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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.
// TODO(rsc): The code having to do with the heap bitmap needs very serious cleanup.
// It has gotten completely out of control.
// 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.
// TODO(austin): The rest of this comment is woefully out of date and
// needs to be rewritten. There is no distinct scan phase any more and
// we allocate black during GC.
// 0. Set phase = GCscan from GCoff.
// 1. Wait for all P's to acknowledge phase change.
// At this point all goroutines have passed through a GC safepoint and
// know we are in the GCscan phase.
// 2. GC scans all goroutine stacks, mark and enqueues all encountered pointers
// (marking avoids most duplicate enqueuing but races may produce benign duplication).
// Preempted goroutines are scanned before P schedules next goroutine.
// 3. Set phase = GCmark.
// 4. Wait for all P's to acknowledge phase change.
// 5. Now write barrier marks and enqueues black, grey, or white to white pointers.
// Malloc still allocates white (non-marked) objects.
// 6. Meanwhile GC transitively walks the heap marking reachable objects.
// 7. When GC finishes marking heap, it preempts P's one-by-one and
// retakes partial wbufs (filled by write barrier or during a stack scan of the goroutine
// currently scheduled on the P).
// 8. Once the GC has exhausted all available marking work it sets phase = marktermination.
// 9. Wait for all P's to acknowledge phase change.
// 10. Malloc now allocates black objects, so number of unmarked reachable objects
// monotonically decreases.
// 11. GC preempts P's one-by-one taking partial wbufs and marks all unmarked yet
// reachable objects.
// 12. When GC completes a full cycle over P's and discovers no new grey
// objects, (which means all reachable objects are marked) set phase = GCoff.
// 13. Wait for all P's to acknowledge phase change.
// 14. Now malloc allocates white (but sweeps spans before use).
// Write barrier becomes nop.
// 15. GC does background sweeping, see description below.
// 16. When sufficient allocation has taken place replay the sequence starting at 0 above,
// see discussion of GC rate below.
// Changing phases.
// Phases are changed by setting the gcphase to the next phase and possibly calling ackgcphase.
// All phase action must be benign in the presence of a change.
// Starting with GCoff
// GCoff to GCscan
// GSscan scans stacks and globals greying them and never marks an object black.
// Once all the P's are aware of the new phase they will scan gs on preemption.
// This means that the scanning of preempted gs can't start until all the Ps
// have acknowledged.
// When a stack is scanned, this phase also installs stack barriers to
// track how much of the stack has been active.
// This transition enables write barriers because stack barriers
// assume that writes to higher frames will be tracked by write
// barriers. Technically this only needs write barriers for writes
// to stack slots, but we enable write barriers in general.
// GCscan to GCmark
// In GCmark, work buffers are drained until there are no more
// pointers to scan.
// No scanning of objects (making them black) can happen until all
// Ps have enabled the write barrier, but that already happened in
// the transition to GCscan.
// GCmark to GCmarktermination
// The only change here is that we start allocating black so the Ps must acknowledge
// the change before we begin the termination algorithm
// GCmarktermination to GSsweep
// Object currently on the freelist must be marked black for this to work.
// Are things on the free lists black or white? How does the sweep phase work?
// 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).
package runtime
import (
const (
_DebugGC = 0
_ConcurrentSweep = true
_FinBlockSize = 4 * 1024
// sweepMinHeapDistance is a lower bound on the heap distance
// (in bytes) reserved for concurrent sweeping between GC
// cycles. This will be scaled by gcpercent/100.
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")
_ = setGCPercent(readgogc())
for datap := &firstmoduledata; datap != nil; datap = {
datap.gcdatamask = progToPointerMask((*byte)(unsafe.Pointer(datap.gcdata)),
datap.gcbssmask = progToPointerMask((*byte)(unsafe.Pointer(datap.gcbss)), datap.ebss-datap.bss)
memstats.next_gc = heapminimum
work.startSema = 1
work.markDoneSema = 1
func readgogc() int32 {
p := gogetenv("GOGC")
if p == "" {
return 100
if p == "off" {
return -1
return int32(atoi(p))
// 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 and enables GC.
func gcenable() {
c := make(chan int, 1)
go bgsweep(c)
memstats.enablegc = true // now that runtime is initialized, GC is okay
//go:linkname setGCPercent runtime/debug.setGCPercent
func setGCPercent(in int32) (out int32) {
out = gcpercent
if in < 0 {
in = -1
gcpercent = in
heapminimum = defaultHeapMinimum * uint64(gcpercent) / 100
if gcController.triggerRatio > float64(gcpercent)/100 {
gcController.triggerRatio = float64(gcpercent) / 100
return out
// Garbage collector phase.
