't trigger below the minimum heap size.
	minTrigger := c.heapMinimum
	if !isSweepDone() {
		// Concurrent sweep happens in the heap growth
		// from gcController.heapLive to trigger, so ensure
		// that concurrent sweep has some heap growth
		// in which to perform sweeping before we
		// start the next GC cycle.
		sweepMin := atomic.Load64(&c.heapLive) + sweepMinHeapDistance
		if sweepMin > minTrigger {
			minTrigger = sweepMin
		}
	}

	// If we let the trigger go too low, then if the application
	// is allocating very rapidly we might end up in a situation
	// where we're allocating black during a nearly always-on GC.
	// The result of this is a growing heap and ultimately an
	// increase in RSS. By capping us at a point >0, we're essentially
	// saying that we're OK using more CPU during the GC to prevent
	// this growth in RSS.
	//
	// The current constant was chosen empirically: given a sufficiently
	// fast/scalable allocator with 48 Ps that could drive the trigger ratio
	// to <0.05, this constant causes applications to retain the same peak
	// RSS compared to not having this allocator.
	if triggerBound := uint64(0.7*float64(goal-c.heapMarked)) + c.heapMarked; minTrigger < triggerBound {
		minTrigger = triggerBound
	}

	// For small heaps, set the max trigger point at 95% of the heap goal.
	// This ensures we always have *some* headroom when the GC actually starts.
	// For larger heaps, set the max trigger point at the goal, minus the
	// minimum heap size.
	// This choice follows from the fact that the minimum heap size is chosen
	// to reflect the costs of a GC with no work to do. With a large heap but
	// very little scan work to perform, this gives us exactly as much runway
	// as we would need, in the worst case.
	maxRunway := uint64(0.95 * float64(goal-c.heapMarked))
	if largeHeapMaxRunway := goal - c.heapMinimum; goal > c.heapMinimum && maxRunway < largeHeapMaxRunway {
		maxRunway = largeHeapMaxRunway
	}
	maxTrigger := maxRunway + c.heapMarked
	if maxTrigger < minTrigger {
		maxTrigger = minTrigger
	}

	// Compute the trigger by using our estimate of the cons/mark ratio.
	//
	// The idea is to take our expected scan work, and multiply it by
	// the cons/mark ratio to determine how long it'll take to complete
	// that scan work in terms of bytes allocated. This gives us our GC's
	// runway.
	//
	// However, the cons/mark ratio is a ratio of rates per CPU-second, but
	// here we care about the relative rates for some division of CPU
	// resources among the mutator and the GC.
	//
	// To summarize, we have B / cpu-ns, and we want B / ns. We get that
	// by multiplying by our desired division of CPU resources. We choose
	// to express CPU resources as GOMAPROCS*fraction. Note that because
	// we're working with a ratio here, we can omit the number of CPU cores,
	// because they'll appear in the numerator and denominator and cancel out.
	// As a result, this is basically just "weighing" the cons/mark ratio by
	// our desired division of resources.
	//
	// Furthermore, by setting the trigger so that CPU resources are divided
	// this way, assuming that the cons/mark ratio is correct, we make that
	// division a reality.
	var trigger uint64
	runway := uint64((c.consMark * (1 - gcGoalUtilization) / (gcGoalUtilization)) * float64(c.lastHeapScan+c.stackScan+c.globalsScan))
	if runway > goal {
		trigger = minTrigger
	} else {
		trigger = goal - runway
	}
	if trigger < minTrigger {
		trigger = minTrigger
	}
	if trigger > maxTrigger {
		trigger = maxTrigger
	}
	if trigger > goal {
		goal = trigger
	}

	// Commit to the trigger and goal.
	c.trigger = trigger
	atomic.Store64(&c.heapGoal, goal)
	if trace.enabled {
		traceHeapGoal()
	}

	// Update mark pacing.
	if gcphase != _GCoff {
		c.revise()
	}
}

// effectiveGrowthRatio returns the current effective heap growth
// ratio (GOGC/100) based on heapMarked from the previous GC and
// heapGoal for the current GC.
//
// This may differ from gcPercent/100 because of various upper and
// lower bounds on gcPercent. For example, if the heap is smaller than
// heapMinimum, this can be higher than gcPercent/100.
//
// mheap_.lock must be held or the world must be stopped.
func (c *gcControllerState) effectiveGrowthRatio() float64 {
	if !c.test {
		assertWorldStoppedOrLockHeld(&mheap_.lock)
	}

	egogc := float64(atomic.Load64(&c.heapGoal)-c.heapMarked) / float64(c.heapMarked)
	if egogc < 0 {
		// Shouldn't happen, but just in case.
		egogc = 0
	}
	return egogc
}

// setGCPercent updates gcPercent and all related pacer state.
// Returns the old value of gcPercent.
//
// Calls gcControllerState.commit.
//
// The world must be stopped, or mheap_.lock must be held.
func (c *gcControllerState) setGCPercent(in int32) int32 {
	if !c.test {
		assertWorldStoppedOrLockHeld(&mheap_.lock)
	}

	out := c.gcPercent.Load()
	if in < 0 {
		in = -1
	}
	c.heapMinimum = defaultHeapMinimum * uint64(in) / 100
	c.gcPercent.Store(in)
	// Update pacing in response to gcPercent change.
	c.commit()

	return out
}

//go:linkname setGCPercent runtime/debug.setGCPercent
func setGCPercent(in int32) (out int32) {
	// Run on the system stack since we grab the heap lock.
	systemstack(func() {
		lock(&mheap_.lock)
		out = gcController.setGCPercent(in)
		gcPaceSweeper(gcController.trigger)
		gcPaceScavenger(gcController.heapGoal, gcController.lastHeapGoal)
		unlock(&mheap_.lock)
	})

