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// Copyright 2009 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.
// Garbage collector (GC).
// The GC runs concurrently with mutator threads, is type accurate (aka precise), allows multiple
// GC thread to run in parallel. It is a concurrent mark and sweep that uses a write barrier. It is
// non-generational and non-compacting. Allocation is done using size segregated per P allocation
// areas to minimize fragmentation while eliminating locks in the common case.
// The algorithm decomposes into several steps.
// This is a high level description of the algorithm being used. For an overview of GC a good
// place to start is Richard Jones'
// The algorithm's intellectual heritage includes Dijkstra's on-the-fly algorithm, see
// Edsger W. Dijkstra, Leslie Lamport, A. J. Martin, C. S. Scholten, and E. F. M. Steffens. 1978.
// On-the-fly garbage collection: an exercise in cooperation. Commun. ACM 21, 11 (November 1978),
// 966-975.
// For journal quality proofs that these steps are complete, correct, and terminate see
// Hudson, R., and Moss, J.E.B. Copying Garbage Collection without stopping the world.
// Concurrency and Computation: Practice and Experience 15(3-5), 2003.
// 1. GC performs sweep termination.
// a. Stop the world. This causes all Ps to reach a GC safe-point.
// b. Sweep any unswept spans. There will only be unswept spans if
// this GC cycle was forced before the expected time.
// 2. GC performs the mark phase.
// a. Prepare for the mark phase by setting gcphase to _GCmark
// (from _GCoff), enabling the write barrier, enabling mutator
// assists, and enqueueing root mark jobs. No objects may be
// scanned until all Ps have enabled the write barrier, which is
// accomplished using STW.
// b. Start the world. From this point, GC work is done by mark
// workers started by the scheduler and by assists performed as
// part of allocation. The write barrier shades both the
// overwritten pointer and the new pointer value for any pointer
// writes (see mbarrier.go for details). Newly allocated objects
// are immediately marked black.
// c. GC performs root marking jobs. This includes scanning all
// stacks, shading all globals, and shading any heap pointers in
// off-heap runtime data structures. Scanning a stack stops a
// goroutine, shades any pointers found on its stack, and then
// resumes the goroutine.
// d. GC drains the work queue of grey objects, scanning each grey
// object to black and shading all pointers found in the object
// (which in turn may add those pointers to the work queue).
// e. Because GC work is spread across local caches, GC uses a
// distributed termination algorithm to detect when there are no
// more root marking jobs or grey objects (see gcMarkDone). At this
// point, GC transitions to mark termination.
// 3. GC performs mark termination.
// a. Stop the world.
// b. Set gcphase to _GCmarktermination, and disable workers and
// assists.
// c. Perform housekeeping like flushing mcaches.
// 4. GC performs the sweep phase.
// a. Prepare for the sweep phase by setting gcphase to _GCoff,
// setting up sweep state and disabling the write barrier.
// b. Start the world. From this point on, newly allocated objects
// are white, and allocating sweeps spans before use if necessary.
// c. GC does concurrent sweeping in the background and in response
// to allocation. See description below.
// 5. When sufficient allocation has taken place, replay the sequence
// starting with 1 above. See discussion of GC rate below.
// Concurrent sweep.
// The sweep phase proceeds concurrently with normal program execution.
// The heap is swept span-by-span both lazily (when a goroutine needs another span)
// and concurrently in a background goroutine (this helps programs that are not CPU bound).
// At the end of STW mark termination all spans are marked as "needs sweeping".
// The background sweeper goroutine simply sweeps spans one-by-one.
// To avoid requesting more OS memory while there are unswept spans, when a
// goroutine needs another span, it first attempts to reclaim that much memory
// by sweeping. When a goroutine needs to allocate a new small-object span, it
// sweeps small-object spans for the same object size until it frees at least
// one object. When a goroutine needs to allocate large-object span from heap,
// it sweeps spans until it frees at least that many pages into heap. There is
// one case where this may not suffice: if a goroutine sweeps and frees two
// nonadjacent one-page spans to the heap, it will allocate a new two-page
// span, but there can still be other one-page unswept spans which could be
// combined into a two-page span.
// It's critical to ensure that no operations proceed on unswept spans (that would corrupt
// mark bits in GC bitmap). During GC all mcaches are flushed into the central cache,
// so they are empty. When a goroutine grabs a new span into mcache, it sweeps it.
// When a goroutine explicitly frees an object or sets a finalizer, it ensures that
// the span is swept (either by sweeping it, or by waiting for the concurrent sweep to finish).
// The finalizer goroutine is kicked off only when all spans are swept.
// When the next GC starts, it sweeps all not-yet-swept spans (if any).
// GC rate.
// Next GC is after we've allocated an extra amount of memory proportional to
// the amount already in use. The proportion is controlled by GOGC environment variable
// (100 by default). If GOGC=100 and we're using 4M, we'll GC again when we get to 8M
// (this mark is tracked in next_gc variable). This keeps the GC cost in linear
// proportion to the allocation cost. Adjusting GOGC just changes the linear constant
// (and also the amount of extra memory used).
// Oblets
// In order to prevent long pauses while scanning large objects and to
// improve parallelism, the garbage collector breaks up scan jobs for
// objects larger than maxObletBytes into "oblets" of at most
// maxObletBytes. When scanning encounters the beginning of a large
// object, it scans only the first oblet and enqueues the remaining
// oblets as new scan jobs.
package runtime
import (
const (
_DebugGC = 0
_ConcurrentSweep = true
_FinBlockSize = 4 * 1024
// debugScanConservative enables debug logging for stack
// frames that are scanned conservatively.
debugScanConservative = false
// sweepMinHeapDistance is a lower bound on the heap distance
// (in bytes) reserved for concurrent sweeping between GC
// cycles.
sweepMinHeapDistance = 1024 * 1024
// 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.
var heapminimum uint64 = defaultHeapMinimum
// defaultHeapMinimum is the value of heapminimum for GOGC==100.
const defaultHeapMinimum = 4 << 20
// Initialized from $GOGC. GOGC=off means no GC.
var gcpercent int32
func gcinit() {
if unsafe.Sizeof(workbuf{}) != _WorkbufSize {
throw("size of Workbuf is suboptimal")
// No sweep on the first cycle.
mheap_.sweepdone = 1
// Set a reasonable initial GC trigger.
memstats.triggerRatio = 7 / 8.0
// Fake a heap_marked value so it looks like a trigger at
// heapminimum is the appropriate growth from heap_marked.
// This will go into computing the initial GC goal.
memstats.heap_marked = uint64(float64(heapminimum) / (1 + memstats.triggerRatio))
// Set gcpercent from the environment. This will also compute
// and set the GC trigger and goal.
_ = setGCPercent(readgogc())
work.startSema = 1
work.markDoneSema = 1
lockInit(&work.sweepWaiters.lock, lockRankSweepWaiters)
lockInit(&work.assistQueue.lock, lockRankAssistQueue)
lockInit(&work.wbufSpans.lock, lockRankWbufSpans)
func readgogc() int32 {
p := gogetenv("GOGC")
if p == "off" {
return -1
if n, ok := atoi32(p); ok {
return n
return 100
// gcenable is called after the bulk of the runtime initialization,
// just before we're about to start letting user code run.