// Indicates to write barrier and sychronization task to preform.
var gcphase uint32
// The compiler knows about this variable.
// If you change it, you must change the compiler too.
var writeBarrier struct {
enabled bool // compiler emits a check of this before calling write barrier
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
// gcBlackenPromptly indicates that optimizations that may
// hide work from the global work queue should be disabled.
// If gcBlackenPromptly is true, per-P gcWork caches should
// be flushed immediately and new objects should be allocated black.
// There is a tension between allocating objects white and
// allocating them black. If white and the objects die before being
// marked they can be collected during this GC cycle. On the other
// hand allocating them black will reduce _GCmarktermination latency
// since more work is done in the mark phase. This tension is resolved
// by allocating white until the mark phase is approaching its end and
// then allocating black for the remainder of the mark phase.
var gcBlackenPromptly bool
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*gcGoalUtilization 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*gcGoalUtilization.
// 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.
// 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.next_gc
// 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
var gcController = gcControllerState{
// Initial trigger ratio guess.
triggerRatio: 7 / 8.0,
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
// heapGoal is the goal memstats.heap_live for when this cycle
// ends. This is computed at the beginning of each cycle.
heapGoal uint64
// 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.
// For example, if the overall mark utilization goal is 25%
// and GOMAXPROCS is 6, one P will be a dedicated mark worker
// and this will be set to 0.5 so that 50% of the time some P
// is in a fractional mark worker. This is computed at the
// beginning of each cycle.
fractionalUtilizationGoal float64
// triggerRatio is the heap growth ratio at which the garbage
// collection cycle should start. 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 is updated
// at the end of of each cycle.
triggerRatio float64
_ [sys.CacheLineSize]byte
// fractionalMarkWorkersNeeded is the number of fractional
// mark workers that need to be started. This is either 0 or
// 1. This is potentially updated atomically at every
// scheduling point (hence it gets its own cache line).
fractionalMarkWorkersNeeded int64
_ [sys.CacheLineSize]byte
// 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
// If this is the first GC cycle or we're operating on a very
// small heap, fake heap_marked so it looks like next_gc is
// the appropriate growth from heap_marked, even though the
// real heap_marked may not have a meaningful value (on the
// first cycle) or may be much smaller (resulting in a large
// error response).
if memstats.next_gc <= heapminimum {
memstats.heap_marked = uint64(float64(memstats.next_gc) / (1 + c.triggerRatio))
memstats.heap_reachable = memstats.heap_marked
// Compute the heap goal for this cycle
c.heapGoal = memstats.heap_reachable + memstats.heap_reachable*uint64(gcpercent)/100
// 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 next_gc is large or if the trigger is really close to
// GOGC. Assist is proportional to this distance, so enforce a
// minimum distance, even if it means going over the GOGC goal
// by a tiny bit.
if c.heapGoal < memstats.heap_live+1024*1024 {
c.heapGoal = memstats.heap_live + 1024*1024
// Compute the total mark utilization goal and divide it among
// dedicated and fractional workers.
totalUtilizationGoal := float64(gomaxprocs) * gcGoalUtilization
c.dedicatedMarkWorkersNeeded = int64(totalUtilizationGoal)
c.fractionalUtilizationGoal = totalUtilizationGoal - float64(c.dedicatedMarkWorkersNeeded)
if c.fractionalUtilizationGoal > 0 {
c.fractionalMarkWorkersNeeded = 1
} else {
c.fractionalMarkWorkersNeeded = 0
// Clear per-P state
for _, p := range &allp {
if p == nil {
p.gcAssistTime = 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, "->",
c.heapGoal>>20, " MB)",
" workers=", c.dedicatedMarkWorkersNeeded,
"+", c.fractionalMarkWorkersNeeded, "\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 or memstats.heap_live 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).
