Source File
mgcpacer.go
Belonging Package
runtime
// Copyright 2021 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package runtime
import (
_ // for go:linkname
)
const (
// gcGoalUtilization is the goal CPU utilization for
// marking as a fraction of GOMAXPROCS.
//
// Increasing the goal utilization will shorten GC cycles as the GC
// has more resources behind it, lessening costs from the write barrier,
// but comes at the cost of increasing mutator latency.
gcGoalUtilization = gcBackgroundUtilization
// gcBackgroundUtilization is the fixed CPU utilization for background
// marking. It must be <= gcGoalUtilization. The difference between
// gcGoalUtilization and gcBackgroundUtilization will be made up by
// mark assists. The scheduler will aim to use within 50% of this
// goal.
//
// As a general rule, there's little reason to set gcBackgroundUtilization
// < gcGoalUtilization. One reason might be in mostly idle applications,
// where goroutines are unlikely to assist at all, so the actual
// utilization will be lower than the goal. But this is moot point
// because the idle mark workers already soak up idle CPU resources.
// These two values are still kept separate however because they are
// distinct conceptually, and in previous iterations of the pacer the
// distinction was more important.
gcBackgroundUtilization = 0.25
// gcCreditSlack is the amount of scan work credit that can
// accumulate locally before updating gcController.heapScanWork and,
// optionally, gcController.bgScanCredit. Lower values give a more
// accurate assist ratio and make it more likely that assists will
// successfully steal background credit. Higher values reduce memory
// contention.
gcCreditSlack = 2000
// gcAssistTimeSlack is the nanoseconds of mutator assist time that
// can accumulate on a P before updating gcController.assistTime.
gcAssistTimeSlack = 5000
// gcOverAssistWork determines how many extra units of scan work a GC
// assist does when an assist happens. This amortizes the cost of an
// assist by pre-paying for this many bytes of future allocations.
gcOverAssistWork = 64 << 10
// defaultHeapMinimum is the value of heapMinimum for GOGC==100.
defaultHeapMinimum = (goexperiment.HeapMinimum512KiBInt)*(512<<10) +
(1-goexperiment.HeapMinimum512KiBInt)*(4<<20)
// maxStackScanSlack is the bytes of stack space allocated or freed
// that can accumulate on a P before updating gcController.stackSize.
maxStackScanSlack = 8 << 10
// memoryLimitMinHeapGoalHeadroom is the minimum amount of headroom the
// pacer gives to the heap goal when operating in the memory-limited regime.
// That is, it'll reduce the heap goal by this many extra bytes off of the
// base calculation, at minimum.
memoryLimitMinHeapGoalHeadroom = 1 << 20
// memoryLimitHeapGoalHeadroomPercent is how headroom the memory-limit-based
// heap goal should have as a percent of the maximum possible heap goal allowed
// to maintain the memory limit.
memoryLimitHeapGoalHeadroomPercent = 3
)
// gcController implements the GC pacing controller that determines
// when to trigger concurrent garbage collection and how much marking
// work to do in mutator assists and background marking.
//
// It calculates the ratio between the allocation rate (in terms of CPU
// time) and the GC scan throughput to determine the heap size at which to
// trigger a GC cycle such that no GC assists are required to finish on time.
// This algorithm thus optimizes GC CPU utilization to the dedicated background
// mark utilization of 25% of GOMAXPROCS by minimizing GC assists.
// GOMAXPROCS. The high-level design of this algorithm is documented
// at https://github.com/golang/proposal/blob/master/design/44167-gc-pacer-redesign.md.
// See https://golang.org/s/go15gcpacing for additional historical context.
var gcController gcControllerState
type gcControllerState struct {
// Initialized from GOGC. GOGC=off means no GC.
gcPercent atomic.Int32
// memoryLimit is the soft memory limit in bytes.
//
// Initialized from GOMEMLIMIT. GOMEMLIMIT=off is equivalent to MaxInt64
// which means no soft memory limit in practice.
//
// This is an int64 instead of a uint64 to more easily maintain parity with
// the SetMemoryLimit API, which sets a maximum at MaxInt64. This value
// should never be negative.
memoryLimit atomic.Int64
// heapMinimum is the minimum heap size at which to trigger GC.
// For small heaps, this overrides the usual GOGC*live set rule.
//
// When there is a very small live set but a lot of allocation, simply
// collecting when the heap reaches GOGC*live results in many GC
// cycles and high total per-GC overhead. This minimum amortizes this
// per-GC overhead while keeping the heap reasonably small.
//
// During initialization this is set to 4MB*GOGC/100. In the case of
// GOGC==0, this will set heapMinimum to 0, resulting in constant
// collection even when the heap size is small, which is useful for
// debugging.
heapMinimum uint64
// runway is the amount of runway in heap bytes allocated by the
// application that we want to give the GC once it starts.
//
// This is computed from consMark during mark termination.
runway atomic.Uint64
// consMark is the estimated per-CPU consMark ratio for the application.
//
// It represents the ratio between the application's allocation
// rate, as bytes allocated per CPU-time, and the GC's scan rate,
// as bytes scanned per CPU-time.
// The units of this ratio are (B / cpu-ns) / (B / cpu-ns).
//
// At a high level, this value is computed as the bytes of memory
// allocated (cons) per unit of scan work completed (mark) in a GC
// cycle, divided by the CPU time spent on each activity.
//
// Updated at the end of each GC cycle, in endCycle.
consMark float64
// lastConsMark is the computed cons/mark value for the previous 4 GC
// cycles. Note that this is *not* the last value of consMark, but the
// measured cons/mark value in endCycle.
lastConsMark [4]float64
// gcPercentHeapGoal is the goal heapLive for when next GC ends derived
// from gcPercent.
//
// Set to ^uint64(0) if gcPercent is disabled.
gcPercentHeapGoal atomic.Uint64
// sweepDistMinTrigger is the minimum trigger to ensure a minimum
// sweep distance.
//
// This bound is also special because it applies to both the trigger
// *and* the goal (all other trigger bounds must be based *on* the goal).