// It kicks off the background sweeper goroutine, the background
// scavenger goroutine, and enables GC.
func gcenable() {
// Kick off sweeping and scavenging.
c := make(chan int, 2)
go bgsweep(c)
go bgscavenge(c)
memstats.enablegc = true // now that runtime is initialized, GC is okay
//go:linkname setGCPercent runtime..z2fdebug.setGCPercent
func setGCPercent(in int32) (out int32) {
// Run on the system stack since we grab the heap lock.
systemstack(func() {
out = gcpercent
if in < 0 {
in = -1
gcpercent = in
heapminimum = defaultHeapMinimum * uint64(gcpercent) / 100
// Update pacing in response to gcpercent change.
// 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
// Garbage collector phase.
// Indicates to write barrier and synchronization task to perform.
var gcphase uint32
// The compiler knows about this variable.
// If you change it, you must change gofrontend/, too.
// If you change the first four bytes, you must also change the write
// barrier insertion code.
var writeBarrier struct {
enabled bool // compiler emits a check of this before calling write barrier
pad [3]byte // compiler uses 32-bit load for "enabled" field
needed bool // whether we need a write barrier for current GC phase
cgo bool // whether we need a write barrier for a cgo check
alignme uint64 // guarantee alignment so that compiler can use a 32 or 64-bit load
// gcBlackenEnabled is 1 if mutator assists and background mark
// workers are allowed to blacken objects. This must only be set when
// gcphase == _GCmark.
var gcBlackenEnabled uint32
const (
_GCoff = iota // GC not running; sweeping in background, write barrier disabled
_GCmark // GC marking roots and workbufs: allocate black, write barrier ENABLED
_GCmarktermination // GC mark termination: allocate black, P's help GC, write barrier ENABLED
func setGCPhase(x uint32) {
atomic.Store(&gcphase, x)
writeBarrier.needed = gcphase == _GCmark || gcphase == _GCmarktermination
writeBarrier.enabled = writeBarrier.needed || writeBarrier.cgo
// gcMarkWorkerMode represents the mode that a concurrent mark worker
// should operate in.
// Concurrent marking happens through four different mechanisms. One
// is mutator assists, which happen in response to allocations and are
// not scheduled. The other three are variations in the per-P mark
// workers and are distinguished by gcMarkWorkerMode.
type gcMarkWorkerMode int
const (
// gcMarkWorkerDedicatedMode indicates that the P of a mark
// worker is dedicated to running that mark worker. The mark
// worker should run without preemption.
gcMarkWorkerDedicatedMode gcMarkWorkerMode = iota
// gcMarkWorkerFractionalMode indicates that a P is currently
// running the "fractional" mark worker. The fractional worker
// is necessary when GOMAXPROCS*gcBackgroundUtilization is not
// an integer. The fractional worker should run until it is
// preempted and will be scheduled to pick up the fractional
// part of GOMAXPROCS*gcBackgroundUtilization.
// gcMarkWorkerIdleMode indicates that a P is running the mark
// worker because it has nothing else to do. The idle worker
// should run until it is preempted and account its time
// against gcController.idleMarkTime.
// gcMarkWorkerModeStrings are the strings labels of gcMarkWorkerModes
// to use in execution traces.
var gcMarkWorkerModeStrings = [...]string{
"GC (dedicated)",
"GC (fractional)",
"GC (idle)",
// 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 memstats.gc_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 {
// scanWork is the total scan work performed this cycle. This
// is updated atomically during the cycle. Updates occur in
// bounded batches, since it is both written and read
// throughout the cycle. At the end of the cycle, this is how
// much of the retained heap is scannable.
// Currently this is the bytes of heap scanned. For most uses,
// this is an opaque unit of work, but for estimation the
// definition is important.
scanWork 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 heap_scan is updated.
assistWorkPerByte float64
// assistBytesPerWork is 1/assistWorkPerByte.
assistBytesPerWork 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
_ cpu.CacheLinePad
// startCycle resets the GC controller's state and computes estimates
// for a new GC cycle. The caller must hold worldsema.
func (c *gcControllerState) startCycle() {
c.scanWork = 0
c.bgScanCredit = 0
c.assistTime = 0
c.dedicatedMarkTime = 0
c.fractionalMarkTime = 0
c.idleMarkTime = 0
// 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 heap_live
// over gc_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 memstats.next_gc < memstats.heap_live+1024*1024 {
memstats.next_gc = memstats.heap_live + 1024*1024
// 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(gomaxprocs) * 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(gomaxprocs)
} else {
c.fractionalUtilizationGoal = 0
// In STW mode, we just want dedicated workers.
if debug.gcstoptheworld > 0 {
c.dedicatedMarkWorkersNeeded = int64(gomaxprocs)
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 {
print("pacer: assist ratio=", c.assistWorkPerByte,
" (scan ", memstats.heap_scan>>20, " MB in ",
work.initialHeapLive>>20, "->",
memstats.next_gc>>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 either under STW or
// whenever memstats.heap_scan, memstats.heap_live, or
// memstats.next_gc is updated (with mheap_.lock held).
// 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 := gcpercent
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(&memstats.heap_live)
// Assume we're under the soft goal. Pace GC to complete at
// next_gc assuming the heap is in steady-state.
heapGoal := int64(memstats.next_gc)
// 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*heap_scan.)
scanWorkExpected := int64(float64(memstats.heap_scan) * 100 / float64(100+gcpercent))
if live > memstats.next_gc || c.scanWork > 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(memstats.next_gc) * maxOvershoot)
// Compute the upper bound on the scan work remaining.
scanWorkExpected = int64(memstats.heap_scan)
// Compute the remaining scan work estimate.
// Note that we currently count allocations during GC as both
// scannable heap (heap_scan) 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 - c.scanWork
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 next_gc, it will
// have done (or stolen) the remaining amount of scan work.
c.assistWorkPerByte = float64(scanWorkRemaining) / float64(heapRemaining)
c.assistBytesPerWork = float64(heapRemaining) / float64(scanWorkRemaining)
// endCycle computes the trigger ratio for the next cycle.
func (c *gcControllerState) endCycle() float64 {
if work.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 memstats.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 := gcEffectiveGrowthRatio()
actualGrowthRatio := float64(memstats.heap_live)/float64(memstats.heap_marked) - 1
assistDuration := nanotime() - 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(gomaxprocs))
triggerError := goalGrowthRatio - memstats.triggerRatio - utilization/gcGoalUtilization*(actualGrowthRatio-memstats.triggerRatio)
// Finally, we adjust the trigger for next time by this error,
// damped by the proportional gain.
triggerRatio := memstats.triggerRatio + triggerGain*triggerError
if debug.gcpacertrace > 0 {
// Print controller state in terms of the design
// document.