// TODO: Consider removing the periodic controller update altogether.
// Since we switched to allocating black, in theory we shouldn't have
// to change the assist ratio. However, this is still a useful hook
// that we've found many uses for when experimenting.
func (c *gcControllerState) revise() {
// Compute the expected scan work remaining.
// Note that we currently count allocations during GC as both
// scannable heap (heap_scan) and scan work completed
// (scanWork), so this difference won't be changed by
// allocations during GC.
// This particular estimate is a strict upper bound on the
// possible remaining scan work for the current heap.
// You might consider dividing this by 2 (or by
// (100+GOGC)/100) to counter this over-estimation, but
// benchmarks show that this has almost no effect on mean
// mutator utilization, heap size, or assist time and it
// introduces the danger of under-estimating and letting the
// mutator outpace the garbage collector.
scanWorkExpected := int64(memstats.heap_scan) - c.scanWork
if scanWorkExpected < 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 expected scan work
// negative.
scanWorkExpected = 1000
// Compute the heap distance remaining.
heapDistance := int64(c.heapGoal) - int64(memstats.heap_live)
if heapDistance <= 0 {
// This shouldn't happen, but if it does, avoid
// dividing by zero or setting the assist negative.
heapDistance = 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(scanWorkExpected) / float64(heapDistance)
c.assistBytesPerWork = float64(heapDistance) / float64(scanWorkExpected)
// endCycle updates the GC controller state at the end of the
// concurrent part of the GC cycle.
func (c *gcControllerState) endCycle() {
h_t := c.triggerRatio // For debugging
// 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.
// TODO(austin): next_gc is based on heap_reachable, not
// heap_marked, which means the actual growth ratio
// technically isn't comparable to the trigger ratio.
goalGrowthRatio := float64(gcpercent) / 100
actualGrowthRatio := float64(memstats.heap_live)/float64(memstats.heap_marked) - 1
assistDuration := nanotime() - c.markStartTime
// Assume background mark hit its utilization goal.
utilization := gcGoalUtilization
// Add assist utilization; avoid divide by zero.
if assistDuration > 0 {
utilization += float64(c.assistTime) / float64(assistDuration*int64(gomaxprocs))
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.
c.triggerRatio += triggerGain * triggerError
if c.triggerRatio < 0 {
// This can happen if the mutator is allocating very
// quickly or the GC is scanning very slowly.
c.triggerRatio = 0
} else if c.triggerRatio > goalGrowthRatio*0.95 {
// Ensure there's always a little margin so that the
// mutator assist ratio isn't infinity.
c.triggerRatio = goalGrowthRatio * 0.95
if debug.gcpacertrace > 0 {
// Print controller state in terms of the design
// document.
H_m_prev := memstats.heap_marked
H_T := memstats.next_gc
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,
// 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 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(fastrand1() % 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
// TODO(austin): This P isn't going to run anything
// else for a while, so kick everything out of its run
// queue.
} else {
if !decIfPositive(&c.fractionalMarkWorkersNeeded) {
// No more workers are need right now.
return nil
// This P has picked the token for the fractional worker.
// Is the GC currently under or at the utilization goal?
// If so, do more work.
// We used to check whether doing one time slice of work
// would remain under the utilization goal, but that has the
// effect of delaying work until the mutator has run for
// enough time slices to pay for the work. During those time
// slices, write barriers are enabled, so the mutator is running slower.
// Now instead we do the work whenever we're under or at the
// utilization work and pay for it by letting the mutator run later.
// This doesn't change the overall utilization averages, but it
// front loads the GC work so that the GC finishes earlier and
// write barriers can be turned off sooner, effectively giving
// the mutator a faster machine.
// The old, slower behavior can be restored by setting
// gcForcePreemptNS = forcePreemptNS.
const gcForcePreemptNS = 0
// TODO(austin): We could fast path this and basically
// eliminate contention on c.fractionalMarkWorkersNeeded by
// precomputing the minimum time at which it's worth
// next scheduling the fractional worker. Then Ps
// don't have to fight in the window where we've
// passed that deadline and no one has started the
// worker yet.