//
// It is computed ahead of time, at commit time. The theory is that,
// absent a sudden change to a parameter like gcPercent, the trigger
// will be chosen to always give the sweeper enough headroom. However,
// such a change might dramatically and suddenly move up the trigger,
// in which case we need to ensure the sweeper still has enough headroom.
sweepDistMinTrigger atomic.Uint64
// triggered is the point at which the current GC cycle actually triggered.
// Only valid during the mark phase of a GC cycle, otherwise set to ^uint64(0).
//
// Updated while the world is stopped.
triggered uint64
// lastHeapGoal is the value of heapGoal at the moment the last GC
// ended. Note that this is distinct from the last value heapGoal had,
// because it could change if e.g. gcPercent changes.
//
// Read and written with the world stopped or with mheap_.lock held.
lastHeapGoal uint64
// heapLive is the number of bytes considered live by the GC.
// That is: retained by the most recent GC plus allocated
// since then. heapLive ≤ memstats.totalAlloc-memstats.totalFree, since
// heapAlloc includes unmarked objects that have not yet been swept (and
// hence goes up as we allocate and down as we sweep) while heapLive
// excludes these objects (and hence only goes up between GCs).
//
// To reduce contention, this is updated only when obtaining a span
// from an mcentral and at this point it counts all of the unallocated
// slots in that span (which will be allocated before that mcache
// obtains another span from that mcentral). Hence, it slightly
// overestimates the "true" live heap size. It's better to overestimate
// than to underestimate because 1) this triggers the GC earlier than
// necessary rather than potentially too late and 2) this leads to a
// conservative GC rate rather than a GC rate that is potentially too
// low.
//
// Whenever this is updated, call traceHeapAlloc() and
// this gcControllerState's revise() method.
heapLive atomic.Uint64
// heapScan is the number of bytes of "scannable" heap. This is the
// live heap (as counted by heapLive), but omitting no-scan objects and
// no-scan tails of objects.
//
// This value is fixed at the start of a GC cycle. It represents the
// maximum scannable heap.
heapScan atomic.Uint64
// lastHeapScan is the number of bytes of heap that were scanned
// last GC cycle. It is the same as heapMarked, but only
// includes the "scannable" parts of objects.
//
// Updated when the world is stopped.
lastHeapScan uint64
// lastStackScan is the number of bytes of stack that were scanned
// last GC cycle.
lastStackScan atomic.Uint64
// maxStackScan is the amount of allocated goroutine stack space in
// use by goroutines.
//
// This number tracks allocated goroutine stack space rather than used
// goroutine stack space (i.e. what is actually scanned) because used
// goroutine stack space is much harder to measure cheaply. By using
// allocated space, we make an overestimate; this is OK, it's better
// to conservatively overcount than undercount.
maxStackScan atomic.Uint64
// globalsScan is the total amount of global variable space
// that is scannable.
globalsScan atomic.Uint64
// heapMarked is the number of bytes marked by the previous
// GC. After mark termination, heapLive == heapMarked, but
// unlike heapLive, heapMarked does not change until the
// next mark termination.
heapMarked uint64
// heapScanWork is the total heap scan work performed this cycle.
// stackScanWork is the total stack scan work performed this cycle.
// globalsScanWork is the total globals scan work performed this cycle.
//
// These are updated atomically during the cycle. Updates occur in
// bounded batches, since they are both written and read
// throughout the cycle. At the end of the cycle, heapScanWork is how
// much of the retained heap is scannable.
//
// Currently these are measured in bytes. For most uses, this is an
// opaque unit of work, but for estimation the definition is important.
//
// Note that stackScanWork includes only stack space scanned, not all
// of the allocated stack.
heapScanWork atomic.Int64
stackScanWork atomic.Int64
globalsScanWork atomic.Int64
// bgScanCredit is the scan work credit accumulated by the concurrent
// background scan. This credit is accumulated by the background scan
// and stolen by mutator assists. Updates occur in bounded batches,
// since it is both written and read throughout the cycle.
bgScanCredit atomic.Int64
// assistTime is the nanoseconds spent in mutator assists
// during this cycle. This is updated atomically, and must also
// be updated atomically even during a STW, because it is read
// by sysmon. Updates occur in bounded batches, since it is both
// written and read throughout the cycle.
assistTime atomic.Int64
// dedicatedMarkTime is the nanoseconds spent in dedicated mark workers
// during this cycle. This is updated at the end of the concurrent mark
// phase.
dedicatedMarkTime atomic.Int64
// fractionalMarkTime is the nanoseconds spent in the fractional mark
// worker during this cycle. This is updated throughout the cycle and
// will be up-to-date if the fractional mark worker is not currently
// running.
fractionalMarkTime atomic.Int64
// idleMarkTime is the nanoseconds spent in idle marking during this
// cycle. This is updated throughout the cycle.
idleMarkTime atomic.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 as dedicated mark workers get started.
dedicatedMarkWorkersNeeded atomic.Int64
// idleMarkWorkers is two packed int32 values in a single uint64.
// These two values are always updated simultaneously.
//
// The bottom int32 is the current number of idle mark workers executing.
//
// The top int32 is the maximum number of idle mark workers allowed to
// execute concurrently. Normally, this number is just gomaxprocs. However,
// during periodic GC cycles it is set to 0 because the system is idle
// anyway; there's no need to go full blast on all of GOMAXPROCS.
//
// The maximum number of idle mark workers is used to prevent new workers
// from starting, but it is not a hard maximum. It is possible (but
// exceedingly rare) for the current number of idle mark workers to
// transiently exceed the maximum. This could happen if the maximum changes
// just after a GC ends, and an M with no P.
//
// Note that if we have no dedicated mark workers, we set this value to
// 1 in this case we only have fractional GC workers which aren't scheduled
// strictly enough to ensure GC progress. As a result, idle-priority mark
// workers are vital to GC progress in these situations.
//
// For example, consider a situation in which goroutines block on the GC
// (such as via runtime.GOMAXPROCS) and only fractional mark workers are
// scheduled (e.g. GOMAXPROCS=1). Without idle-priority mark workers, the
// last running M might skip scheduling a fractional mark worker if its
// utilization goal is met, such that once it goes to sleep (because there's
// nothing to do), there will be nothing else to spin up a new M for the
// fractional worker in the future, stalling GC progress and causing a
// deadlock. However, idle-priority workers will *always* run when there is
// nothing left to do, ensuring the GC makes progress.
