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// 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 (
const (
// gcGoalUtilization is the goal CPU utilization for
// marking as a fraction of GOMAXPROCS.
gcGoalUtilization = goexperiment.PacerRedesignInt*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.
// Setting this to < gcGoalUtilization avoids saturating the trigger
// feedback controller when there are no assists, which allows it to
// better control CPU and heap growth. However, the larger the gap,
// the more mutator assists are expected to happen, which impact
// mutator latency.
// If goexperiment.PacerRedesign, the trigger feedback controller
// is replaced with an estimate of the mark/cons ratio that doesn't
// have the same saturation issues, so this is set equal to
// gcGoalUtilization.
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) +
// 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 {
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 uses a feedback control algorithm to adjust the gcController.trigger
// trigger based on the heap growth and GC CPU utilization each cycle.
// This algorithm optimizes for heap growth to match GOGC and for CPU
// utilization between assist and background marking to be 25% of
// GOMAXPROCS. The high-level design of this algorithm is documented
// at
// All fields of gcController are used only during a single mark
// cycle.
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
// triggerRatio is the heap growth ratio that triggers marking.
// E.g., if this is 0.6, then GC should start when the live
// heap has reached 1.6 times the heap size marked by the
// previous cycle. This should be ≤ GOGC/100 so the trigger
// heap size is less than the goal heap size. This is set
// during mark termination for the next cycle's trigger.
// Protected by mheap_.lock or a STW.
// Used if !goexperiment.PacerRedesign.
triggerRatio float64
// 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 triggerRatio 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.
// For goexperiment.PacerRedesign.
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.
// For goexperiment.PacerRedesign.
consMarkController piController
// 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.
// For !goexperiment.PacerRedesign: Whenever this is updated,
// call this gcControllerState's revise() method. It is read
// and written atomically or with the world stopped.
// For goexperiment.PacerRedesign: 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.
// For !goexperiment.PacerRedesign, stackScanWork and globalsScanWork
// are always zero.
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
// 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
if goexperiment.PacerRedesign {
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,
} else {
// Set a reasonable initial GC trigger.
c.triggerRatio = 7 / 8.0
// Fake a heapMarked value so it looks like a trigger at
// heapMinimum is the appropriate growth from heapMarked.
// This will go into computing the initial GC goal.
c.heapMarked = uint64(float64(c.heapMinimum) / (1 + c.triggerRatio))
// This will also compute and set the GC trigger and goal.
// 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) {
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 goexperiment.PacerRedesign {
if c.heapGoal < c.heapLive+64<<10 {
c.heapGoal = c.heapLive + 64<<10
} else {
if c.heapGoal < c.heapLive+1<<20 {
c.heapGoal = c.heapLive + 1<<20
// 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.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
// Compute initial values for controls that are updated
// throughout the cycle.
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))
var scanWorkExpected int64
if goexperiment.PacerRedesign {
// 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
} else {
// Compute the expected scan work remaining.
// This is estimated based on the expected
// steady-state scannable heap. For example, with
// GOGC=100, only half of the scannable heap is
// expected to be live, so that's what we target.
// (This is a float calculation to avoid overflowing on
// 100*heapScan.)
scanWorkExpected = int64(float64(scan) * 100 / float64(100+gcPercent))
if int64(live) > heapGoal || work > scanWorkExpected {
// We're past the soft goal, or we've already done more scan
// work than we expected. Pace GC so that in the worst case it
// will complete by the hard goal.
const maxOvershoot = 1.1
heapGoal = int64(float64(heapGoal) * maxOvershoot)
// Compute the upper bound on the scan work remaining.
scanWorkExpected = int64(scan)
// 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)
// endCycle computes the trigger ratio (!goexperiment.PacerRedesign)
// or the consMark estimate (goexperiment.PacerRedesign) for the next cycle.
// Returns the trigger ratio if application, or 0 (goexperiment.PacerRedesign).
