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runtime: break out GC pacer into its own file

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This change breaks out the GC pacer into its own file so that it'll be
easier to see the full implementation and change it. It also suggests an
obvious place to put tests (mgcpacer_test.go).

This includes all of gcControllerState, gcSetTriggerRatio, anything
related to GOGC, and all related globals and constants.

This is almost a clean move, except that globals and constants are
formatted into blocks instead of having a separate "var" declaration for
each one.

For #44167.

Change-Id: I85aa84ce85c6cfbe0b33e8a3c91cbe9dc41de8cb
Reviewed-on: https://go-review.googlesource.com/c/go/+/306596
Trust: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Michael Pratt <mpratt@google.com>
This commit is contained in:
Michael Anthony Knyszek 2021-03-31 21:47:41 +00:00 committed by Michael Knyszek
parent 9913f821e2
commit f5f7647107
2 changed files with 735 additions and 720 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

720
src/runtime/mgc.go
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@ -149,26 +149,6 @@ const (
sweepMinHeapDistance = 1024 * 1024 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() { func gcinit() {
if unsafe.Sizeof(workbuf{}) != _WorkbufSize { if unsafe.Sizeof(workbuf{}) != _WorkbufSize {
throw("size of Workbuf is suboptimal") throw("size of Workbuf is suboptimal")
@ -196,17 +176,6 @@ func gcinit() {
lockInit(&work.wbufSpans.lock, lockRankWbufSpans) lockInit(&work.wbufSpans.lock, lockRankWbufSpans)
} }
func readgogc() int32 {
p := gogetenv("GOGC")
if p == "off" {
return -1
}
if n, ok := atoi32(p); ok {
return n
}
return 100
}
// Temporary in order to enable register ABI work. // Temporary in order to enable register ABI work.
// TODO(register args): convert back to local chan in gcenabled, passed to "go" stmts. // TODO(register args): convert back to local chan in gcenabled, passed to "go" stmts.
var gcenable_setup chan int var gcenable_setup chan int
@ -226,31 +195,6 @@ func gcenable() {
memstats.enablegc = true // now that runtime is initialized, GC is okay memstats.enablegc = true // now that runtime is initialized, GC is okay
} }
//go:linkname setGCPercent runtime/debug.setGCPercent
func setGCPercent(in int32) (out int32) {
// Run on the system stack since we grab the heap lock.
systemstack(func() {
lock(&mheap_.lock)
out = gcpercent
if in < 0 {
in = -1
}
gcpercent = in
heapminimum = defaultHeapMinimum * uint64(gcpercent) / 100
// Update pacing in response to gcpercent change.
gcSetTriggerRatio(memstats.triggerRatio)
unlock(&mheap_.lock)
})
// If we just disabled GC, wait for any concurrent GC mark to
// finish so we always return with no GC running.
if in < 0 {
gcWaitOnMark(atomic.Load(&work.cycles))
}
return out
}
// Garbage collector phase. // Garbage collector phase.
// Indicates to write barrier and synchronization task to perform. // Indicates to write barrier and synchronization task to perform.
var gcphase uint32 var gcphase uint32
@ -330,473 +274,6 @@ var gcMarkWorkerModeStrings = [...]string{
"GC (idle)", "GC (idle)",
} }
// gcController implements the GC pacing controller that determines
// when to trigger concurrent garbage collection and how much marking
// work to do in mutator assists and background marking.
//
// It uses a feedback control algorithm to adjust the memstats.gc_trigger
// trigger based on the heap growth and GC CPU utilization each cycle.
// This algorithm optimizes for heap growth to match GOGC and for CPU
// utilization between assist and background marking to be 25% of
// GOMAXPROCS. The high-level design of this algorithm is documented
// at https://golang.org/s/go15gcpacing.
//
// All fields of gcController are used only during a single mark
// cycle.
var gcController gcControllerState
type gcControllerState struct {
// scanWork is the total scan work performed this cycle. This
// is updated atomically during the cycle. Updates occur in
// bounded batches, since it is both written and read
// throughout the cycle. At the end of the cycle, this is how
// much of the retained heap is scannable.