//
// See github.com/golang/go/issues/44163 for more details.
idleMarkWorkers atomic.Uint64
// assistWorkPerByte is the ratio of scan work to allocated
// bytes that should be performed by mutator assists. This is
// computed at the beginning of each cycle and updated every
// time heapScan is updated.
assistWorkPerByte atomic.Float64
// assistBytesPerWork is 1/assistWorkPerByte.
//
// Note that because this is read and written independently
// from assistWorkPerByte users may notice a skew between
// the two values, and such a state should be safe.
assistBytesPerWork atomic.Float64
// fractionalUtilizationGoal is the fraction of wall clock
// time that should be spent in the fractional mark worker on
// each P that isn't running a dedicated worker.
//
// For example, if the utilization goal is 25% and there are
// no dedicated workers, this will be 0.25. If the goal is
// 25%, there is one dedicated worker, and GOMAXPROCS is 5,
// this will be 0.05 to make up the missing 5%.
//
// If this is zero, no fractional workers are needed.
fractionalUtilizationGoal float64
// These memory stats are effectively duplicates of fields from
// memstats.heapStats but are updated atomically or with the world
// stopped and don't provide the same consistency guarantees.
//
// Because the runtime is responsible for managing a memory limit, it's
// useful to couple these stats more tightly to the gcController, which
// is intimately connected to how that memory limit is maintained.
heapInUse sysMemStat // bytes in mSpanInUse spans
heapReleased sysMemStat // bytes released to the OS
heapFree sysMemStat // bytes not in any span, but not released to the OS
totalAlloc atomic.Uint64 // total bytes allocated
totalFree atomic.Uint64 // total bytes freed
mappedReady atomic.Uint64 // total virtual memory in the Ready state (see mem.go).
// test indicates that this is a test-only copy of gcControllerState.
test bool
_ cpu.CacheLinePad
}
func ( *gcControllerState) ( int32, int64) {
.heapMinimum = defaultHeapMinimum
.triggered = ^uint64(0)
.setGCPercent()
.setMemoryLimit()
.commit(true) // No sweep phase in the first GC cycle.
// N.B. Don't bother calling traceHeapGoal. Tracing is never enabled at
// initialization time.
// N.B. No need to call revise; there's no GC enabled during
// initialization.
}
// startCycle resets the GC controller's state and computes estimates
// for a new GC cycle. The caller must hold worldsema and the world
// must be stopped.
func ( *gcControllerState) ( int64, int, gcTrigger) {
.heapScanWork.Store(0)
.stackScanWork.Store(0)
.globalsScanWork.Store(0)
.bgScanCredit.Store(0)
.assistTime.Store(0)
.dedicatedMarkTime.Store(0)
.fractionalMarkTime.Store(0)
.idleMarkTime.Store(0)
.markStartTime =
.triggered = .heapLive.Load()
// 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.
:= float64() * gcBackgroundUtilization
:= int64( + 0.5)
:= float64()/ - 1
const = 0.3
if < - || > {
// 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() > {
// Too many dedicated workers.
--
}
.fractionalUtilizationGoal = ( - float64()) / float64()
} else {
.fractionalUtilizationGoal = 0
}
// In STW mode, we just want dedicated workers.
if debug.gcstoptheworld > 0 {
= int64()
.fractionalUtilizationGoal = 0
}
// Clear per-P state
for , := range allp {
.gcAssistTime = 0
.gcFractionalMarkTime = 0
}
if .kind == gcTriggerTime {
// During a periodic GC cycle, reduce the number of idle mark workers
// required. However, we need at least one dedicated mark worker or
// idle GC worker to ensure GC progress in some scenarios (see comment
// on maxIdleMarkWorkers).
if > 0 {
.setMaxIdleMarkWorkers(0)
} else {
// TODO(mknyszek): The fundamental reason why we need this is because
// we can't count on the fractional mark worker to get scheduled.
// Fix that by ensuring it gets scheduled according to its quota even
// if the rest of the application is idle.
.setMaxIdleMarkWorkers(1)
}
} else {
// N.B. gomaxprocs and dedicatedMarkWorkersNeeded are guaranteed not to
// change during a GC cycle.
.setMaxIdleMarkWorkers(int32() - int32())
}
// Compute initial values for controls that are updated
// throughout the cycle.
.dedicatedMarkWorkersNeeded.Store()
.revise()
if debug.gcpacertrace > 0 {
:= .heapGoal()
:= .assistWorkPerByte.Load()
print("pacer: assist ratio=", ,
" (scan ", gcController.heapScan.Load()>>20, " MB in ",
work.initialHeapLive>>20, "->",
>>20, " MB)",
" workers=", ,
"+", .fractionalUtilizationGoal, "\n")
}
}
// revise updates the assist ratio during the GC cycle to account for
// improved estimates. This should be called whenever gcController.heapScan,
// gcController.heapLive, or if any inputs to gcController.heapGoal are
// updated. It is safe to call concurrently, but it may race with other
// calls to revise.
//
// The result of this race is that the two assist ratio values may not line
// up or may be stale. In practice this is OK because the assist ratio
// moves slowly throughout a GC cycle, and the assist ratio is a best-effort
// heuristic anyway. Furthermore, no part of the heuristic depends on
// the two assist ratio values being exact reciprocals of one another, since
// the two values are used to convert values from different sources.
//
// The worst case result of this raciness is that we may miss a larger shift
// in the ratio (say, if we decide to pace more aggressively against the
// hard heap goal) but even this "hard goal" is best-effort (see #40460).
// The dedicated GC should ensure we don't exceed the hard goal by too much
// in the rare case we do exceed it.
//
// It should only be called when gcBlackenEnabled != 0 (because this
// is when assists are enabled and the necessary statistics are
// available).
func ( *gcControllerState) () {
:= .gcPercent.Load()
if < 0 {
// If GC is disabled but we're running a forced GC,
// act like GOGC is huge for the below calculations.