H_m_prev := memstats.heap_marked
h_t := memstats.triggerRatio
H_T := memstats.gc_trigger
h_a := actualGrowthRatio
H_a := memstats.heap_live
h_g := goalGrowthRatio
H_g := int64(float64(H_m_prev) * (1 + h_g))
u_a := utilization
u_g := gcGoalUtilization
W_a := c.scanWork
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 the 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 _p_.gcBgMarkWorker == 0 {
// The mark worker associated with this P is blocked
// performing a mark transition. We can't run it
// because it may be on some other run or wait queue.
return nil
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
decIfPositive := func(ptr *int64) bool {
if *ptr > 0 {
if atomic.Xaddint64(ptr, -1) >= 0 {
return true
// We lost a race
atomic.Xaddint64(ptr, +1)
return false
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() - gcController.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 := _p_.gcBgMarkWorker.ptr()
casgstatus(gp, _Gwaiting, _Grunnable)
if trace.enabled {
traceGoUnpark(gp, 0)
return gp
// pollFractionalWorkerExit reports whether a fractional mark worker
// should self-preempt. It assumes it is called from the fractional
// worker.
func pollFractionalWorkerExit() bool {
// This should be kept in sync with the fractional worker
// scheduler logic in findRunnableGCWorker.
now := nanotime()
delta := now - gcController.markStartTime
if delta <= 0 {
return true
p := getg().m.p.ptr()
selfTime := p.gcFractionalMarkTime + (now - p.gcMarkWorkerStartTime)
// Add some slack to the utilization goal so that the
// fractional worker isn't behind again the instant it exits.
return float64(selfTime)/float64(delta) > 1.2*gcController.fractionalUtilizationGoal
// gcSetTriggerRatio 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, memstats.heap_marked, and
// memstats.heap_live. These must be up to date.
// mheap_.lock must be held or the world must be stopped.
func gcSetTriggerRatio(triggerRatio float64) {
// 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 = memstats.heap_marked + memstats.heap_marked*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
memstats.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(memstats.heap_marked) * (1 + triggerRatio))
// Don't trigger below the minimum heap size.
minTrigger := heapminimum
if !isSweepDone() {
// Concurrent sweep happens in the heap growth
// from heap_live to gc_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(&memstats.heap_live) + sweepMinHeapDistance
if sweepMin > minTrigger {
minTrigger = sweepMin
if trigger < minTrigger {
trigger = minTrigger
if int64(trigger) < 0 {
print("runtime: next_gc=", memstats.next_gc, " heap_marked=", memstats.heap_marked, " heap_live=", memstats.heap_live, " initialHeapLive=", work.initialHeapLive, "triggerRatio=", triggerRatio, " minTrigger=", minTrigger, "\n")
throw("gc_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.
memstats.gc_trigger = trigger
memstats.next_gc = goal
if trace.enabled {
// Update mark pacing.
if gcphase != _GCoff {
// Update sweep pacing.
if isSweepDone() {
mheap_.sweepPagesPerByte = 0
} else {
// Concurrent sweep needs to sweep all of the in-use
// pages by the time the allocated heap reaches the GC
// trigger. Compute the ratio of in-use pages to sweep
// per byte allocated, accounting for the fact that
// some might already be swept.
heapLiveBasis := atomic.Load64(&memstats.heap_live)
heapDistance := int64(trigger) - int64(heapLiveBasis)
// Add a little margin so rounding errors and
// concurrent sweep are less likely to leave pages
// unswept when GC starts.
heapDistance -= 1024 * 1024
if heapDistance < _PageSize {
// Avoid setting the sweep ratio extremely high
heapDistance = _PageSize
pagesSwept := atomic.Load64(&mheap_.pagesSwept)
pagesInUse := atomic.Load64(&mheap_.pagesInUse)
sweepDistancePages := int64(pagesInUse) - int64(pagesSwept)
if sweepDistancePages <= 0 {
mheap_.sweepPagesPerByte = 0
} else {
mheap_.sweepPagesPerByte = float64(sweepDistancePages) / float64(heapDistance)
mheap_.sweepHeapLiveBasis = heapLiveBasis
// Write pagesSweptBasis last, since this
// signals concurrent sweeps to recompute
// their debt.
atomic.Store64(&mheap_.pagesSweptBasis, pagesSwept)
// gcEffectiveGrowthRatio returns the current effective heap growth
// ratio (GOGC/100) based on heap_marked from the previous GC and
// next_gc 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 gcEffectiveGrowthRatio() float64 {
egogc := float64(memstats.next_gc-memstats.heap_marked) / float64(memstats.heap_marked)
if egogc < 0 {
// Shouldn't happen, but just in case.
egogc = 0
return egogc
// gcGoalUtilization is the goal CPU utilization for
// marking as a fraction of GOMAXPROCS.
const gcGoalUtilization = 0.30
// 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.
const gcBackgroundUtilization = 0.25
// gcCreditSlack is the amount of scan work credit that can
// accumulate locally before updating gcController.scanWork 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.
const gcCreditSlack = 2000
// gcAssistTimeSlack is the nanoseconds of mutator assist time that
// can accumulate on a P before updating gcController.assistTime.
const 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.
const gcOverAssistWork = 64 << 10
var work struct {
full lfstack // lock-free list of full blocks workbuf
empty lfstack // lock-free list of empty blocks workbuf
pad0 cpu.CacheLinePad // prevents false-sharing between full/empty and nproc/nwait
wbufSpans struct {
lock mutex
// free is a list of spans dedicated to workbufs, but
// that don't currently contain any workbufs.
free mSpanList
// busy is a list of all spans containing workbufs on
// one of the workbuf lists.
busy mSpanList
// Restore 64-bit alignment on 32-bit.
_ uint32
// bytesMarked is the number of bytes marked this cycle. This
// includes bytes blackened in scanned objects, noscan objects
// that go straight to black, and permagrey objects scanned by
// markroot during the concurrent scan phase. This is updated
// atomically during the cycle. Updates may be batched
// arbitrarily, since the value is only read at the end of the
// cycle.
// Because of benign races during marking, this number may not
// be the exact number of marked bytes, but it should be very
// close.
// Put this field here because it needs 64-bit atomic access
// (and thus 8-byte alignment even on 32-bit architectures).
bytesMarked uint64
markrootNext uint32 // next markroot job
markrootJobs uint32 // number of markroot jobs
nproc uint32
tstart int64
nwait uint32
ndone uint32
// Number of roots of various root types. Set by gcMarkRootPrepare.
nFlushCacheRoots int
nDataRoots, nSpanRoots, nStackRoots int
// Each type of GC state transition is protected by a lock.
// Since multiple threads can simultaneously detect the state
// transition condition, any thread that detects a transition
// condition must acquire the appropriate transition lock,
// re-check the transition condition and return if it no
// longer holds or perform the transition if it does.
// Likewise, any transition must invalidate the transition
// condition before releasing the lock. This ensures that each
// transition is performed by exactly one thread and threads
// that need the transition to happen block until it has
// happened.