//
// Currently this is the bytes of heap scanned. For most uses,
// this is an opaque unit of work, but for estimation the
// definition is important.
scanWork int64
// bgScanCredit is the scan work credit accumulated by the
// concurrent background scan. This credit is accumulated by
// the background scan and stolen by mutator assists. This is
// updated atomically. Updates occur in bounded batches, since
// it is both written and read throughout the cycle.
bgScanCredit int64
// assistTime is the nanoseconds spent in mutator assists
// during this cycle. This is updated atomically. Updates
// occur in bounded batches, since it is both written and read
// throughout the cycle.
assistTime int64
// dedicatedMarkTime is the nanoseconds spent in dedicated
// mark workers during this cycle. This is updated atomically
// at the end of the concurrent mark phase.
dedicatedMarkTime int64
// fractionalMarkTime is the nanoseconds spent in the
// fractional mark worker during this cycle. This is updated
// atomically throughout the cycle and will be up-to-date if
// the fractional mark worker is not currently running.
fractionalMarkTime int64
// idleMarkTime is the nanoseconds spent in idle marking
// during this cycle. This is updated atomically throughout
// the cycle.
idleMarkTime int64
// markStartTime is the absolute start time in nanoseconds
// that assists and background mark workers started.
markStartTime int64
// dedicatedMarkWorkersNeeded is the number of dedicated mark
// workers that need to be started. This is computed at the
// beginning of each cycle and decremented atomically as
// dedicated mark workers get started.
dedicatedMarkWorkersNeeded int64
// assistWorkPerByte is the ratio of scan work to allocated
// bytes that should be performed by mutator assists. This is
// computed at the beginning of each cycle and updated every
// time heap_scan is updated.
//
// Stored as a uint64, but it's actually a float64. Use
// float64frombits to get the value.
//
// Read and written atomically.
assistWorkPerByte uint64
// assistBytesPerWork is 1/assistWorkPerByte.
//
// Stored as a uint64, but it's actually a float64. Use
// float64frombits to get the value.
//
// Read and written atomically.
//
// 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 uint64
// fractionalUtilizationGoal is the fraction of wall clock
// time that should be spent in the fractional mark worker on
// each P that isn't running a dedicated worker.
//
// For example, if the utilization goal is 25% and there are
// no dedicated workers, this will be 0.25. If the goal is
// 25%, there is one dedicated worker, and GOMAXPROCS is 5,
// this will be 0.05 to make up the missing 5%.
//
// If this is zero, no fractional workers are needed.
fractionalUtilizationGoal float64
_ cpu.CacheLinePad
}
// startCycle resets the GC controller's state and computes estimates
// for a new GC cycle. The caller must hold worldsema and the world
// must be stopped.
func (c *gcControllerState) startCycle() {
c.scanWork = 0
c.bgScanCredit = 0
c.assistTime = 0
c.dedicatedMarkTime = 0
c.fractionalMarkTime = 0
c.idleMarkTime = 0
// Ensure that the heap goal is at least a little larger than
// the current live heap size. This may not be the case if GC
// start is delayed or if the allocation that pushed heap_live
// over gc_trigger is large or if the trigger is really close to
// GOGC. Assist is proportional to this distance, so enforce a
// minimum distance, even if it means going over the GOGC goal
// by a tiny bit.
if memstats.next_gc < memstats.heap_live+1024*1024 {
memstats.next_gc = memstats.heap_live + 1024*1024
}
// Compute the background mark utilization goal. In general,
// this may not come out exactly. We round the number of
// dedicated workers so that the utilization is closest to
// 25%. For small GOMAXPROCS, this would introduce too much
// error, so we add fractional workers in that case.
totalUtilizationGoal := float64(gomaxprocs) * gcBackgroundUtilization
c.dedicatedMarkWorkersNeeded = int64(totalUtilizationGoal + 0.5)
utilError := float64(c.dedicatedMarkWorkersNeeded)/totalUtilizationGoal - 1
const maxUtilError = 0.3
if utilError < -maxUtilError || utilError > maxUtilError {
// Rounding put us more than 30% off our goal. With
// gcBackgroundUtilization of 25%, this happens for
// GOMAXPROCS<=3 or GOMAXPROCS=6. Enable fractional
// workers to compensate.
if float64(c.dedicatedMarkWorkersNeeded) > totalUtilizationGoal {
// Too many dedicated workers.
c.dedicatedMarkWorkersNeeded--
}
c.fractionalUtilizationGoal = (totalUtilizationGoal - float64(c.dedicatedMarkWorkersNeeded)) / float64(gomaxprocs)
} else {
c.fractionalUtilizationGoal = 0
}
// In STW mode, we just want dedicated workers.