= 100000
}
:= .heapLive.Load()
:= .heapScan.Load()
:= .heapScanWork.Load() + .stackScanWork.Load() + .globalsScanWork.Load()
// Assume we're under the soft goal. Pace GC to complete at
// heapGoal assuming the heap is in steady-state.
:= int64(.heapGoal())
// The expected scan work is computed as the amount of bytes scanned last
// GC cycle (both heap and stack), plus our estimate of globals work for this cycle.
:= int64(.lastHeapScan + .lastStackScan.Load() + .globalsScan.Load())
// maxScanWork is a worst-case estimate of the amount of scan work that
// needs to be performed in this GC cycle. Specifically, it represents
// the case where *all* scannable memory turns out to be live, and
// *all* allocated stack space is scannable.
:= .maxStackScan.Load()
:= int64( + + .globalsScan.Load())
if > {
// We've already done more scan work than expected. Because our expectation
// is based on a steady-state scannable heap size, we assume this means our
// heap is growing. Compute a new heap goal that takes our existing runway
// computed for scanWorkExpected and extrapolates it to maxScanWork, the worst-case
// scan work. This keeps our assist ratio stable if the heap continues to grow.
//
// The effect of this mechanism is that assists stay flat in the face of heap
// growths. It's OK to use more memory this cycle to scan all the live heap,
// because the next GC cycle is inevitably going to use *at least* that much
// memory anyway.
:= int64(float64(-int64(.triggered))/float64()*float64()) + int64(.triggered)
=
// hardGoal is a hard limit on the amount that we're willing to push back the
// heap goal, and that's twice the heap goal (i.e. if GOGC=100 and the heap and/or
// stacks and/or globals grow to twice their size, this limits the current GC cycle's
// growth to 4x the original live heap's size).
//
// This maintains the invariant that we use no more memory than the next GC cycle
// will anyway.
:= int64((1.0 + float64()/100.0) * float64())
if > {
=
}
=
}
if int64() > {
// We're already past our heap goal, even the extrapolated one.
// Leave ourselves some extra runway, so in the worst case we
// finish by that point.
const = 1.1
= int64(float64() * )
// Compute the upper bound on the scan work remaining.
=
}
// Compute the remaining scan work estimate.
//
// Note that we currently count allocations during GC as both
// scannable heap (heapScan) and scan work completed
// (scanWork), so allocation will change this difference
// slowly in the soft regime and not at all in the hard
// regime.
:= -
if < 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.
= 1000
}
// Compute the heap distance remaining.
:= - int64()
if <= 0 {
// This shouldn't happen, but if it does, avoid
// dividing by zero or setting the assist negative.
= 1
}
// Compute the mutator assist ratio so by the time the mutator
// allocates the remaining heap bytes up to heapGoal, it will
// have done (or stolen) the remaining amount of scan work.
// Note that the assist ratio values are updated atomically
// but not together. This means there may be some degree of
// skew between the two values. This is generally OK as the
// values shift relatively slowly over the course of a GC
// cycle.
:= float64() / float64()
:= float64() / float64()
.assistWorkPerByte.Store()
.assistBytesPerWork.Store()
}
// endCycle computes the consMark estimate for the next cycle.
// userForced indicates whether the current GC cycle was forced
// by the application.
func ( *gcControllerState) ( int64, int, bool) {
// Record last heap goal for the scavenger.
// We'll be updating the heap goal soon.
gcController.lastHeapGoal = .heapGoal()
// Compute the duration of time for which assists were turned on.
:= - .markStartTime
// Assume background mark hit its utilization goal.
:= gcBackgroundUtilization
// Add assist utilization; avoid divide by zero.
if > 0 {
+= float64(.assistTime.Load()) / float64(*int64())
}
if .heapLive.Load() <= .triggered {
// Shouldn't happen, but let's be very safe about this in case the
// GC is somehow extremely short.
//
// In this case though, the only reasonable value for c.heapLive-c.triggered
// would be 0, which isn't really all that useful, i.e. the GC was so short
// that it didn't matter.
//
// Ignore this case and don't update anything.
return
}
:= 0.0
if > 0 {
= float64(.idleMarkTime.Load()) / float64(*int64())
}
// Determine the cons/mark ratio.
//
// The units we want for the numerator and denominator are both B / cpu-ns.
// We get this by taking the bytes allocated or scanned, and divide by the amount of
// CPU time it took for those operations. For allocations, that CPU time is
//
// assistDuration * procs * (1 - utilization)
//
// Where utilization includes just background GC workers and assists. It does *not*
// include idle GC work time, because in theory the mutator is free to take that at
// any point.
//
// For scanning, that CPU time is
//
// assistDuration * procs * (utilization + idleUtilization)
//
// In this case, we *include* idle utilization, because that is additional CPU time that
// the GC had available to it.
//
// In effect, idle GC time is sort of double-counted here, but it's very weird compared
// to other kinds of GC work, because of how fluid it is. Namely, because the mutator is
// *always* free to take it.
//
// So this calculation is really:
// (heapLive-trigger) / (assistDuration * procs * (1-utilization)) /
// (scanWork) / (assistDuration * procs * (utilization+idleUtilization))
//
// Note that because we only care about the ratio, assistDuration and procs cancel out.
:= .heapScanWork.Load() + .stackScanWork.Load() + .globalsScanWork.Load()
:= (float64(.heapLive.Load()-.triggered) * ( + )) /
(float64() * (1 - ))
// Update our cons/mark estimate. This is the maximum of the value we just computed and the last
// 4 cons/mark values we measured. The reason we take the maximum here is to bias a noisy
// cons/mark measurement toward fewer assists at the expense of additional GC cycles (starting
// earlier).