// startSema protects the transition from "off" to mark or
// mark termination.
startSema uint32
// markDoneSema protects transitions from mark to mark termination.
markDoneSema uint32
bgMarkReady note // signal background mark worker has started
bgMarkDone uint32 // cas to 1 when at a background mark completion point
// Background mark completion signaling
// mode is the concurrency mode of the current GC cycle.
mode gcMode
// userForced indicates the current GC cycle was forced by an
// explicit user call.
userForced bool
// totaltime is the CPU nanoseconds spent in GC since the
// program started if debug.gctrace > 0.
totaltime int64
// initialHeapLive is the value of memstats.heap_live at the
// beginning of this GC cycle.
initialHeapLive uint64
// assistQueue is a queue of assists that are blocked because
// there was neither enough credit to steal or enough work to
// do.
assistQueue struct {
lock mutex
q gQueue
// sweepWaiters is a list of blocked goroutines to wake when
// we transition from mark termination to sweep.
sweepWaiters struct {
lock mutex
list gList
// cycles is the number of completed GC cycles, where a GC
// cycle is sweep termination, mark, mark termination, and
// sweep. This differs from memstats.numgc, which is
// incremented at mark termination.
cycles uint32
// Timing/utilization stats for this cycle.
stwprocs, maxprocs int32
tSweepTerm, tMark, tMarkTerm, tEnd int64 // nanotime() of phase start
pauseNS int64 // total STW time this cycle
pauseStart int64 // nanotime() of last STW
// debug.gctrace heap sizes for this cycle.
heap0, heap1, heap2, heapGoal uint64
// GC runs a garbage collection and blocks the caller until the
// garbage collection is complete. It may also block the entire
// program.
func GC() {
// We consider a cycle to be: sweep termination, mark, mark
// termination, and sweep. This function shouldn't return
// until a full cycle has been completed, from beginning to
// end. Hence, we always want to finish up the current cycle
// and start a new one. That means:
// 1. In sweep termination, mark, or mark termination of cycle
// N, wait until mark termination N completes and transitions
// to sweep N.
// 2. In sweep N, help with sweep N.
// At this point we can begin a full cycle N+1.
// 3. Trigger cycle N+1 by starting sweep termination N+1.
// 4. Wait for mark termination N+1 to complete.
// 5. Help with sweep N+1 until it's done.
// This all has to be written to deal with the fact that the
// GC may move ahead on its own. For example, when we block
// until mark termination N, we may wake up in cycle N+2.
// Wait until the current sweep termination, mark, and mark
// termination complete.
n := atomic.Load(&work.cycles)
// We're now in sweep N or later. Trigger GC cycle N+1, which
// will first finish sweep N if necessary and then enter sweep
// termination N+1.
gcStart(gcTrigger{kind: gcTriggerCycle, n: n + 1})
// Wait for mark termination N+1 to complete.
gcWaitOnMark(n + 1)
// Finish sweep N+1 before returning. We do this both to
// complete the cycle and because runtime.GC() is often used
// as part of tests and benchmarks to get the system into a
// relatively stable and isolated state.
for atomic.Load(&work.cycles) == n+1 && sweepone() != ^uintptr(0) {
// Callers may assume that the heap profile reflects the
// just-completed cycle when this returns (historically this
// happened because this was a STW GC), but right now the
// profile still reflects mark termination N, not N+1.
// As soon as all of the sweep frees from cycle N+1 are done,
// we can go ahead and publish the heap profile.
// First, wait for sweeping to finish. (We know there are no
// more spans on the sweep queue, but we may be concurrently
// sweeping spans, so we have to wait.)
for atomic.Load(&work.cycles) == n+1 && atomic.Load(&mheap_.sweepers) != 0 {
// Now we're really done with sweeping, so we can publish the
// stable heap profile. Only do this if we haven't already hit
// another mark termination.
mp := acquirem()
cycle := atomic.Load(&work.cycles)
if cycle == n+1 || (gcphase == _GCmark && cycle == n+2) {
// gcWaitOnMark blocks until GC finishes the Nth mark phase. If GC has
// already completed this mark phase, it returns immediately.
func gcWaitOnMark(n uint32) {
for {
// Disable phase transitions.
nMarks := atomic.Load(&work.cycles)
if gcphase != _GCmark {
// We've already completed this cycle's mark.
if nMarks > n {
// We're done.
// Wait until sweep termination, mark, and mark
// termination of cycle N complete.
goparkunlock(&work.sweepWaiters.lock, waitReasonWaitForGCCycle, traceEvGoBlock, 1)
// gcMode indicates how concurrent a GC cycle should be.
type gcMode int
const (
gcBackgroundMode gcMode = iota // concurrent GC and sweep
gcForceMode // stop-the-world GC now, concurrent sweep
gcForceBlockMode // stop-the-world GC now and STW sweep (forced by user)
// A gcTrigger is a predicate for starting a GC cycle. Specifically,
// it is an exit condition for the _GCoff phase.
type gcTrigger struct {
kind gcTriggerKind
now int64 // gcTriggerTime: current time
n uint32 // gcTriggerCycle: cycle number to start
type gcTriggerKind int
const (
// gcTriggerHeap indicates that a cycle should be started when
// the heap size reaches the trigger heap size computed by the
// controller.
gcTriggerHeap gcTriggerKind = iota
// gcTriggerTime indicates that a cycle should be started when
// it's been more than forcegcperiod nanoseconds since the
// previous GC cycle.
// gcTriggerCycle indicates that a cycle should be started if
// we have not yet started cycle number gcTrigger.n (relative
// to work.cycles).
// test reports whether the trigger condition is satisfied, meaning
// that the exit condition for the _GCoff phase has been met. The exit
// condition should be tested when allocating.
func (t gcTrigger) test() bool {
if !memstats.enablegc || panicking != 0 || gcphase != _GCoff {
return false
switch t.kind {
case gcTriggerHeap:
// Non-atomic access to heap_live for performance. If
// we are going to trigger on this, this thread just
// atomically wrote heap_live anyway and we'll see our
// own write.
return memstats.heap_live >= memstats.gc_trigger
case gcTriggerTime:
if gcpercent < 0 {
return false
lastgc := int64(atomic.Load64(&memstats.last_gc_nanotime))
return lastgc != 0 && > forcegcperiod
case gcTriggerCycle:
// t.n > work.cycles, but accounting for wraparound.
return int32(t.n-work.cycles) > 0
return true
// gcStart starts the GC. It transitions from _GCoff to _GCmark (if
// debug.gcstoptheworld == 0) or performs all of GC (if
// debug.gcstoptheworld != 0).
// This may return without performing this transition in some cases,
// such as when called on a system stack or with locks held.
func gcStart(trigger gcTrigger) {
// Since this is called from malloc and malloc is called in
// the guts of a number of libraries that might be holding
// locks, don't attempt to start GC in non-preemptible or
// potentially unstable situations.
mp := acquirem()
if gp := getg(); gp == mp.g0 || mp.locks > 1 || mp.preemptoff != "" {
mp = nil
// Pick up the remaining unswept/not being swept spans concurrently
// This shouldn't happen if we're being invoked in background
// mode since proportional sweep should have just finished
// sweeping everything, but rounding errors, etc, may leave a
// few spans unswept. In forced mode, this is necessary since
// GC can be forced at any point in the sweeping cycle.
// We check the transition condition continuously here in case
// this G gets delayed in to the next GC cycle.
for trigger.test() && sweepone() != ^uintptr(0) {
// Perform GC initialization and the sweep termination
// transition.