// userForced indicates whether the current GC cycle was forced
// by the application.
func (c *gcControllerState) endCycle(now int64, procs int, userForced bool) float64 {
// 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 goexperiment.PacerRedesign {
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 0
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.
// Period for this is 1 GC cycle.
oldConsMark := c.consMark
c.consMark =, currentConsMark, 1.0)
if debug.gcpacertrace > 0 {
print("pacer: ", int(utilization*100), "% CPU (", int(gcGoalUtilization*100), " 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, ")")
return 0
// !goexperiment.PacerRedesign below.
if userForced {
// Forced GC means this cycle didn't start at the
// trigger, so where it finished isn't good
// information about how to adjust the trigger.
// Just leave it where it is.
return c.triggerRatio
// Proportional response gain for the trigger controller. Must
// be in [0, 1]. Lower values smooth out transient effects but
// take longer to respond to phase changes. Higher values
// react to phase changes quickly, but are more affected by
// transient changes. Values near 1 may be unstable.
const triggerGain = 0.5
// Compute next cycle trigger ratio. First, this computes the
// "error" for this cycle; that is, how far off the trigger
// was from what it should have been, accounting for both heap
// growth and GC CPU utilization. We compute the actual heap
// growth during this cycle and scale that by how far off from
// the goal CPU utilization we were (to estimate the heap
// growth if we had the desired CPU utilization). The
// difference between this estimate and the GOGC-based goal
// heap growth is the error.
goalGrowthRatio := c.effectiveGrowthRatio()
actualGrowthRatio := float64(c.heapLive)/float64(c.heapMarked) - 1
triggerError := goalGrowthRatio - c.triggerRatio - utilization/gcGoalUtilization*(actualGrowthRatio-c.triggerRatio)
// Finally, we adjust the trigger for next time by this error,
// damped by the proportional gain.
triggerRatio := c.triggerRatio + triggerGain*triggerError
if debug.gcpacertrace > 0 {
// Print controller state in terms of the design
// document.
H_m_prev := c.heapMarked
h_t := c.triggerRatio
H_T := c.trigger
h_a := actualGrowthRatio
H_a := c.heapLive
h_g := goalGrowthRatio
H_g := int64(float64(H_m_prev) * (1 + h_g))
u_a := utilization
u_g := gcGoalUtilization
W_a := c.heapScanWork.Load()
print("pacer: H_m_prev=", H_m_prev,
" h_t=", h_t, " H_T=", H_T,
" h_a=", h_a, " H_a=", H_a,
" h_g=", h_g, " H_g=", H_g,
" u_a=", u_a, " u_g=", u_g,
" W_a=", W_a,
" goalΔ=", goalGrowthRatio-h_t,
" actualΔ=", h_a-h_t,
" u_a/u_g=", u_a/u_g,
return triggerRatio
// 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.
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
// 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 {
// Pick a random other P to preempt.
if gomaxprocs <= 1 {
gp := getg()
if gp == nil || gp.m == nil || gp.m.p == 0 {
myID := gp.m.p.ptr().id
for tries := 0; tries < 5; tries++ {
id := int32(fastrandn(uint32(gomaxprocs - 1)))
if id >= myID {
p := allp[id]
if p.status != _Prunning {
if preemptone(p) {
// 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.
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.
return nil
// Run a fractional worker.
_p_.gcMarkWorkerMode = gcMarkWorkerFractionalMode
// Run the background mark worker.
gp :=
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 {
// logWorkTime updates mark work accounting in the controller by a duration of
// work in nanoseconds.
// Safe to execute at any time.
func (c *gcControllerState) logWorkTime(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)
throw("logWorkTime: 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.
// Only update heapScan in the new pacer redesign if we're not
// currently in a GC.
if !goexperiment.PacerRedesign || gcBlackenEnabled == 0 {
if dHeapScan != 0 {
atomic.Xadd64(&gcController.heapScan, dHeapScan)
if gcBlackenEnabled != 0 {
// gcController.heapLive and heapScan changed.
func (c *gcControllerState) addScannableStack(pp *p, amount int64) {
if pp == nil {
atomic.Xadd64(&c.scannableStackSize, amount)
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.