if debug.gcstoptheworld > 0 {
c.dedicatedMarkWorkersNeeded = int64(gomaxprocs)
c.fractionalUtilizationGoal = 0
}
// Clear per-P state
for _, p := range allp {
p.gcAssistTime = 0
p.gcFractionalMarkTime = 0
}
// Compute initial values for controls that are updated
// throughout the cycle.
c.revise()
if debug.gcpacertrace > 0 {
assistRatio := float64frombits(atomic.Load64(&c.assistWorkPerByte))
print("pacer: assist ratio=", assistRatio,
" (scan ", memstats.heap_scan>>20, " MB in ",
work.initialHeapLive>>20, "->",
memstats.next_gc>>20, " MB)",
" workers=", c.dedicatedMarkWorkersNeeded,
"+", c.fractionalUtilizationGoal, "\n")
}
}
// revise updates the assist ratio during the GC cycle to account for
// improved estimates. This should be called whenever memstats.heap_scan,
// memstats.heap_live, or memstats.next_gc is updated. It is safe to
// call concurrently, but it may race with other calls to revise.
//
// The result of this race is that the two assist ratio values may not line
// up or may be stale. In practice this is OK because the assist ratio
// moves slowly throughout a GC cycle, and the assist ratio is a best-effort
// heuristic anyway. Furthermore, no part of the heuristic depends on
// the two assist ratio values being exact reciprocals of one another, since
// the two values are used to convert values from different sources.
//
// The worst case result of this raciness is that we may miss a larger shift
// in the ratio (say, if we decide to pace more aggressively against the
// hard heap goal) but even this "hard goal" is best-effort (see #40460).
// The dedicated GC should ensure we don't exceed the hard goal by too much
// in the rare case we do exceed it.
//
// It should only be called when gcBlackenEnabled != 0 (because this
// is when assists are enabled and the necessary statistics are
// available).
func (c *gcControllerState) revise() {
gcpercent := gcpercent
if gcpercent < 0 {
// If GC is disabled but we're running a forced GC,
// act like GOGC is huge for the below calculations.
gcpercent = 100000
}
live := atomic.Load64(&memstats.heap_live)
scan := atomic.Load64(&memstats.heap_scan)
work := atomic.Loadint64(&c.scanWork)
// Assume we're under the soft goal. Pace GC to complete at
// next_gc assuming the heap is in steady-state.
heapGoal := int64(atomic.Load64(&memstats.next_gc))
// Compute the expected scan work remaining.
//
// This is estimated based on the expected
// steady-state scannable heap. For example, with
// GOGC=100, only half of the scannable heap is
// expected to be live, so that's what we target.
//
// (This is a float calculation to avoid overflowing on
// 100*heap_scan.)
scanWorkExpected := int64(float64(scan) * 100 / float64(100+gcpercent))
if int64(live) > heapGoal || work > scanWorkExpected {
// We're past the soft goal, or we've already done more scan
// work than we expected. Pace GC so that in the worst case it
// will complete by the hard goal.
const maxOvershoot = 1.1
heapGoal = int64(float64(heapGoal) * maxOvershoot)
// Compute the upper bound on the scan work remaining.
scanWorkExpected = int64(scan)
}
// Compute the remaining scan work estimate.
//
// Note that we currently count allocations during GC as both
// scannable heap (heap_scan) and scan work completed
// (scanWork), so allocation will change this difference
// slowly in the soft regime and not at all in the hard
// regime.
scanWorkRemaining := scanWorkExpected - work
if scanWorkRemaining < 1000 {
// We set a somewhat arbitrary lower bound on
// remaining scan work since if we aim a little high,
// we can miss by a little.
//
// We *do* need to enforce that this is at least 1,
// since marking is racy and double-scanning objects
// may legitimately make the remaining scan work
// negative, even in the hard goal regime.
scanWorkRemaining = 1000
}
// Compute the heap distance remaining.
heapRemaining := heapGoal - int64(live)
if heapRemaining <= 0 {
// This shouldn't happen, but if it does, avoid
// dividing by zero or setting the assist negative.
heapRemaining = 1
}
// Compute the mutator assist ratio so by the time the mutator
// allocates the remaining heap bytes up to next_gc, it will
// have done (or stolen) the remaining amount of scan work.