:= .consMark
.consMark =
for := range .lastConsMark {
if .lastConsMark[] > .consMark {
.consMark = .lastConsMark[]
}
}
copy(.lastConsMark[:], .lastConsMark[1:])
.lastConsMark[len(.lastConsMark)-1] =
if debug.gcpacertrace > 0 {
printlock()
:= gcGoalUtilization * 100
print("pacer: ", int(*100), "% CPU (", int(), " exp.) for ")
print(.heapScanWork.Load(), "+", .stackScanWork.Load(), "+", .globalsScanWork.Load(), " B work (", .lastHeapScan+.lastStackScan.Load()+.globalsScan.Load(), " B exp.) ")
:= .heapLive.Load()
print("in ", .triggered, " B -> ", , " B (∆goal ", int64()-int64(.lastHeapGoal), ", cons/mark ", , ")")
println()
printunlock()
}
}
// enlistWorker encourages another dedicated mark worker to start on
// another P if there are spare worker slots. It is used by putfull
// when more work is made available.
//
//go:nowritebarrier
func ( *gcControllerState) () {
// If there are idle Ps, wake one so it will run an idle worker.
// NOTE: This is suspected of causing deadlocks. See golang.org/issue/19112.
//
// if sched.npidle.Load() != 0 && sched.nmspinning.Load() == 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 .dedicatedMarkWorkersNeeded.Load() <= 0 {
return
}
// Pick a random other P to preempt.
if gomaxprocs <= 1 {
return
}
:= getg()
if == nil || .m == nil || .m.p == 0 {
return
}
:= .m.p.ptr().id
for := 0; < 5; ++ {
:= int32(cheaprandn(uint32(gomaxprocs - 1)))
if >= {
++
}
:= allp[]
if .status != _Prunning {
continue
}
if preemptone() {
return
}
}
}
// findRunnableGCWorker returns a background mark worker for pp if it
// should be run. This must only be called when gcBlackenEnabled != 0.
func ( *gcControllerState) ( *p, int64) (*g, int64) {
if gcBlackenEnabled == 0 {
throw("gcControllerState.findRunnable: blackening not enabled")
}
// Since we have the current time, check if the GC CPU limiter
// hasn't had an update in a while. This check is necessary in
// case the limiter is on but hasn't been checked in a while and
// so may have left sufficient headroom to turn off again.
if == 0 {
= nanotime()
}
if gcCPULimiter.needUpdate() {
gcCPULimiter.update()
}
if !gcMarkWorkAvailable() {
// No work to be done right now. This can happen at
// the end of the mark phase when there are still
// assists tapering off. Don't bother running a worker
// now because it'll just return immediately.
return nil,
}
// Grab a worker before we commit to running below.
:= (*gcBgMarkWorkerNode)(gcBgMarkWorkerPool.pop())
if == nil {
// There is at least one worker per P, so normally there are
// enough workers to run on all Ps, if necessary. However, once
// a worker enters gcMarkDone it may park without rejoining the
// pool, thus freeing a P with no corresponding worker.
// gcMarkDone never depends on another worker doing work, so it
// is safe to simply do nothing here.
//
// If gcMarkDone bails out without completing the mark phase,
// it will always do so with queued global work. Thus, that P
// will be immediately eligible to re-run the worker G it was
// just using, ensuring work can complete.
return nil,
}
:= func( *atomic.Int64) bool {
for {
:= .Load()
if <= 0 {
return false
}
if .CompareAndSwap(, -1) {
return true
}
}
}
if (&.dedicatedMarkWorkersNeeded) {
// This P is now dedicated to marking until the end of
// the concurrent mark phase.
.gcMarkWorkerMode = gcMarkWorkerDedicatedMode
} else if .fractionalUtilizationGoal == 0 {
// No need for fractional workers.
gcBgMarkWorkerPool.push(&.node)
return nil,
} else {
// Is this P behind on the fractional utilization
// goal?
//
// This should be kept in sync with pollFractionalWorkerExit.
:= - .markStartTime
if > 0 && float64(.gcFractionalMarkTime)/float64() > .fractionalUtilizationGoal {
// Nope. No need to run a fractional worker.
gcBgMarkWorkerPool.push(&.node)
return nil,
}
// Run a fractional worker.
.gcMarkWorkerMode = gcMarkWorkerFractionalMode
}
// Run the background mark worker.
:= .gp.ptr()
:= traceAcquire()
casgstatus(, _Gwaiting, _Grunnable)
if .ok() {
.GoUnpark(, 0)
traceRelease()
}
return ,
}
// resetLive sets up the controller state for the next mark phase after the end
// of the previous one. Must be called after endCycle and before commit, before
// the world is started.
//
// The world must be stopped.
func ( *gcControllerState) ( uint64) {
.heapMarked =
.heapLive.Store()
.heapScan.Store(uint64(.heapScanWork.Load()))
.lastHeapScan = uint64(.heapScanWork.Load())
.lastStackScan.Store(uint64(.stackScanWork.Load()))
.triggered = ^uint64(0) // Reset triggered.
// heapLive was updated, so emit a trace event.
:= traceAcquire()
if .ok() {
.HeapAlloc()
traceRelease()
}
}
// markWorkerStop must be called whenever a mark worker stops executing.
//
// It updates mark work accounting in the controller by a duration of
// work in nanoseconds and other bookkeeping.
//
// Safe to execute at any time.
func ( *gcControllerState) ( gcMarkWorkerMode, int64) {
switch {
case gcMarkWorkerDedicatedMode:
.dedicatedMarkTime.Add()
.dedicatedMarkWorkersNeeded.Add(1)
case gcMarkWorkerFractionalMode:
.fractionalMarkTime.Add()
case gcMarkWorkerIdleMode:
.idleMarkTime.Add()
.removeIdleMarkWorker()
default:
throw("markWorkerStop: unknown mark worker mode")
}
}
func ( *gcControllerState) (, int64) {
if != 0 {
:= traceAcquire()
:= gcController.heapLive.Add()
if .ok() {
// gcController.heapLive changed.
.HeapAlloc()
traceRelease()
}
}
if gcBlackenEnabled == 0 {
// Update heapScan when we're not in a current GC. It is fixed
// at the beginning of a cycle.
if != 0 {
gcController.heapScan.Add()
}
} else {
// gcController.heapLive changed.