// If goexperiment.PacerRedesign is true, triggerRatio is ignored.
// 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(triggerRatio float64) {
if !c.test {
if !goexperiment.PacerRedesign {
// 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 {
// Update mark pacing.
if gcphase != _GCoff {
// oldCommit sets the trigger ratio and updates everything
// derived from it: the 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.
// For !goexperiment.PacerRedesign.
func (c *gcControllerState) oldCommit(triggerRatio float64) {
gcPercent := c.gcPercent.Load()
// Compute the next GC goal, which is when the allocated heap
// has grown by GOGC/100 over the heap marked by the last
// cycle.
goal := ^uint64(0)
if gcPercent >= 0 {
goal = c.heapMarked + c.heapMarked*uint64(gcPercent)/100
// Set the trigger ratio, capped to reasonable bounds.
if gcPercent >= 0 {
scalingFactor := float64(gcPercent) / 100
// Ensure there's always a little margin so that the
// mutator assist ratio isn't infinity.
maxTriggerRatio := 0.95 * scalingFactor
if triggerRatio > maxTriggerRatio {
triggerRatio = maxTriggerRatio
// If we let triggerRatio 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.
minTriggerRatio := 0.6 * scalingFactor
if triggerRatio < minTriggerRatio {
triggerRatio = minTriggerRatio
} else if triggerRatio < 0 {
// gcPercent < 0, so just make sure we're not getting a negative
// triggerRatio. This case isn't expected to happen in practice,
// and doesn't really matter because if gcPercent < 0 then we won't
// ever consume triggerRatio further on in this function, but let's
// just be defensive here; the triggerRatio being negative is almost
// certainly undesirable.
triggerRatio = 0
c.triggerRatio = triggerRatio
// Compute the absolute GC trigger from the trigger ratio.
// We trigger the next GC cycle when the allocated heap has
// grown by the trigger ratio over the marked heap size.
trigger := ^uint64(0)
if gcPercent >= 0 {
trigger = uint64(float64(c.heapMarked) * (1 + triggerRatio))
// 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 trigger < minTrigger {
trigger = minTrigger
if int64(trigger) < 0 {
print("runtime: heapGoal=", c.heapGoal, " heapMarked=", c.heapMarked, " gcController.heapLive=", c.heapLive, " initialHeapLive=", work.initialHeapLive, "triggerRatio=", triggerRatio, " minTrigger=", minTrigger, "\n")
throw("trigger underflow")
if trigger > goal {
// The trigger ratio is always less than GOGC/100, but
// other bounds on the trigger may have raised it.
// Push up the goal, too.
goal = trigger
// Commit to the trigger and goal.
c.trigger = trigger
atomic.Store64(&c.heapGoal, goal)
if trace.enabled {
// Update mark pacing.
if gcphase != _GCoff {
// 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 {
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 {
out := c.gcPercent.Load()
if in < 0 {
in = -1
c.heapMinimum = defaultHeapMinimum * uint64(in) / 100
// Update pacing in response to gcPercent change.
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() {
out = gcController.setGCPercent(in)
gcPaceScavenger(gcController.heapGoal, gcController.lastHeapGoal)
// If we just disabled GC, wait for any concurrent GC mark to
// finish so we always return with no GC running.
if in < 0 {
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.
func (c *piController) next(input, setpoint, period float64) float64 {
// Compute the raw output value.
prop := * (setpoint - input)
rawOutput := prop + c.errIntegral
// Clamp rawOutput into output.
output := rawOutput
if output < c.min {
output = c.min
} else if output > c.max {
output = c.max
// Update the controller's state.
if c.ti != 0 && != 0 {
c.errIntegral += (*period/c.ti)*(setpoint-input) + (period/*(output-rawOutput)
return output