// Note that the assist ratio values are updated atomically
// but not together. This means there may be some degree of
// skew between the two values. This is generally OK as the
// values shift relatively slowly over the course of a GC
// cycle.
assistWorkPerByte := float64(scanWorkRemaining) / float64(heapRemaining)
assistBytesPerWork := float64(heapRemaining) / float64(scanWorkRemaining)
atomic.Store64(&c.assistWorkPerByte, float64bits(assistWorkPerByte))
atomic.Store64(&c.assistBytesPerWork, float64bits(assistBytesPerWork))
}
// endCycle computes the trigger ratio for the next cycle.
func (c *gcControllerState) endCycle() float64 {
if work.userForced {
// Forced GC means this cycle didn't start at the
// trigger, so where it finished isn't good
// information about how to adjust the trigger.
// Just leave it where it is.
return memstats.triggerRatio
}
// Proportional response gain for the trigger controller. Must
// be in [0, 1]. Lower values smooth out transient effects but
// take longer to respond to phase changes. Higher values
// react to phase changes quickly, but are more affected by
// transient changes. Values near 1 may be unstable.
const triggerGain = 0.5
// Compute next cycle trigger ratio. First, this computes the
// "error" for this cycle; that is, how far off the trigger
// was from what it should have been, accounting for both heap
// growth and GC CPU utilization. We compute the actual heap
// growth during this cycle and scale that by how far off from
// the goal CPU utilization we were (to estimate the heap
// growth if we had the desired CPU utilization). The
// difference between this estimate and the GOGC-based goal
// heap growth is the error.
goalGrowthRatio := gcEffectiveGrowthRatio()
actualGrowthRatio := float64(memstats.heap_live)/float64(memstats.heap_marked) - 1
assistDuration := nanotime() - c.markStartTime
// Assume background mark hit its utilization goal.
utilization := gcBackgroundUtilization
// Add assist utilization; avoid divide by zero.
if assistDuration > 0 {
utilization += float64(c.assistTime) / float64(assistDuration*int64(gomaxprocs))
}
triggerError := goalGrowthRatio - memstats.triggerRatio - utilization/gcGoalUtilization*(actualGrowthRatio-memstats.triggerRatio)
// Finally, we adjust the trigger for next time by this error,
// damped by the proportional gain.
triggerRatio := memstats.triggerRatio + triggerGain*triggerError
if debug.gcpacertrace > 0 {
// Print controller state in terms of the design
// document.
H_m_prev := memstats.heap_marked
h_t := memstats.triggerRatio
H_T := memstats.gc_trigger
h_a := actualGrowthRatio
H_a := memstats.heap_live
h_g := goalGrowthRatio
H_g := int64(float64(H_m_prev) * (1 + h_g))
u_a := utilization
u_g := gcGoalUtilization
W_a := c.scanWork
print("pacer: H_m_prev=", H_m_prev,
" h_t=", h_t, " H_T=", H_T,
" h_a=", h_a, " H_a=", H_a,
" h_g=", h_g, " H_g=", H_g,
" u_a=", u_a, " u_g=", u_g,
" W_a=", W_a,
" goalΔ=", goalGrowthRatio-h_t,
" actualΔ=", h_a-h_t,
" u_a/u_g=", u_a/u_g,
"\n")
}
return triggerRatio
}
// enlistWorker encourages another dedicated mark worker to start on
// another P if there are spare worker slots. It is used by putfull
// when more work is made available.
//
//go:nowritebarrier
func (c *gcControllerState) enlistWorker() {
// If there are idle Ps, wake one so it will run an idle worker.
// NOTE: This is suspected of causing deadlocks. See golang.org/issue/19112.
//
// if atomic.Load(&sched.npidle) != 0 && atomic.Load(&sched.nmspinning) == 0 {
// wakep()
// return
// }
// There are no idle Ps. If we need more dedicated workers,
// try to preempt a running P so it will switch to a worker.
if c.dedicatedMarkWorkersNeeded <= 0 {
return
}
// Pick a random other P to preempt.
if gomaxprocs <= 1 {
return
}
gp := getg()
if gp == nil || gp.m == nil || gp.m.p == 0 {
return
}
myID := gp.m.p.ptr().id
for tries := 0; tries < 5; tries++ {
id := int32(fastrandn(uint32(gomaxprocs - 1)))
if id >= myID {
id++
}
p := allp[id]
if p.status != _Prunning {
continue
}
if preemptone(p) {
return
}
}
}
// findRunnableGCWorker returns a background mark worker for _p_ if it
// should be run. This must only be called when gcBlackenEnabled != 0.