	// If we just disabled GC, wait for any concurrent GC mark to
	// finish so we always return with no GC running.
	if in < 0 {
		gcWaitOnMark(atomic.Load(&work.cycles))
	}

	return out
}

func readGOGC() int32 {
	p := gogetenv("GOGC")
	if p == "off" {
		return -1
	}
	if n, ok := atoi32(p); ok {
		return n
	}
	return 100
}

type piController struct {
	kp float64 // Proportional constant.
	ti float64 // Integral time constant.
	tt float64 // Reset time.

	min, max float64 // Output boundaries.

	// PI controller state.

	errIntegral float64 // Integral of the error from t=0 to now.

	// Error flags.
	errOverflow   bool // Set if errIntegral ever overflowed.
	inputOverflow bool // Set if an operation with the input overflowed.
}

// next provides a new sample to the controller.
//
// input is the sample, setpoint is the desired point, and period is how much
// time (in whatever unit makes the most sense) has passed since the last sample.
//
// Returns a new value for the variable it's controlling, and whether the operation
// completed successfully. One reason this might fail is if error has been growing
// in an unbounded manner, to the point of overflow.
//
// In the specific case of an error overflow occurs, the errOverflow field will be
// set and the rest of the controller's internal state will be fully reset.
func (c *piController) next(input, setpoint, period float64) (float64, bool) {
	// Compute the raw output value.
	prop := c.kp * (setpoint - input)
	rawOutput := prop + c.errIntegral

	// Clamp rawOutput into output.
	output := rawOutput
	if isInf(output) || isNaN(output) {
		// The input had a large enough magnitude that either it was already
		// overflowed, or some operation with it overflowed.
		// Set a flag and reset. That's the safest thing to do.
		c.reset()
		c.inputOverflow = true
		return c.min, false
	}
	if output < c.min {
		output = c.min
	} else if output > c.max {
		output = c.max
	}

	// Update the controller's state.
	if c.ti != 0 && c.tt != 0 {
		c.errIntegral += (c.kp*period/c.ti)*(setpoint-input) + (period/c.tt)*(output-rawOutput)
		if isInf(c.errIntegral) || isNaN(c.errIntegral) {
			// So much error has accumulated that we managed to overflow.
			// The assumptions around the controller have likely broken down.
			// Set a flag and reset. That's the safest thing to do.
			c.reset()
			c.errOverflow = true
			return c.min, false
		}
	}
	return output, true
}

// reset resets the controller state, except for controller error flags.
func (c *piController) reset() {
	c.errIntegral = 0
}

// addIdleMarkWorker attempts to add a new idle mark worker.
//
// If this returns true, the caller must become an idle mark worker unless
// there's no background mark worker goroutines in the pool. This case is
// harmless because there are already background mark workers running.
// If this returns false, the caller must NOT become an idle mark worker.
//
// nosplit because it may be called without a P.
//go:nosplit
func (c *gcControllerState) addIdleMarkWorker() bool {
	for {
		old := c.idleMarkWorkers.Load()
		n, max := int32(old&uint64(^uint32(0))), int32(old>>32)
		if n >= max {
			// See the comment on idleMarkWorkers for why
			// n > max is tolerated.
			return false
		}
		if n < 0 {
			print("n=", n, " max=", max, "\n")
			throw("negative idle mark workers")
		}
		new := uint64(uint32(n+1)) | (uint64(max) << 32)
		if c.idleMarkWorkers.CompareAndSwap(old, new) {
			return true
		}
	}
}

// needIdleMarkWorker is a hint as to whether another idle mark worker is needed.
//
// The caller must still call addIdleMarkWorker to become one. This is mainly
// useful for a quick check before an expensive operation.
//
// nosplit because it may be called without a P.
//go:nosplit
func (c *gcControllerState) needIdleMarkWorker() bool {
	p := c.idleMarkWorkers.Load()
	n, max := int32(p&uint64(^uint32(0))), int32(p>>32)
	return n < max
}

// removeIdleMarkWorker must be called when an new idle mark worker stops executing.
func (c *gcControllerState) removeIdleMarkWorker() {
	for {
		old := c.idleMarkWorkers.Load()
		n, max := int32(old&uint64(^uint32(0))), int32(old>>32)
		if n-1 < 0 {
			print("n=", n, " max=", max, "\n")
			throw("negative idle mark workers")
		}
		new := uint64(uint32(n-1)) | (uint64(max) << 32)
		if c.idleMarkWorkers.CompareAndSwap(old, new) {
			return
		}
	}
}

// setMaxIdleMarkWorkers sets the maximum number of idle mark workers allowed.
//
// This method is optimistic in that it does not wait for the number of
// idle mark workers to reduce to max before returning; it assumes the workers
// will deschedule themselves.
func (c *gcControllerState) setMaxIdleMarkWorkers(max int32) {
	for {
		old := c.idleMarkWorkers.Load()
		n := int32(old & uint64(^uint32(0)))
		if n < 0 {
			print("n=", n, " max=", max, "\n")
			throw("negative idle mark workers")
		}
		new := uint64(uint32(n)) | (uint64(max) << 32)
		if c.idleMarkWorkers.CompareAndSwap(old, new) {
			return
		}
	}
}