.revise()
}
}
func ( *gcControllerState) ( *p, int64) {
if == nil {
.maxStackScan.Add()
return
}
.maxStackScanDelta +=
if .maxStackScanDelta >= maxStackScanSlack || .maxStackScanDelta <= -maxStackScanSlack {
.maxStackScan.Add(.maxStackScanDelta)
.maxStackScanDelta = 0
}
}
func ( *gcControllerState) ( int64) {
.globalsScan.Add()
}
// heapGoal returns the current heap goal.
func ( *gcControllerState) () uint64 {
, := .heapGoalInternal()
return
}
// heapGoalInternal is the implementation of heapGoal which returns additional
// information that is necessary for computing the trigger.
//
// The returned minTrigger is always <= goal.
func ( *gcControllerState) () (, uint64) {
// Start with the goal calculated for gcPercent.
= .gcPercentHeapGoal.Load()
// Check if the memory-limit-based goal is smaller, and if so, pick that.
if := .memoryLimitHeapGoal(); < {
=
} else {
// We're not limited by the memory limit goal, so perform a series of
// adjustments that might move the goal forward in a variety of circumstances.
:= .sweepDistMinTrigger.Load()
if > {
// Set the goal to maintain a minimum sweep distance since
// the last call to commit. Note that we never want to do this
// if we're in the memory limit regime, because it could push
// the goal up.
=
}
// Since we ignore the sweep distance trigger in the memory
// limit regime, we need to ensure we don't propagate it to
// the trigger, because it could cause a violation of the
// invariant that the trigger < goal.
=
// Ensure that the heap goal is at least a little larger than
// the point at which we triggered. This may not be the case if GC
// start is delayed or if the allocation that pushed gcController.heapLive
// over trigger is large or if the trigger is really close to
// GOGC. Assist is proportional to this distance, so enforce a
// minimum distance, even if it means going over the GOGC goal
// by a tiny bit.
//
// Ignore this if we're in the memory limit regime: we'd prefer to
// have the GC respond hard about how close we are to the goal than to
// push the goal back in such a manner that it could cause us to exceed
// the memory limit.
const = 64 << 10
if .triggered != ^uint64(0) && < .triggered+ {
= .triggered +
}
}
return
}
// memoryLimitHeapGoal returns a heap goal derived from memoryLimit.
func ( *gcControllerState) () uint64 {
// Start by pulling out some values we'll need. Be careful about overflow.
var , , uint64
for {
= .heapFree.load() // Free and unscavenged memory.
= .totalAlloc.Load() - .totalFree.Load() // Heap object bytes in use.
= .mappedReady.Load() // Total unreleased mapped memory.
if + <= {
break
}
// It is impossible for total unreleased mapped memory to exceed heap memory, but
// because these stats are updated independently, we may observe a partial update
// including only some values. Thus, we appear to break the invariant. However,
// this condition is necessarily transient, so just try again. In the case of a
// persistent accounting error, we'll deadlock here.
}
// Below we compute a goal from memoryLimit. There are a few things to be aware of.
// Firstly, the memoryLimit does not easily compare to the heap goal: the former
// is total mapped memory by the runtime that hasn't been released, while the latter is
// only heap object memory. Intuitively, the way we convert from one to the other is to
// subtract everything from memoryLimit that both contributes to the memory limit (so,
// ignore scavenged memory) and doesn't contain heap objects. This isn't quite what
// lines up with reality, but it's a good starting point.
//
// In practice this computation looks like the following:
//
// goal := memoryLimit - ((mappedReady - heapFree - heapAlloc) + max(mappedReady - memoryLimit, 0))
// ^1 ^2
// goal -= goal / 100 * memoryLimitHeapGoalHeadroomPercent
// ^3
//
// Let's break this down.
//
// The first term (marker 1) is everything that contributes to the memory limit and isn't
// or couldn't become heap objects. It represents, broadly speaking, non-heap overheads.
// One oddity you may have noticed is that we also subtract out heapFree, i.e. unscavenged
// memory that may contain heap objects in the future.
//
// Let's take a step back. In an ideal world, this term would look something like just
// the heap goal. That is, we "reserve" enough space for the heap to grow to the heap
// goal, and subtract out everything else. This is of course impossible; the definition
// is circular! However, this impossible definition contains a key insight: the amount
// we're *going* to use matters just as much as whatever we're currently using.
//
// Consider if the heap shrinks to 1/10th its size, leaving behind lots of free and
// unscavenged memory. mappedReady - heapAlloc will be quite large, because of that free
// and unscavenged memory, pushing the goal down significantly.
//
// heapFree is also safe to exclude from the memory limit because in the steady-state, it's
// just a pool of memory for future heap allocations, and making new allocations from heapFree
// memory doesn't increase overall memory use. In transient states, the scavenger and the
// allocator actively manage the pool of heapFree memory to maintain the memory limit.
//
// The second term (marker 2) is the amount of memory we've exceeded the limit by, and is
// intended to help recover from such a situation. By pushing the heap goal down, we also
// push the trigger down, triggering and finishing a GC sooner in order to make room for
// other memory sources. Note that since we're effectively reducing the heap goal by X bytes,
// we're actually giving more than X bytes of headroom back, because the heap goal is in
// terms of heap objects, but it takes more than X bytes (e.g. due to fragmentation) to store
// X bytes worth of objects.
//
// The final adjustment (marker 3) reduces the maximum possible memory limit heap goal by
// memoryLimitHeapGoalPercent. As the name implies, this is to provide additional headroom in
// the face of pacing inaccuracies, and also to leave a buffer of unscavenged memory so the
// allocator isn't constantly scavenging. The reduction amount also has a fixed minimum
// (memoryLimitMinHeapGoalHeadroom, not pictured) because the aforementioned pacing inaccuracies
// disproportionately affect small heaps: as heaps get smaller, the pacer's inputs get fuzzier.
// Shorter GC cycles and less GC work means noisy external factors like the OS scheduler have a
// greater impact.
:= uint64(.memoryLimit.Load())
// Compute term 1.
:= - -
// Compute term 2.
var uint64
if > {
= -
}
if + >= {
// We're at a point where non-heap memory exceeds the memory limit on its own.
// There's honestly not much we can do here but just trigger GCs continuously
// and let the CPU limiter reign that in. Something has to give at this point.
// Set it to heapMarked, the lowest possible goal.
return .heapMarked
}
// Compute the goal.