// Re-check transition condition under transition lock.
if !trigger.test() {
// For stats, check if this GC was forced by the user.
work.userForced = trigger.kind == gcTriggerCycle
// In gcstoptheworld debug mode, upgrade the mode accordingly.
// We do this after re-checking the transition condition so
// that multiple goroutines that detect the heap trigger don't
// start multiple STW GCs.
mode := gcBackgroundMode
if debug.gcstoptheworld == 1 {
mode = gcForceMode
} else if debug.gcstoptheworld == 2 {
mode = gcForceBlockMode
// Ok, we're doing it! Stop everybody else
if trace.enabled {
// Check that all Ps have finished deferred mcache flushes.
for _, p := range allp {
if fg := atomic.Load(&p.mcache.flushGen); fg != mheap_.sweepgen {
println("runtime: p",, "flushGen", fg, "!= sweepgen", mheap_.sweepgen)
throw("p mcache not flushed")
work.stwprocs, work.maxprocs = gomaxprocs, gomaxprocs
if work.stwprocs > ncpu {
// This is used to compute CPU time of the STW phases,
// so it can't be more than ncpu, even if GOMAXPROCS is.
work.stwprocs = ncpu
work.heap0 = atomic.Load64(&memstats.heap_live)
work.pauseNS = 0
work.mode = mode
now := nanotime()
work.tSweepTerm = now
work.pauseStart = now
if trace.enabled {
// Finish sweep before we start concurrent scan.
systemstack(func() {
// clearpools before we start the GC. If we wait they memory will not be
// reclaimed until the next GC cycle.
work.heapGoal = memstats.next_gc
// In STW mode, disable scheduling of user Gs. This may also
// disable scheduling of this goroutine, so it may block as
// soon as we start the world again.
if mode != gcBackgroundMode {
// Enter concurrent mark phase and enable
// write barriers.
// Because the world is stopped, all Ps will
// observe that write barriers are enabled by
// the time we start the world and begin
// scanning.
// Write barriers must be enabled before assists are
// enabled because they must be enabled before
// any non-leaf heap objects are marked. Since
// allocations are blocked until assists can
// happen, we want enable assists as early as
// possible.
gcBgMarkPrepare() // Must happen before assist enable.
// Mark all active tinyalloc blocks. Since we're
// allocating from these, they need to be black like
// other allocations. The alternative is to blacken
// the tiny block on every allocation from it, which
// would slow down the tiny allocator.
// At this point all Ps have enabled the write
// barrier, thus maintaining the no white to
// black invariant. Enable mutator assists to
// put back-pressure on fast allocating
// mutators.
atomic.Store(&gcBlackenEnabled, 1)
// Assists and workers can start the moment we start
// the world.
gcController.markStartTime = now
// In STW mode, we could block the instant systemstack
// returns, so make sure we're not preemptible.
mp = acquirem()
// Concurrent mark.
systemstack(func() {
now = startTheWorldWithSema(trace.enabled)
work.pauseNS += now - work.pauseStart
work.tMark = now
// Release the world sema before Gosched() in STW mode
// because we will need to reacquire it later but before
// this goroutine becomes runnable again, and we could
// self-deadlock otherwise.
// Make sure we block instead of returning to user code
// in STW mode.
if mode != gcBackgroundMode {
// gcMarkDoneFlushed counts the number of P's with flushed work.
// Ideally this would be a captured local in gcMarkDone, but forEachP
// escapes its callback closure, so it can't capture anything.
// This is protected by markDoneSema.
var gcMarkDoneFlushed uint32
// debugCachedWork enables extra checks for debugging premature mark
// termination.
// For debugging issue #27993.
const debugCachedWork = false
// gcWorkPauseGen is for debugging the mark completion algorithm.
// gcWork put operations spin while gcWork.pauseGen == gcWorkPauseGen.
// Only used if debugCachedWork is true.
// For debugging issue #27993.
var gcWorkPauseGen uint32 = 1
// gcMarkDone transitions the GC from mark to mark termination if all
// reachable objects have been marked (that is, there are no grey
// objects and can be no more in the future). Otherwise, it flushes
// all local work to the global queues where it can be discovered by
// other workers.
// This should be called when all local mark work has been drained and
// there are no remaining workers. Specifically, when
// work.nwait == work.nproc && !gcMarkWorkAvailable(p)
// The calling context must be preemptible.
// Flushing local work is important because idle Ps may have local
// work queued. This is the only way to make that work visible and
// drive GC to completion.
// It is explicitly okay to have write barriers in this function. If
// it does transition to mark termination, then all reachable objects
// have been marked, so the write barrier cannot shade any more
// objects.
func gcMarkDone() {
// Ensure only one thread is running the ragged barrier at a
// time.
// Re-check transition condition under transition lock.
// It's critical that this checks the global work queues are
// empty before performing the ragged barrier. Otherwise,
// there could be global work that a P could take after the P
// has passed the ragged barrier.
if !(gcphase == _GCmark && work.nwait == work.nproc && !gcMarkWorkAvailable(nil)) {
// forEachP needs worldsema to execute, and we'll need it to
// stop the world later, so acquire worldsema now.
// Flush all local buffers and collect flushedWork flags.
gcMarkDoneFlushed = 0
systemstack(func() {
gp := getg().m.curg
// Mark the user stack as preemptible so that it may be scanned.
// Otherwise, our attempt to force all P's to a safepoint could
// result in a deadlock as we attempt to preempt a worker that's
// trying to preempt us (e.g. for a stack scan).
casgstatus(gp, _Grunning, _Gwaiting)
forEachP(func(_p_ *p) {
// Flush the write barrier buffer, since this may add
// work to the gcWork.
// For debugging, shrink the write barrier
// buffer so it flushes immediately.
// wbBuf.reset will keep it at this size as
// long as throwOnGCWork is set.
if debugCachedWork {
b := &_p_.wbBuf
b.end = uintptr(unsafe.Pointer(&b.buf[wbBufEntryPointers]))
b.debugGen = gcWorkPauseGen
// Flush the gcWork, since this may create global work
// and set the flushedWork flag.
// TODO(austin): Break up these workbufs to
// better distribute work.
// Collect the flushedWork flag.
if _p_.gcw.flushedWork {
atomic.Xadd(&gcMarkDoneFlushed, 1)
_p_.gcw.flushedWork = false
} else if debugCachedWork {
// For debugging, freeze the gcWork
// until we know whether we've reached
// completion or not. If we think
// we've reached completion, but
// there's a paused gcWork, then
// that's a bug.