// TODO(austin): Shorter preemption interval for mark
// worker to improve fairness and give this
// finer-grained control over schedule?
now := nanotime() - gcController.markStartTime
then := now + gcForcePreemptNS
timeUsed := c.fractionalMarkTime + gcForcePreemptNS
if then > 0 && float64(timeUsed)/float64(then) > c.fractionalUtilizationGoal {
// Nope, we'd overshoot the utilization goal
atomic.Xaddint64(&c.fractionalMarkWorkersNeeded, +1)
return nil
_p_.gcMarkWorkerMode = gcMarkWorkerFractionalMode
// Run the background mark worker
gp := _p_.gcBgMarkWorker.ptr()
casgstatus(gp, _Gwaiting, _Grunnable)
if trace.enabled {
traceGoUnpark(gp, 0)
return gp
// gcGoalUtilization is the goal CPU utilization for background
// marking as a fraction of GOMAXPROCS.
const gcGoalUtilization = 0.25
// gcCreditSlack is the amount of scan work credit that can 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 uint64 // lock-free list of full blocks workbuf
empty uint64 // lock-free list of empty blocks workbuf
pad0 [sys.CacheLineSize]uint8 // prevents false-sharing between full/empty and nproc/nwait
markrootNext uint32 // next markroot job
markrootJobs uint32 // number of markroot jobs
nproc uint32
tstart int64
nwait uint32
ndone uint32
alldone note
// Number of roots of various root types. Set by gcMarkRootPrepare.
nDataRoots, nBSSRoots, nSpanRoots, nStackRoots, nRescanRoots int
// markrootDone indicates that roots have been marked at least
// once during the current GC cycle. This is checked by root
// marking operations that have to happen only during the
// first root marking pass, whether that's during the
// concurrent mark phase in current GC or mark termination in
// STW GC.
markrootDone bool
// 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 1 to mark 2 and
// from mark 2 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
// Copy of mheap.allspans for marker or sweeper.
spans []*mspan
// totaltime is the CPU nanoseconds spent in GC since the
// program started if debug.gctrace > 0.
totaltime int64
// 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.
bytesMarked uint64
// 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
head, tail guintptr
// rescan is a list of G's that need to be rescanned during
// mark termination. A G adds itself to this list when it
// first invalidates its stack scan.
rescan struct {
lock mutex
list []guintptr
// 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() {
gcStart(gcForceBlockMode, false)
// 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
// gcShouldStart returns true if the exit condition for the _GCoff
// phase has been met. The exit condition should be tested when
// allocating.
// If forceTrigger is true, it ignores the current heap size, but
// checks all other conditions. In general this should be false.
func gcShouldStart(forceTrigger bool) bool {
return gcphase == _GCoff && (forceTrigger || memstats.heap_live >= memstats.next_gc) && memstats.enablegc && panicking == 0 && gcpercent >= 0
// gcStart transitions the GC from _GCoff to _GCmark (if mode ==
// gcBackgroundMode) or _GCmarktermination (if mode !=
// gcBackgroundMode) by performing sweep termination and GC
// initialization.
// This may return without performing this transition in some cases,
// such as when called on a system stack or with locks held.
func gcStart(mode gcMode, forceTrigger bool) {
// 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 (mode != gcBackgroundMode || gcShouldStart(forceTrigger)) && gosweepone() != ^uintptr(0) {
// Perform GC initialization and the sweep termination
// transition.
// If this is a forced GC, don't acquire the transition lock
// or re-check the transition condition because we
// specifically *don't* want to share the transition with
// another thread.
useStartSema := mode == gcBackgroundMode
if useStartSema {
semacquire(&work.startSema, false)
// Re-check transition condition under transition lock.
if !gcShouldStart(forceTrigger) {
// 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.
if mode == gcBackgroundMode {
if debug.gcstoptheworld == 1 {
mode = gcForceMode
} else if debug.gcstoptheworld == 2 {
mode = gcForceBlockMode
// Ok, we're doing it! Stop everybody else
semacquire(&worldsema, false)
if trace.enabled {
if mode == gcBackgroundMode {
now := nanotime()
work.stwprocs, work.maxprocs = gcprocs(), gomaxprocs
work.tSweepTerm = now
work.heap0 = memstats.heap_live
work.pauseNS = 0
work.mode = mode
work.pauseStart = now
// 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.
if mode == gcBackgroundMode { // Do as much work concurrently as possible
work.heapGoal = gcController.heapGoal
// 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.