func (c *gcControllerState) findRunnableGCWorker(_p_ *p) *g {
if gcBlackenEnabled == 0 {
throw("gcControllerState.findRunnable: blackening not enabled")
}
if !gcMarkWorkAvailable(_p_) {
// No work to be done right now. This can happen at
// the end of the mark phase when there are still
// assists tapering off. Don't bother running a worker
// now because it'll just return immediately.
return nil
}
// Grab a worker before we commit to running below.
node := (*gcBgMarkWorkerNode)(gcBgMarkWorkerPool.pop())
if node == nil {
// There is at least one worker per P, so normally there are
// enough workers to run on all Ps, if necessary. However, once
// a worker enters gcMarkDone it may park without rejoining the
// pool, thus freeing a P with no corresponding worker.
// gcMarkDone never depends on another worker doing work, so it
// is safe to simply do nothing here.
//
// If gcMarkDone bails out without completing the mark phase,
// it will always do so with queued global work. Thus, that P
// will be immediately eligible to re-run the worker G it was
// just using, ensuring work can complete.
return nil
}
decIfPositive := func(ptr *int64) bool {
for {
v := atomic.Loadint64(ptr)
if v <= 0 {
return false
}
if atomic.Casint64(ptr, v, v-1) {
return true
}
}
}
if decIfPositive(&c.dedicatedMarkWorkersNeeded) {
// This P is now dedicated to marking until the end of
// the concurrent mark phase.
_p_.gcMarkWorkerMode = gcMarkWorkerDedicatedMode
} else if c.fractionalUtilizationGoal == 0 {
// No need for fractional workers.
gcBgMarkWorkerPool.push(&node.node)
return nil
} else {
// Is this P behind on the fractional utilization
// goal?
//
// This should be kept in sync with pollFractionalWorkerExit.
delta := nanotime() - gcController.markStartTime
if delta > 0 && float64(_p_.gcFractionalMarkTime)/float64(delta) > c.fractionalUtilizationGoal {
// Nope. No need to run a fractional worker.
gcBgMarkWorkerPool.push(&node.node)
return nil
}
// Run a fractional worker.
_p_.gcMarkWorkerMode = gcMarkWorkerFractionalMode
}
// Run the background mark worker.
gp := node.gp.ptr()
casgstatus(gp, _Gwaiting, _Grunnable)
if trace.enabled {
traceGoUnpark(gp, 0)
}
return gp
}
// pollFractionalWorkerExit reports whether a fractional mark worker // pollFractionalWorkerExit reports whether a fractional mark worker
// should self-preempt. It assumes it is called from the fractional // should self-preempt. It assumes it is called from the fractional
// worker. // worker.
@ -815,203 +292,6 @@ func pollFractionalWorkerExit() bool {
return float64(selfTime)/float64(delta) > 1.2*gcController.fractionalUtilizationGoal return float64(selfTime)/float64(delta) > 1.2*gcController.fractionalUtilizationGoal
} }
// gcSetTriggerRatio sets the trigger ratio and updates everything
// derived from it: the absolute trigger, the heap goal, mark pacing,
// and sweep pacing.
//
// This can be called any time. If GC is the in the middle of a
// concurrent phase, it will adjust the pacing of that phase.
//
// This depends on gcpercent, memstats.heap_marked, and
// memstats.heap_live. These must be up to date.
//
// mheap_.lock must be held or the world must be stopped.
func gcSetTriggerRatio(triggerRatio float64) {
assertWorldStoppedOrLockHeld(&mheap_.lock)
// Compute the next GC goal, which is when the allocated heap
// has grown by GOGC/100 over the heap marked by the last
// cycle.
goal := ^uint64(0)
if gcpercent >= 0 {
goal = memstats.heap_marked + memstats.heap_marked*uint64(gcpercent)/100
}
// Set the trigger ratio, capped to reasonable bounds.
if gcpercent >= 0 {
scalingFactor := float64(gcpercent) / 100
// Ensure there's always a little margin so that the
// mutator assist ratio isn't infinity.
maxTriggerRatio := 0.95 * scalingFactor
if triggerRatio > maxTriggerRatio {
triggerRatio = maxTriggerRatio
}
// If we let triggerRatio go too low, then if the application
// is allocating very rapidly we might end up in a situation
// where we're allocating black during a nearly always-on GC.