:= - ( + )
// Apply some headroom to the goal to account for pacing inaccuracies and to reduce
// the impact of scavenging at allocation time in response to a high allocation rate
// when GOGC=off. See issue #57069. Also, be careful about small limits.
:= / 100 * memoryLimitHeapGoalHeadroomPercent
if < memoryLimitMinHeapGoalHeadroom {
// Set a fixed minimum to deal with the particularly large effect pacing inaccuracies
// have for smaller heaps.
= memoryLimitMinHeapGoalHeadroom
}
if < || - < {
=
} else {
= -
}
// Don't let us go below the live heap. A heap goal below the live heap doesn't make sense.
if < .heapMarked {
= .heapMarked
}
return
}
const (
// These constants determine the bounds on the GC trigger as a fraction
// of heap bytes allocated between the start of a GC (heapLive == heapMarked)
// and the end of a GC (heapLive == heapGoal).
//
// The constants are obscured in this way for efficiency. The denominator
// of the fraction is always a power-of-two for a quick division, so that
// the numerator is a single constant integer multiplication.
triggerRatioDen = 64
// The minimum trigger 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.
minTriggerRatioNum = 45 // ~0.7
// The maximum trigger constant is chosen somewhat arbitrarily, but the
// current constant has served us well over the years.
maxTriggerRatioNum = 61 // ~0.95
)
// trigger returns the current point at which a GC should trigger along with
// the heap goal.
//
// The returned value may be compared against heapLive to determine whether
// the GC should trigger. Thus, the GC trigger condition should be (but may
// not be, in the case of small movements for efficiency) checked whenever
// the heap goal may change.
func ( *gcControllerState) () (uint64, uint64) {
, := .heapGoalInternal()
// Invariant: the trigger must always be less than the heap goal.
//
// Note that the memory limit sets a hard maximum on our heap goal,
// but the live heap may grow beyond it.
if .heapMarked >= {
// The goal should never be smaller than heapMarked, but let's be
// defensive about it. The only reasonable trigger here is one that
// causes a continuous GC cycle at heapMarked, but respect the goal
// if it came out as smaller than that.
return ,
}
// Below this point, c.heapMarked < goal.
// heapMarked is our absolute minimum, and it's possible the trigger
// bound we get from heapGoalinternal is less than that.
if < .heapMarked {
= .heapMarked
}
// If we let the trigger go too low, then if the application
// is allocating very rapidly we might end up in a situation
// where we're allocating black during a nearly always-on GC.
// The result of this is a growing heap and ultimately an
// increase in RSS. By capping us at a point >0, we're essentially
// saying that we're OK using more CPU during the GC to prevent
// this growth in RSS.
:= ((-.heapMarked)/triggerRatioDen)*minTriggerRatioNum + .heapMarked
if < {
=
}
// For small heaps, set the max trigger point at maxTriggerRatio of the way
// from the live heap to the heap goal. This ensures we always have *some*
// headroom when the GC actually starts. For larger heaps, set the max trigger
// point at the goal, minus the minimum heap size.
//
// This choice follows from the fact that the minimum heap size is chosen
// to reflect the costs of a GC with no work to do. With a large heap but
// very little scan work to perform, this gives us exactly as much runway
// as we would need, in the worst case.
:= ((-.heapMarked)/triggerRatioDen)*maxTriggerRatioNum + .heapMarked
if > defaultHeapMinimum && -defaultHeapMinimum > {
= - defaultHeapMinimum
}
= max(, )
// Compute the trigger from our bounds and the runway stored by commit.
var uint64
:= .runway.Load()
if > {
=
} else {
= -
}
= max(, )
= min(, )
if > {
print("trigger=", , " heapGoal=", , "\n")
print("minTrigger=", , " maxTrigger=", , "\n")
throw("produced a trigger greater than the heap goal")
}
return ,
}
// commit recomputes all pacing parameters needed to derive the
// trigger and the heap goal. Namely, the gcPercent-based heap goal,
// and the amount of runway we want to give the GC this cycle.
//
// 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.
//
// isSweepDone should be the result of calling isSweepDone(),
// unless we're testing or we know we're executing during a GC cycle.
//
// This depends on gcPercent, gcController.heapMarked, and
// gcController.heapLive. These must be up to date.
//
// Callers must call gcControllerState.revise after calling this
// function if the GC is enabled.
//
// mheap_.lock must be held or the world must be stopped.
func ( *gcControllerState) ( bool) {
if !.test {
assertWorldStoppedOrLockHeld(&mheap_.lock)
}
if {
// The sweep is done, so there aren't any restrictions on the trigger
// we need to think about.
.sweepDistMinTrigger.Store(0)
} else {
// Concurrent sweep happens in the heap growth
// from gcController.heapLive to trigger. Make sure we
// give the sweeper some runway if it doesn't have enough.
.sweepDistMinTrigger.Store(.heapLive.Load() + sweepMinHeapDistance)
}
// Compute the next GC goal, which is when the allocated heap
// has grown by GOGC/100 over where it started the last cycle,
// plus additional runway for non-heap sources of GC work.
:= ^uint64(0)
if := .gcPercent.Load(); >= 0 {
= .heapMarked + (.heapMarked+.lastStackScan.Load()+.globalsScan.Load())*uint64()/100
}
// Apply the minimum heap size here. It's defined in terms of gcPercent
// and is only updated by functions that call commit.
if < .heapMinimum {
= .heapMinimum
}
.gcPercentHeapGoal.Store()
// Compute the amount of runway we want the GC to have by using our
// estimate of the cons/mark ratio.
//
// The idea is to take our expected scan work, and multiply it by
// the cons/mark ratio to determine how long it'll take to complete
// that scan work in terms of bytes allocated. This gives us our GC's
// runway.
//
// However, the cons/mark ratio is a ratio of rates per CPU-second, but
// here we care about the relative rates for some division of CPU
// resources among the mutator and the GC.
//
// To summarize, we have B / cpu-ns, and we want B / ns. We get that
// by multiplying by our desired division of CPU resources. We choose
// to express CPU resources as GOMAPROCS*fraction. Note that because
// we're working with a ratio here, we can omit the number of CPU cores,
// because they'll appear in the numerator and denominator and cancel out.