// It's necessary to enable write barriers
// during the scan phase for several reasons:
// They must be enabled for writes to higher
// stack frames before we scan stacks and
// install stack barriers because this is how
// we track writes to inactive stack frames.
// (Alternatively, we could not install stack
// barriers over frame boundaries with
// up-pointers).
// They 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.
// markrootSpans uses work.spans, so make sure
// it is up to date.
gcBgMarkPrepare() // Must happen before assist enable.
// 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
// Concurrent mark.
now = nanotime()
work.pauseNS += now - work.pauseStart
work.tMark = now
} else {
t := nanotime()
work.tMark, work.tMarkTerm = t, t
work.heapGoal = work.heap0
// Perform mark termination. This will restart the world.
if useStartSema {
// gcMarkDone transitions the GC from mark 1 to mark 2 and from mark 2
// to mark termination.
// This should be called when all mark work has been drained. In mark
// 1, this includes all root marking jobs, global work buffers, and
// active work buffers in assists and background workers; however,
// work may still be cached in per-P work buffers. In mark 2, per-P
// caches are disabled.
// The calling context must be preemptible.
// Note that it is explicitly okay to have write barriers in this
// function because completion of concurrent mark is best-effort
// anyway. Any work created by write barriers here will be cleaned up
// by mark termination.
func gcMarkDone() {
semacquire(&work.markDoneSema, false)
// Re-check transition condition under transition lock.
if !(gcphase == _GCmark && work.nwait == work.nproc && !gcMarkWorkAvailable(nil)) {
// Disallow starting new workers so that any remaining workers
// in the current mark phase will drain out.
// TODO(austin): Should dedicated workers keep an eye on this
// and exit gcDrain promptly?
atomic.Xaddint64(&gcController.dedicatedMarkWorkersNeeded, -0xffffffff)
atomic.Xaddint64(&gcController.fractionalMarkWorkersNeeded, -0xffffffff)
if !gcBlackenPromptly {
// Transition from mark 1 to mark 2.
// The global work list is empty, but there can still be work
// sitting in the per-P work caches and there can be more
// objects reachable from global roots since they don't have write
// barriers. Rescan some roots and flush work caches.
// Disallow caching workbufs and indicate that we're in mark 2.
gcBlackenPromptly = true
// Prevent completion of mark 2 until we've flushed
// cached workbufs.
atomic.Xadd(&work.nwait, -1)
// GC is set up for mark 2. Let Gs blocked on the
// transition lock go while we flush caches.
systemstack(func() {
// Flush all currently cached workbufs and
// ensure all Ps see gcBlackenPromptly. This
// also blocks until any remaining mark 1
// workers have exited their loop so we can
// start new mark 2 workers that will observe
// the new root marking jobs.
forEachP(func(_p_ *p) {
// Now we can start up mark 2 workers.
atomic.Xaddint64(&gcController.dedicatedMarkWorkersNeeded, 0xffffffff)
atomic.Xaddint64(&gcController.fractionalMarkWorkersNeeded, 0xffffffff)
incnwait := atomic.Xadd(&work.nwait, +1)
if incnwait == work.nproc && !gcMarkWorkAvailable(nil) {
// This loop will make progress because
// gcBlackenPromptly is now true, so it won't
// take this same "if" branch.
goto top
} else {
// Transition to mark termination.
now := nanotime()
work.tMarkTerm = now
work.pauseStart = now
getg().m.preemptoff = "gcing"
// 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.
// Record that one root marking pass has completed.
work.markrootDone = true
// Disable assists and background workers. We must do
// this before waking blocked assists.
atomic.Store(&gcBlackenEnabled, 0)
// Flush the gcWork caches. This must be done before
// endCycle since endCycle depends on statistics kept
// in these caches.
// 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.
// Perform mark termination. This will restart the world.
func gcMarkTermination() {
// World is stopped.
// Start marktermination which includes enabling the write barrier.
atomic.Store(&gcBlackenEnabled, 0)
gcBlackenPromptly = false
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 = "garbage collection"
// 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 stop-the-world mark using checkmark bits,
// to check that we didn't forget to mark anything during
// the concurrent mark process.