// The result of this is a growing heap and ultimately an
// increase in RSS. By capping us at a point >0, we're essentially
// saying that we're OK using more CPU during the GC to prevent
// this growth in RSS.
//
// The current constant was chosen empirically: given a sufficiently
// fast/scalable allocator with 48 Ps that could drive the trigger ratio
// to <0.05, this constant causes applications to retain the same peak
// RSS compared to not having this allocator.
minTriggerRatio := 0.6 * scalingFactor
if triggerRatio < minTriggerRatio {
triggerRatio = minTriggerRatio
}
} else if triggerRatio < 0 {
// gcpercent < 0, so just make sure we're not getting a negative
// triggerRatio. This case isn't expected to happen in practice,
// and doesn't really matter because if gcpercent < 0 then we won't
// ever consume triggerRatio further on in this function, but let's
// just be defensive here; the triggerRatio being negative is almost
// certainly undesirable.
triggerRatio = 0
}
memstats.triggerRatio = triggerRatio
// Compute the absolute GC trigger from the trigger ratio.
//
// We trigger the next GC cycle when the allocated heap has
// grown by the trigger ratio over the marked heap size.
trigger := ^uint64(0)
if gcpercent >= 0 {
trigger = uint64(float64(memstats.heap_marked) * (1 + triggerRatio))
// Don't trigger below the minimum heap size.
minTrigger := heapminimum
if !isSweepDone() {
// Concurrent sweep happens in the heap growth
// from heap_live to gc_trigger, so ensure
// that concurrent sweep has some heap growth
// in which to perform sweeping before we
// start the next GC cycle.
sweepMin := atomic.Load64(&memstats.heap_live) + sweepMinHeapDistance
if sweepMin > minTrigger {
minTrigger = sweepMin
}
}
if trigger < minTrigger {
trigger = minTrigger
}
if int64(trigger) < 0 {
print("runtime: next_gc=", memstats.next_gc, " heap_marked=", memstats.heap_marked, " heap_live=", memstats.heap_live, " initialHeapLive=", work.initialHeapLive, "triggerRatio=", triggerRatio, " minTrigger=", minTrigger, "\n")
throw("gc_trigger underflow")
}
if trigger > goal {
// The trigger ratio is always less than GOGC/100, but
// other bounds on the trigger may have raised it.
// Push up the goal, too.
goal = trigger
}
}
// Commit to the trigger and goal.
memstats.gc_trigger = trigger
atomic.Store64(&memstats.next_gc, goal)
if trace.enabled {
traceNextGC()
}
// Update mark pacing.
if gcphase != _GCoff {
gcController.revise()
}
// Update sweep pacing.
if isSweepDone() {
mheap_.sweepPagesPerByte = 0
} else {
// Concurrent sweep needs to sweep all of the in-use
// pages by the time the allocated heap reaches the GC
// trigger. Compute the ratio of in-use pages to sweep
// per byte allocated, accounting for the fact that
// some might already be swept.
heapLiveBasis := atomic.Load64(&memstats.heap_live)
heapDistance := int64(trigger) - int64(heapLiveBasis)
// Add a little margin so rounding errors and
// concurrent sweep are less likely to leave pages
// unswept when GC starts.
heapDistance -= 1024 * 1024
if heapDistance < _PageSize {
// Avoid setting the sweep ratio extremely high
heapDistance = _PageSize
}
pagesSwept := atomic.Load64(&mheap_.pagesSwept)
pagesInUse := atomic.Load64(&mheap_.pagesInUse)
sweepDistancePages := int64(pagesInUse) - int64(pagesSwept)
if sweepDistancePages <= 0 {
mheap_.sweepPagesPerByte = 0
} else {
mheap_.sweepPagesPerByte = float64(sweepDistancePages) / float64(heapDistance)
mheap_.sweepHeapLiveBasis = heapLiveBasis
// Write pagesSweptBasis last, since this
// signals concurrent sweeps to recompute
// their debt.
atomic.Store64(&mheap_.pagesSweptBasis, pagesSwept)
}
}
gcPaceScavenger()
}
// gcEffectiveGrowthRatio returns the current effective heap growth
// ratio (GOGC/100) based on heap_marked from the previous GC and
// next_gc for the current GC.
//
// This may differ from gcpercent/100 because of various upper and
// lower bounds on gcpercent. For example, if the heap is smaller than
// heapminimum, this can be higher than gcpercent/100.