_p_.gcw.pauseGen = gcWorkPauseGen
// Capture the G's stack.
for i := range _p_.gcw.pauseStack {
_p_.gcw.pauseStack[i].pc = 0
callers(1, _p_.gcw.pauseStack[:])
casgstatus(gp, _Gwaiting, _Grunning)
if gcMarkDoneFlushed != 0 {
if debugCachedWork {
// Release paused gcWorks.
atomic.Xadd(&gcWorkPauseGen, 1)
// More grey objects were discovered since the
// previous termination check, so there may be more
// work to do. Keep going. It's possible the
// transition condition became true again during the
// ragged barrier, so re-check it.
goto top
if debugCachedWork {
throwOnGCWork = true
// Release paused gcWorks. If there are any, they
// should now observe throwOnGCWork and panic.
atomic.Xadd(&gcWorkPauseGen, 1)
// There was no global work, no local work, and no Ps
// communicated work since we took markDoneSema. Therefore
// there are no grey objects and no more objects can be
// shaded. Transition to mark termination.
now := nanotime()
work.tMarkTerm = now
work.pauseStart = now
getg().m.preemptoff = "gcing"
if trace.enabled {
// The gcphase is _GCmark, it will transition to _GCmarktermination
// below. The important thing is that the wb remains active until
// all marking is complete. This includes writes made by the GC.
if debugCachedWork {
// For debugging, double check that no work was added after we
// went around above and disable write barrier buffering.
for _, p := range allp {
gcw := &p.gcw
if !gcw.empty() {
print("runtime: P ",, " flushedWork ", gcw.flushedWork)
if gcw.wbuf1 == nil {
print(" wbuf1=<nil>")
} else {
print(" wbuf1.n=", gcw.wbuf1.nobj)
if gcw.wbuf2 == nil {
print(" wbuf2=<nil>")
} else {
print(" wbuf2.n=", gcw.wbuf2.nobj)
if gcw.pauseGen == gcw.putGen {
println("runtime: checkPut already failed at this generation")
} else {
// For unknown reasons (see issue #27993), there is
// sometimes work left over when we enter mark
// termination. Detect this and resume concurrent
// mark. This is obviously unfortunate.
// Switch to the system stack to call wbBufFlush1,
// though in this case it doesn't matter because we're
// non-preemptible anyway.
restart := false
systemstack(func() {
for _, p := range allp {
if !p.gcw.empty() {
restart = true
if restart {
getg().m.preemptoff = ""
systemstack(func() {
now := startTheWorldWithSema(true)
work.pauseNS += now - work.pauseStart
goto top
// Disable assists and background workers. We must do
// this before waking blocked assists.
atomic.Store(&gcBlackenEnabled, 0)
// Wake all blocked assists. These will run when we
// start the world again.
// Likewise, release the transition lock. Blocked
// workers and assists will run when we start the
// world again.
// In STW mode, re-enable user goroutines. These will be
// queued to run after we start the world.
// endCycle depends on all gcWork cache stats being flushed.
// The termination algorithm above ensured that up to
// allocations since the ragged barrier.
nextTriggerRatio := gcController.endCycle()
// Perform mark termination. This will restart the world.
func gcMarkTermination(nextTriggerRatio float64) {
// World is stopped.
// Start marktermination which includes enabling the write barrier.
atomic.Store(&gcBlackenEnabled, 0)
work.heap1 = memstats.heap_live
startTime := nanotime()
mp := acquirem()
mp.preemptoff = "gcing"
_g_ := getg()
_g_.m.traceback = 2
gp := _g_.m.curg
casgstatus(gp, _Grunning, _Gwaiting)
gp.waitreason = waitReasonGarbageCollection
// Run gc on the g0 stack. We do this so that the g stack
// we're currently running on will no longer change. Cuts
// the root set down a bit (g0 stacks are not scanned, and
// we don't need to scan gc's internal state). We also
// need to switch to g0 so we can shrink the stack.
systemstack(func() {
// Must return immediately.
// The outer function's stack may have moved
// during gcMark (it shrinks stacks, including the
// outer function's stack), so we must not refer
// to any of its variables. Return back to the
// non-system stack to pick up the new addresses
// before continuing.
systemstack(func() {
work.heap2 = work.bytesMarked
if debug.gccheckmark > 0 {
// Run a full non-parallel, stop-the-world
// mark using checkmark bits, to check that we
// didn't forget to mark anything during the
// concurrent mark process.
gcw := &getg().m.p.ptr().gcw
gcDrain(gcw, 0)
// marking is complete so we can turn the write barrier off
_g_.m.traceback = 0
casgstatus(gp, _Gwaiting, _Grunning)
if trace.enabled {
// all done
mp.preemptoff = ""
if gcphase != _GCoff {
throw("gc done but gcphase != _GCoff")
// Record next_gc and heap_inuse for scavenger.
memstats.last_next_gc = memstats.next_gc
memstats.last_heap_inuse = memstats.heap_inuse
// Update GC trigger and pacing for the next cycle.
// Update timing memstats
now := nanotime()
sec, nsec, _ := time_now()
unixNow := sec*1e9 + int64(nsec)
work.pauseNS += now - work.pauseStart
work.tEnd = now
atomic.Store64(&memstats.last_gc_unix, uint64(unixNow)) // must be Unix time to make sense to user
atomic.Store64(&memstats.last_gc_nanotime, uint64(now)) // monotonic time for us
memstats.pause_ns[memstats.numgc%uint32(len(memstats.pause_ns))] = uint64(work.pauseNS)
memstats.pause_end[memstats.numgc%uint32(len(memstats.pause_end))] = uint64(unixNow)
memstats.pause_total_ns += uint64(work.pauseNS)
// Update work.totaltime.
sweepTermCpu := int64(work.stwprocs) * (work.tMark - work.tSweepTerm)
// We report idle marking time below, but omit it from the
// overall utilization here since it's "free".
markCpu := gcController.assistTime + gcController.dedicatedMarkTime + gcController.fractionalMarkTime
markTermCpu := int64(work.stwprocs) * (work.tEnd - work.tMarkTerm)
cycleCpu := sweepTermCpu + markCpu + markTermCpu
work.totaltime += cycleCpu
// Compute overall GC CPU utilization.
totalCpu := sched.totaltime + (now-sched.procresizetime)*int64(gomaxprocs)
memstats.gc_cpu_fraction = float64(work.totaltime) / float64(totalCpu)
// Reset sweep state.
sweep.nbgsweep = 0
sweep.npausesweep = 0
if work.userForced {
// Bump GC cycle count and wake goroutines waiting on sweep.
// Finish the current heap profiling cycle and start a new
// heap profiling cycle. We do this before starting the world
// so events don't leak into the wrong cycle.
systemstack(func() { startTheWorldWithSema(true) })
// Flush the heap profile so we can start a new cycle next GC.
// This is relatively expensive, so we don't do it with the
// world stopped.
// Prepare workbufs for freeing by the sweeper. We do this
// asynchronously because it can take non-trivial time.
// Ensure all mcaches are flushed. Each P will flush its own
// mcache before allocating, but idle Ps may not. Since this
// is necessary to sweep all spans, we need to ensure all
// mcaches are flushed before we start the next GC cycle.
systemstack(func() {
forEachP(func(_p_ *p) {
// Print gctrace before dropping worldsema. As soon as we drop
// worldsema another cycle could start and smash the stats
// we're trying to print.
if debug.gctrace > 0 {
util := int(memstats.gc_cpu_fraction * 100)
var sbuf [24]byte
print("gc ", memstats.numgc,
" @", string(itoaDiv(sbuf[:], uint64(work.tSweepTerm-runtimeInitTime)/1e6, 3)), "s ",
util, "%: ")
prev := work.tSweepTerm
for i, ns := range []int64{work.tMark, work.tMarkTerm, work.tEnd} {
if i != 0 {
print(string(fmtNSAsMS(sbuf[:], uint64(ns-prev))))
prev = ns
print(" ms clock, ")
for i, ns := range []int64{sweepTermCpu, gcController.assistTime, gcController.dedicatedMarkTime + gcController.fractionalMarkTime, gcController.idleMarkTime, markTermCpu} {
if i == 2 || i == 3 {
// Separate mark time components with /.