// marking is complete so we can turn the write barrier off
if debug.gctrace > 1 {
startTime = nanotime()
// The g stacks have been scanned so
// they have gcscanvalid==true and gcworkdone==true.
// Reset these so that all stacks will be rescanned.
// Still in STW but gcphase is _GCoff, reset to _GCmarktermination
// At this point all objects will be found during the gcMark which
// does a complete STW mark and object scan.
setGCPhase(_GCoff) // marking is done, turn off wb.
_g_.m.traceback = 0
casgstatus(gp, _Gwaiting, _Grunning)
if trace.enabled {
// all done
mp.preemptoff = ""
if gcphase != _GCoff {
throw("gc done but gcphase != _GCoff")
// Update timing memstats
now, unixNow := nanotime(), unixnanotime()
work.pauseNS += now - work.pauseStart
work.tEnd = now
atomic.Store64(&memstats.last_gc, uint64(unixNow)) // must be Unix time to make sense to user
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
// Free stack spans. This must be done between GC cycles.
// Best-effort remove stack barriers so they don't get in the
// way of things like GDB and perf.
myallgs := allgs
// 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.mode != gcBackgroundMode {
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 == nil || p.status == _Pdead {
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), "GC worker (idle)", 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()
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")
switch _p_.gcMarkWorkerMode {
throw("gcBgMarkWorker: unexpected gcMarkWorkerMode")
case gcMarkWorkerDedicatedMode:
gcDrain(&_p_.gcw, gcDrainNoBlock|gcDrainFlushBgCredit)
case gcMarkWorkerFractionalMode, gcMarkWorkerIdleMode:
gcDrain(&_p_.gcw, gcDrainUntilPreempt|gcDrainFlushBgCredit)
// If we are nearing the end of mark, dispose
// of the cache promptly. We must do this
// before signaling that we're no longer
// working so that other workers can't observe
// no workers and no work while we have this
// cached, and before we compute done.
if gcBlackenPromptly {
// 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(&gcController.fractionalMarkWorkersNeeded, 1)
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 returns true if 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 atomic.Load64(&work.full) != 0 {
return true // global work available
if work.markrootNext < work.markrootJobs {
return true // root scan work available
return false
// gcFlushGCWork disposes the gcWork caches of all Ps. The world must
// be stopped.
func gcFlushGCWork() {
// Gather all cached GC work. All other Ps are stopped, so
// it's safe to manipulate their GC work caches.
for i := 0; i < int(gomaxprocs); i++ {
// gcMark runs the mark (or, for concurrent GC, mark termination)
// STW is in effect at this point.
//TODO go:nowritebarrier
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
gcCopySpans() // TODO(rlh): should this be hoisted and done only once? Right now it is done for normal marking and also for checkmarking.
// Make sure the per-P gcWork caches are empty. During mark
// termination, these caches can still be used temporarily,
// but must be disposed to the global lists immediately.
// Queue root marking jobs.
work.nwait = 0
work.ndone = 0
work.nproc = uint32(gcprocs())
if trace.enabled {
if work.nproc > 1 {
gcw := &getg().m.p.ptr().gcw
gcDrain(gcw, gcDrainBlock)
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")
if work.nproc > 1 {
// Record that at least one root marking pass has completed.
work.markrootDone = true
for i := 0; i < int(gomaxprocs); i++ {
gcw := &allp[i].gcw
if !gcw.empty() {
throw("P has cached GC work at end of mark termination")
if gcw.scanWork != 0 || gcw.bytesMarked != 0 {
throw("P has unflushed stats at end of mark termination")
if trace.enabled {
// Update the reachable heap stat.
memstats.heap_reachable = work.bytesMarked
// Trigger the next GC cycle when the allocated heap has grown
// by triggerRatio over the reachable heap size. Assume that
// we're in steady state, so the reachable heap size is the
// same now as it was at the beginning of the GC cycle.
memstats.next_gc = uint64(float64(memstats.heap_reachable) * (1 + gcController.triggerRatio))
if memstats.next_gc < heapminimum {
memstats.next_gc = heapminimum
if int64(memstats.next_gc) < 0 {
print("next_gc=", memstats.next_gc, " bytesMarked=", work.bytesMarked, " heap_live=", memstats.heap_live, " initialHeapLive=", work.initialHeapLive, "\n")
throw("next_gc underflow")
// 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_marked = work.bytesMarked
memstats.heap_scan = uint64(gcController.scanWork)
minNextGC := memstats.heap_live + sweepMinHeapDistance*uint64(gcpercent)/100
if memstats.next_gc < minNextGC {
// The allocated heap is already past the trigger.