//
// mheap_.lock must be held or the world must be stopped.
func gcEffectiveGrowthRatio() float64 {
assertWorldStoppedOrLockHeld(&mheap_.lock)
egogc := float64(atomic.Load64(&memstats.next_gc)-memstats.heap_marked) / float64(memstats.heap_marked)
if egogc < 0 {
// Shouldn't happen, but just in case.
egogc = 0
}
return egogc
}
// gcGoalUtilization is the goal CPU utilization for
// marking as a fraction of GOMAXPROCS.
const gcGoalUtilization = 0.30
// gcBackgroundUtilization is the fixed CPU utilization for background
// marking. It must be <= gcGoalUtilization. The difference between
// gcGoalUtilization and gcBackgroundUtilization will be made up by
// mark assists. The scheduler will aim to use within 50% of this
// goal.
//
// Setting this to < gcGoalUtilization avoids saturating the trigger
// feedback controller when there are no assists, which allows it to
// better control CPU and heap growth. However, the larger the gap,
// the more mutator assists are expected to happen, which impact
// mutator latency.
const gcBackgroundUtilization = 0.25
// gcCreditSlack is the amount of scan work credit that can
// accumulate locally before updating gcController.scanWork and,
// optionally, gcController.bgScanCredit. Lower values give a more
// accurate assist ratio and make it more likely that assists will
// successfully steal background credit. Higher values reduce memory
// contention.
const gcCreditSlack = 2000
// gcAssistTimeSlack is the nanoseconds of mutator assist time that
// can accumulate on a P before updating gcController.assistTime.
const gcAssistTimeSlack = 5000
// gcOverAssistWork determines how many extra units of scan work a GC
// assist does when an assist happens. This amortizes the cost of an
// assist by pre-paying for this many bytes of future allocations.
const gcOverAssistWork = 64 << 10
var work struct { var work struct {
full lfstack // lock-free list of full blocks workbuf full lfstack // lock-free list of full blocks workbuf
empty lfstack // lock-free list of empty blocks workbuf empty lfstack // lock-free list of empty blocks workbuf

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// Copyright 2021 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package runtime
import (
"internal/cpu"
"runtime/internal/atomic"
_ "unsafe" // for linkname
)
const (
// gcGoalUtilization is the goal CPU utilization for
// marking as a fraction of GOMAXPROCS.
gcGoalUtilization = 0.30
// gcBackgroundUtilization is the fixed CPU utilization for background
// marking. It must be <= gcGoalUtilization. The difference between
// gcGoalUtilization and gcBackgroundUtilization will be made up by
// mark assists. The scheduler will aim to use within 50% of this
// goal.
//
// Setting this to < gcGoalUtilization avoids saturating the trigger
// feedback controller when there are no assists, which allows it to
// better control CPU and heap growth. However, the larger the gap,
// the more mutator assists are expected to happen, which impact
// mutator latency.
gcBackgroundUtilization = 0.25
// gcCreditSlack is the amount of scan work credit that can
// accumulate locally before updating gcController.scanWork and,
// optionally, gcController.bgScanCredit. Lower values give a more
// accurate assist ratio and make it more likely that assists will
// successfully steal background credit. Higher values reduce memory
// contention.
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 = 4 << 20
)
var (
// 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 = defaultHeapMinimum
// Initialized from $GOGC. GOGC=off means no GC.
gcpercent int32
)
// gcController implements the GC pacing controller that determines
// when to trigger concurrent garbage collection and how much marking
// work to do in mutator assists and background marking.
//
// It uses a feedback control algorithm to adjust the memstats.gc_trigger
// trigger based on the heap growth and GC CPU utilization each cycle.
// This algorithm optimizes for heap growth to match GOGC and for CPU
// utilization between assist and background marking to be 25% of
// GOMAXPROCS. The high-level design of this algorithm is documented
// at https://golang.org/s/go15gcpacing.
//
// All fields of gcController are used only during a single mark
// cycle.
var gcController gcControllerState
type gcControllerState struct {
// scanWork is the total scan work performed this cycle. This
// is updated atomically during the cycle. Updates occur in
// bounded batches, since it is both written and read
// throughout the cycle. At the end of the cycle, this is how
// much of the retained heap is scannable.