// As a result, this is basically just "weighing" the cons/mark ratio by
// our desired division of resources.
//
// Furthermore, by setting the runway so that CPU resources are divided
// this way, assuming that the cons/mark ratio is correct, we make that
// division a reality.
.runway.Store(uint64((.consMark * (1 - gcGoalUtilization) / (gcGoalUtilization)) * float64(.lastHeapScan+.lastStackScan.Load()+.globalsScan.Load())))
}
// setGCPercent updates gcPercent. commit must be called after.
// Returns the old value of gcPercent.
//
// The world must be stopped, or mheap_.lock must be held.
func ( *gcControllerState) ( int32) int32 {
if !.test {
assertWorldStoppedOrLockHeld(&mheap_.lock)
}
:= .gcPercent.Load()
if < 0 {
= -1
}
.heapMinimum = defaultHeapMinimum * uint64() / 100
.gcPercent.Store()
return
}
//go:linkname setGCPercent runtime/debug.setGCPercent
func setGCPercent( int32) ( int32) {
// Run on the system stack since we grab the heap lock.
systemstack(func() {
lock(&mheap_.lock)
= gcController.setGCPercent()
gcControllerCommit()
unlock(&mheap_.lock)
})
// If we just disabled GC, wait for any concurrent GC mark to
// finish so we always return with no GC running.
if < 0 {
gcWaitOnMark(work.cycles.Load())
}
return
}
func readGOGC() int32 {
:= gogetenv("GOGC")
if == "off" {
return -1
}
if , := atoi32(); {
return
}
return 100
}
// setMemoryLimit updates memoryLimit. commit must be called after
// Returns the old value of memoryLimit.
//
// The world must be stopped, or mheap_.lock must be held.
func ( *gcControllerState) ( int64) int64 {
if !.test {
assertWorldStoppedOrLockHeld(&mheap_.lock)
}
:= .memoryLimit.Load()
if >= 0 {
.memoryLimit.Store()
}
return
}
//go:linkname setMemoryLimit runtime/debug.setMemoryLimit
func setMemoryLimit( int64) ( int64) {
// Run on the system stack since we grab the heap lock.
systemstack(func() {
lock(&mheap_.lock)
= gcController.setMemoryLimit()
if < 0 || == {
// If we're just checking the value or not changing
// it, there's no point in doing the rest.
unlock(&mheap_.lock)
return
}
gcControllerCommit()
unlock(&mheap_.lock)
})
return
}
func readGOMEMLIMIT() int64 {
:= gogetenv("GOMEMLIMIT")
if == "" || == "off" {
return maxInt64
}
, := parseByteCount()
if ! {
print("GOMEMLIMIT=", , "\n")
throw("malformed GOMEMLIMIT; see `go doc runtime/debug.SetMemoryLimit`")
}
return
}
// addIdleMarkWorker attempts to add a new idle mark worker.
//
// If this returns true, the caller must become an idle mark worker unless
// there's no background mark worker goroutines in the pool. This case is
// harmless because there are already background mark workers running.
// If this returns false, the caller must NOT become an idle mark worker.
//
// nosplit because it may be called without a P.
//
//go:nosplit
func ( *gcControllerState) () bool {
for {
:= .idleMarkWorkers.Load()
, := int32(&uint64(^uint32(0))), int32(>>32)
if >= {
// See the comment on idleMarkWorkers for why
// n > max is tolerated.
return false
}
if < 0 {
print("n=", , " max=", , "\n")
throw("negative idle mark workers")
}
:= uint64(uint32(+1)) | (uint64() << 32)
if .idleMarkWorkers.CompareAndSwap(, ) {
return true
}
}
}
// needIdleMarkWorker is a hint as to whether another idle mark worker is needed.
//
// The caller must still call addIdleMarkWorker to become one. This is mainly
// useful for a quick check before an expensive operation.
//
// nosplit because it may be called without a P.
//
//go:nosplit
func ( *gcControllerState) () bool {
:= .idleMarkWorkers.Load()
, := int32(&uint64(^uint32(0))), int32(>>32)
return <
}
// removeIdleMarkWorker must be called when a new idle mark worker stops executing.
func ( *gcControllerState) () {
for {
:= .idleMarkWorkers.Load()
, := int32(&uint64(^uint32(0))), int32(>>32)
if -1 < 0 {
print("n=", , " max=", , "\n")
throw("negative idle mark workers")
}
:= uint64(uint32(-1)) | (uint64() << 32)
if .idleMarkWorkers.CompareAndSwap(, ) {
return
}
}
}
// setMaxIdleMarkWorkers sets the maximum number of idle mark workers allowed.
//
// This method is optimistic in that it does not wait for the number of
// idle mark workers to reduce to max before returning; it assumes the workers
// will deschedule themselves.
func ( *gcControllerState) ( int32) {
for {
:= .idleMarkWorkers.Load()
:= int32( & uint64(^uint32(0)))
if < 0 {
print("n=", , " max=", , "\n")
throw("negative idle mark workers")
}
:= uint64(uint32()) | (uint64() << 32)
if .idleMarkWorkers.CompareAndSwap(, ) {
return
}
}
}
// gcControllerCommit is gcController.commit, but passes arguments from live
// (non-test) data. It also updates any consumers of the GC pacing, such as
// sweep pacing and the background scavenger.
//
// Calls gcController.commit.
//
// The heap lock must be held, so this must be executed on the system stack.
//
//go:systemstack
func gcControllerCommit() {
assertWorldStoppedOrLockHeld(&mheap_.lock)
gcController.commit(isSweepDone())
// Update mark pacing.
if gcphase != _GCoff {
gcController.revise()
}
// TODO(mknyszek): This isn't really accurate any longer because the heap
// goal is computed dynamically. Still useful to snapshot, but not as useful.
:= traceAcquire()
if .ok() {
.HeapGoal()
traceRelease()
}
, := gcController.trigger()
gcPaceSweeper()
gcPaceScavenger(gcController.memoryLimit.Load(), , gcController.lastHeapGoal)
}
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