// This can happen if the triggerRatio is very low and
// the reachable heap estimate is less than the live
// heap size.
// Concurrent sweep happens in the heap growth from
// heap_live to next_gc, so bump next_gc up to ensure
// that concurrent sweep has some heap growth in which
// to perform sweeping before we start the next GC
// cycle.
memstats.next_gc = minNextGC
if trace.enabled {
func gcSweep(mode gcMode) {
if gcphase != _GCoff {
throw("gcSweep being done but phase is not GCoff")
mheap_.sweepgen += 2
mheap_.sweepdone = 0
sweep.spanidx = 0
if !_ConcurrentSweep || mode == gcForceBlockMode {
// Special case synchronous sweep.
// Record that no proportional sweeping has to happen.
mheap_.sweepPagesPerByte = 0
mheap_.pagesSwept = 0
// Sweep all spans eagerly.
for sweepone() != ^uintptr(0) {
// Do an additional mProf_GC, because all 'free' events are now real as well.
// 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.
heapDistance := int64(memstats.next_gc) - int64(memstats.heap_live)
// 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
mheap_.sweepPagesPerByte = float64(mheap_.pagesInUse) / float64(heapDistance)
mheap_.pagesSwept = 0
mheap_.spanBytesAlloc = 0
// Background sweep.
if sweep.parked {
sweep.parked = false
ready(sweep.g, 0, true)
func gcCopySpans() {
// Cache runtime.mheap_.allspans in work.spans to avoid conflicts with
// resizing/freeing allspans.
// New spans can be created while GC progresses, but they are not garbage for
// this round:
// - new stack spans can be created even while the world is stopped.
// - new malloc spans can be created during the concurrent sweep
// Even if this is stop-the-world, a concurrent exitsyscall can allocate a stack from heap.
// Free the old cached mark array if necessary.
if work.spans != nil && &work.spans[0] != &h_allspans[0] {
sysFree(unsafe.Pointer(&work.spans[0]), uintptr(len(work.spans))*unsafe.Sizeof(work.spans[0]), &memstats.other_sys)
// Cache the current array for sweeping.
mheap_.gcspans = mheap_.allspans
work.spans = h_allspans
// 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.
func gcResetMarkState() {
// This may be called during a concurrent phase, so make sure
// allgs doesn't change.
if !(gcphase == _GCoff || gcphase == _GCmarktermination) {
// Accessing gcRescan is unsafe.
throw("bad GC phase")
for _, gp := range allgs {
gp.gcscandone = false // set to true in gcphasework
gp.gcscanvalid = false // stack has not been scanned
gp.gcRescan = -1
gp.gcAssistBytes = 0
// Clear rescan list.
work.rescan.list = work.rescan.list[:0]
work.bytesMarked = 0
work.initialHeapLive = memstats.heap_live
work.markrootDone = false
// 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.
for i := range sched.deferpool {
// 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[i]; d != nil; d = dlink {
dlink = = nil
sched.deferpool[i] = nil
// Timing
func gchelper() {
_g_ := getg()
_g_.m.traceback = 2
if trace.enabled {
// Parallel mark over GC roots and heap
if gcphase == _GCmarktermination {
gcw := &_g_.m.p.ptr().gcw
gcDrain(gcw, gcDrainBlock) // blocks in getfull
if trace.enabled {
nproc := work.nproc // work.nproc can change right after we increment work.ndone
if atomic.Xadd(&work.ndone, +1) == nproc-1 {
_g_.m.traceback = 0
func gchelperstart() {
_g_ := getg()
if _g_.m.helpgc < 0 || _g_.m.helpgc >= _MaxGcproc {
throw("gchelperstart: bad m->helpgc")
if _g_ != _g_.m.g0 {
throw("gchelper not running on g0 stack")
// 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)