} else if i != 0 {
print(string(fmtNSAsMS(sbuf[:], uint64(ns))))
print(" ms cpu, ",
work.heap0>>20, "->", work.heap1>>20, "->", work.heap2>>20, " MB, ",
work.heapGoal>>20, " MB goal, ",
work.maxprocs, " P")
if work.userForced {
print(" (forced)")
// Careful: another GC cycle may start now.
mp = nil
// now that gc is done, kick off finalizer thread if needed
if !concurrentSweep {
// give the queued finalizers, if any, a chance to run
// gcBgMarkStartWorkers prepares background mark worker goroutines.
// These goroutines will not run until the mark phase, but they must
// be started while the work is not stopped and from a regular G
// stack. The caller must hold worldsema.
func gcBgMarkStartWorkers() {
// Background marking is performed by per-P G's. Ensure that
// each P has a background GC G.
for _, p := range allp {
if p.gcBgMarkWorker == 0 {
go gcBgMarkWorker(p)
notetsleepg(&work.bgMarkReady, -1)
// gcBgMarkPrepare sets up state for background marking.
// Mutator assists must not yet be enabled.
func gcBgMarkPrepare() {
// Background marking will stop when the work queues are empty
// and there are no more workers (note that, since this is
// concurrent, this may be a transient state, but mark
// termination will clean it up). Between background workers
// and assists, we don't really know how many workers there
// will be, so we pretend to have an arbitrarily large number
// of workers, almost all of which are "waiting". While a
// worker is working it decrements nwait. If nproc == nwait,
// there are no workers.
work.nproc = ^uint32(0)
work.nwait = ^uint32(0)
func gcBgMarkWorker(_p_ *p) {
gp := getg()
type parkInfo struct {
m muintptr // Release this m on park.
attach puintptr // If non-nil, attach to this p on park.
// We pass park to a gopark unlock function, so it can't be on
// the stack (see gopark). Prevent deadlock from recursively
// starting GC by disabling preemption.
gp.m.preemptoff = "GC worker init"
park := new(parkInfo)
gp.m.preemptoff = ""
// Inform gcBgMarkStartWorkers that this worker is ready.
// After this point, the background mark worker is scheduled
// cooperatively by gcController.findRunnable. Hence, it must
// never be preempted, as this would put it into _Grunnable
// and put it on a run queue. Instead, when the preempt flag
// is set, this puts itself into _Gwaiting to be woken up by
// gcController.findRunnable at the appropriate time.
for {
// Go to sleep until woken by gcController.findRunnable.
// We can't releasem yet since even the call to gopark
// may be preempted.
gopark(func(g *g, parkp unsafe.Pointer) bool {
park := (*parkInfo)(parkp)
// The worker G is no longer running, so it's
// now safe to allow preemption.
// If the worker isn't attached to its P,
// attach now. During initialization and after
// a phase change, the worker may have been
// running on a different P. As soon as we
// attach, the owner P may schedule the
// worker, so this must be done after the G is
// stopped.
if park.attach != 0 {
p := park.attach.ptr()
// cas the worker because we may be
// racing with a new worker starting
// on this P.
if !p.gcBgMarkWorker.cas(0, guintptr(unsafe.Pointer(g))) {
// The P got a new worker.
// Exit this worker.
return false
return true
}, unsafe.Pointer(park), waitReasonGCWorkerIdle, traceEvGoBlock, 0)
// Loop until the P dies and disassociates this
// worker (the P may later be reused, in which case
// it will get a new worker) or we failed to associate.
if _p_.gcBgMarkWorker.ptr() != gp {
// Disable preemption so we can use the gcw. If the
// scheduler wants to preempt us, we'll stop draining,
// dispose the gcw, and then preempt.
if gcBlackenEnabled == 0 {
throw("gcBgMarkWorker: blackening not enabled")
startTime := nanotime()
_p_.gcMarkWorkerStartTime = startTime
decnwait := atomic.Xadd(&work.nwait, -1)
if decnwait == work.nproc {
println("runtime: work.nwait=", decnwait, "work.nproc=", work.nproc)
throw("work.nwait was > work.nproc")
systemstack(func() {
// Mark our goroutine preemptible so its stack
// can be scanned. This lets two mark workers
// scan each other (otherwise, they would
// deadlock). We must not modify anything on
// the G stack. However, stack shrinking is
// disabled for mark workers, so it is safe to
// read from the G stack.
casgstatus(gp, _Grunning, _Gwaiting)
switch _p_.gcMarkWorkerMode {
throw("gcBgMarkWorker: unexpected gcMarkWorkerMode")
case gcMarkWorkerDedicatedMode:
gcDrain(&_p_.gcw, gcDrainUntilPreempt|gcDrainFlushBgCredit)
if gp.preempt {
// We were preempted. This is
// a useful signal to kick
// everything out of the run
// queue so it can run
// somewhere else.
for {
gp, _ := runqget(_p_)
if gp == nil {
// Go back to draining, this time
// without preemption.
gcDrain(&_p_.gcw, gcDrainFlushBgCredit)
case gcMarkWorkerFractionalMode:
gcDrain(&_p_.gcw, gcDrainFractional|gcDrainUntilPreempt|gcDrainFlushBgCredit)
case gcMarkWorkerIdleMode:
gcDrain(&_p_.gcw, gcDrainIdle|gcDrainUntilPreempt|gcDrainFlushBgCredit)
casgstatus(gp, _Gwaiting, _Grunning)
// Account for time.
duration := nanotime() - startTime
switch _p_.gcMarkWorkerMode {
case gcMarkWorkerDedicatedMode:
atomic.Xaddint64(&gcController.dedicatedMarkTime, duration)
atomic.Xaddint64(&gcController.dedicatedMarkWorkersNeeded, 1)
case gcMarkWorkerFractionalMode:
atomic.Xaddint64(&gcController.fractionalMarkTime, duration)
atomic.Xaddint64(&_p_.gcFractionalMarkTime, duration)
case gcMarkWorkerIdleMode:
atomic.Xaddint64(&gcController.idleMarkTime, duration)
// Was this the last worker and did we run out
// of work?
incnwait := atomic.Xadd(&work.nwait, +1)
if incnwait > work.nproc {
println("runtime: p.gcMarkWorkerMode=", _p_.gcMarkWorkerMode,
"work.nwait=", incnwait, "work.nproc=", work.nproc)
throw("work.nwait > work.nproc")
// If this worker reached a background mark completion
// point, signal the main GC goroutine.
if incnwait == work.nproc && !gcMarkWorkAvailable(nil) {
// Make this G preemptible and disassociate it
// as the worker for this P so
// findRunnableGCWorker doesn't try to
// schedule it.
// Disable preemption and prepare to reattach
// to the P.
// We may be running on a different P at this
// point, so we can't reattach until this G is
// parked.
// gcMarkWorkAvailable reports whether executing a mark worker
// on p is potentially useful. p may be nil, in which case it only
// checks the global sources of work.
func gcMarkWorkAvailable(p *p) bool {
if p != nil && !p.gcw.empty() {
return true
if !work.full.empty() {
return true // global work available
if work.markrootNext < work.markrootJobs {
return true // root scan work available
return false
// gcMark runs the mark (or, for concurrent GC, mark termination)
// All gcWork caches must be empty.
// STW is in effect at this point.
func gcMark(start_time int64) {
if debug.allocfreetrace > 0 {
if gcphase != _GCmarktermination {
throw("in gcMark expecting to see gcphase as _GCmarktermination")
work.tstart = start_time
// Check that there's no marking work remaining.
if work.full != 0 || work.markrootNext < work.markrootJobs {
print("runtime: full=", hex(work.full), " next=", work.markrootNext, " jobs=", work.markrootJobs, " nDataRoots=", work.nDataRoots, " nSpanRoots=", work.nSpanRoots, " nStackRoots=", work.nStackRoots, "\n")
panic("non-empty mark queue after concurrent mark")
if debug.gccheckmark > 0 {
// This is expensive when there's a large number of
// Gs, so only do it if checkmark is also enabled.
if work.full != 0 {
throw("work.full != 0")
// Clear out buffers and double-check that all gcWork caches
// are empty. This should be ensured by gcMarkDone before we
// enter mark termination.
// TODO: We could clear out buffers just before mark if this
// has a non-negligible impact on STW time.
for _, p := range allp {
// The write barrier may have buffered pointers since
// the gcMarkDone barrier. However, since the barrier
// ensured all reachable objects were marked, all of
// these must be pointers to black objects. Hence we
// can just discard the write barrier buffer.
if debug.gccheckmark > 0 || throwOnGCWork {
// For debugging, flush the buffer and make
// sure it really was all marked.
} else {
gcw := &p.gcw
if !gcw.empty() {
print("runtime: P ",, " flushedWork ", gcw.flushedWork)
if gcw.wbuf1 == nil {
print(" wbuf1=<nil>")
} else {
print(" wbuf1.n=", gcw.wbuf1.nobj)
if gcw.wbuf2 == nil {
print(" wbuf2=<nil>")
} else {
print(" wbuf2.n=", gcw.wbuf2.nobj)
throw("P has cached GC work at end of mark termination")
// There may still be cached empty buffers, which we
// need to flush since we're going to free them. Also,
// there may be non-zero stats because we allocated
// black after the gcMarkDone barrier.
throwOnGCWork = false
// Update the marked heap stat.
memstats.heap_marked = work.bytesMarked
// Update other GC heap size stats. This must happen after
// cachestats (which flushes local statistics to these) and
// flushallmcaches (which modifies heap_live).
memstats.heap_live = work.bytesMarked
memstats.heap_scan = uint64(gcController.scanWork)
if trace.enabled {
// gcSweep must be called on the system stack because it acquires the heap
// lock. See mheap for details.
// The world must be stopped.
func gcSweep(mode gcMode) {
if gcphase != _GCoff {
throw("gcSweep being done but phase is not GCoff")
mheap_.sweepgen += 2
mheap_.sweepdone = 0
if !go115NewMCentralImpl && mheap_.sweepSpans[mheap_.sweepgen/2%2].index != 0 {
// We should have drained this list during the last
// sweep phase. We certainly need to start this phase
// with an empty swept list.
throw("non-empty swept list")
mheap_.pagesSwept = 0
mheap_.sweepArenas = mheap_.allArenas
mheap_.reclaimIndex = 0
mheap_.reclaimCredit = 0
if go115NewMCentralImpl {
if !_ConcurrentSweep || mode == gcForceBlockMode {
// Special case synchronous sweep.
// Record that no proportional sweeping has to happen.
mheap_.sweepPagesPerByte = 0
// Sweep all spans eagerly.
for sweepone() != ^uintptr(0) {
// Free workbufs eagerly.
for freeSomeWbufs(false) {
// All "free" events for this mark/sweep cycle have
// now happened, so we can make this profile cycle
// available immediately.
// Background sweep.
if sweep.parked {
sweep.parked = false
ready(sweep.g, 0, true)
// gcResetMarkState resets global state prior to marking (concurrent
// or STW) and resets the stack scan state of all Gs.
// This is safe to do without the world stopped because any Gs created
// during or after this will start out in the reset state.
// gcResetMarkState must be called on the system stack because it acquires
// the heap lock. See mheap for details.
func gcResetMarkState() {
// This may be called during a concurrent phase, so make sure
// allgs doesn't change.
for _, gp := range allgs {
gp.gcscandone = false // set to true in gcphasework
gp.gcAssistBytes = 0
// Clear page marks. This is just 1MB per 64GB of heap, so the
// time here is pretty trivial.
arenas := mheap_.allArenas
for _, ai := range arenas {
ha := mheap_.arenas[ai.l1()][ai.l2()]
for i := range ha.pageMarks {
ha.pageMarks[i] = 0
work.bytesMarked = 0
work.initialHeapLive = atomic.Load64(&memstats.heap_live)
// Hooks for other packages
var poolcleanup func()
//go:linkname sync_runtime_registerPoolCleanup sync.runtime_registerPoolCleanup
func sync_runtime_registerPoolCleanup(f func()) {
poolcleanup = f
func clearpools() {
// clear sync.Pools
if poolcleanup != nil {
// Clear central sudog cache.
// Leave per-P caches alone, they have strictly bounded size.
// Disconnect cached list before dropping it on the floor,
// so that a dangling ref to one entry does not pin all of them.
var sg, sgnext *sudog
for sg = sched.sudogcache; sg != nil; sg = sgnext {
sgnext = = nil
sched.sudogcache = nil
// Clear central defer pools.
// Leave per-P pools alone, they have strictly bounded size.
// disconnect cached list before dropping it on the floor,
// so that a dangling ref to one entry does not pin all of them.
var d, dlink *_defer
for d = sched.deferpool; d != nil; d = dlink {
dlink = = nil
sched.deferpool = nil
// Timing
// itoaDiv formats val/(10**dec) into buf.
func itoaDiv(buf []byte, val uint64, dec int) []byte {
i := len(buf) - 1
idec := i - dec
for val >= 10 || i >= idec {
buf[i] = byte(val%10 + '0')
if i == idec {
buf[i] = '.'
val /= 10
buf[i] = byte(val + '0')
return buf[i:]
// fmtNSAsMS nicely formats ns nanoseconds as milliseconds.
func fmtNSAsMS(buf []byte, ns uint64) []byte {
if ns >= 10e6 {
// Format as whole milliseconds.
return itoaDiv(buf, ns/1e6, 0)
// Format two digits of precision, with at most three decimal places.
x := ns / 1e3
if x == 0 {
buf[0] = '0'
return buf[:1]
dec := 3
for x >= 100 {
x /= 10
return itoaDiv(buf, x, dec)