//
// Currently this is the bytes of heap scanned. For most uses,
// this is an opaque unit of work, but for estimation the
// definition is important.
scanWork int64
// bgScanCredit is the scan work credit accumulated by the
// concurrent background scan. This credit is accumulated by
// the background scan and stolen by mutator assists. This is
// updated atomically. Updates occur in bounded batches, since
// it is both written and read throughout the cycle.
bgScanCredit int64
// assistTime is the nanoseconds spent in mutator assists
// during this cycle. This is updated atomically. Updates
// occur in bounded batches, since it is both written and read
// throughout the cycle.
assistTime int64
// dedicatedMarkTime is the nanoseconds spent in dedicated
// mark workers during this cycle. This is updated atomically
// at the end of the concurrent mark phase.
dedicatedMarkTime int64
// fractionalMarkTime is the nanoseconds spent in the
// fractional mark worker during this cycle. This is updated
// atomically throughout the cycle and will be up-to-date if
// the fractional mark worker is not currently running.
fractionalMarkTime int64
// idleMarkTime is the nanoseconds spent in idle marking
// during this cycle. This is updated atomically throughout
// the cycle.
idleMarkTime int64
// markStartTime is the absolute start time in nanoseconds
// that assists and background mark workers started.
markStartTime int64
// dedicatedMarkWorkersNeeded is the number of dedicated mark
// workers that need to be started. This is computed at the
// beginning of each cycle and decremented atomically as
// dedicated mark workers get started.
dedicatedMarkWorkersNeeded int64
// assistWorkPerByte is the ratio of scan work to allocated
// bytes that should be performed by mutator assists. This is
// computed at the beginning of each cycle and updated every
// time heap_scan is updated.
//
// Stored as a uint64, but it's actually a float64. Use
// float64frombits to get the value.
//
// Read and written atomically.
assistWorkPerByte uint64
// assistBytesPerWork is 1/assistWorkPerByte.
//
// Stored as a uint64, but it's actually a float64. Use
// float64frombits to get the value.
//
// Read and written atomically.
//
// 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 uint64
// fractionalUtilizationGoal is the fraction of wall clock
// time that should be spent in the fractional mark worker on
// each P that isn't running a dedicated worker.
//
// For example, if the utilization goal is 25% and there are
// no dedicated workers, this will be 0.25. If the goal is
// 25%, there is one dedicated worker, and GOMAXPROCS is 5,
// this will be 0.05 to make up the missing 5%.
//
// If this is zero, no fractional workers are needed.
fractionalUtilizationGoal float64
_ cpu.CacheLinePad
}
// startCycle resets the GC controller's state and computes estimates
// for a new GC cycle. The caller must hold worldsema and the world
// must be stopped.
func (c *gcControllerState) startCycle() {
c.scanWork = 0
c.bgScanCredit = 0
c.assistTime = 0
c.dedicatedMarkTime = 0
c.fractionalMarkTime = 0
c.idleMarkTime = 0
// Ensure that the heap goal is at least a little larger than
// the current live heap size. This may not be the case if GC
// start is delayed or if the allocation that pushed heap_live
// over gc_trigger is large or if the trigger is really close to
// GOGC. Assist is proportional to this distance, so enforce a
// minimum distance, even if it means going over the GOGC goal
// by a tiny bit.
if memstats.next_gc < memstats.heap_live+1024*1024 {
memstats.next_gc = memstats.heap_live + 1024*1024
}
// Compute the background mark utilization goal. In general,
// this may not come out exactly. We round the number of
// dedicated workers so that the utilization is closest to
// 25%. For small GOMAXPROCS, this would introduce too much
// error, so we add fractional workers in that case.
totalUtilizationGoal := float64(gomaxprocs) * gcBackgroundUtilization
c.dedicatedMarkWorkersNeeded = int64(totalUtilizationGoal + 0.5)
utilError := float64(c.dedicatedMarkWorkersNeeded)/totalUtilizationGoal - 1
const maxUtilError = 0.3
if utilError < -maxUtilError || utilError > maxUtilError {
// Rounding put us more than 30% off our goal. With
// gcBackgroundUtilization of 25%, this happens for
// GOMAXPROCS<=3 or GOMAXPROCS=6. Enable fractional
// workers to compensate.
if float64(c.dedicatedMarkWorkersNeeded) > totalUtilizationGoal {
// Too many dedicated workers.
c.dedicatedMarkWorkersNeeded--
}
c.fractionalUtilizationGoal = (totalUtilizationGoal - float64(c.dedicatedMarkWorkersNeeded)) / float64(gomaxprocs)
} else {
c.fractionalUtilizationGoal = 0
}
// In STW mode, we just want dedicated workers.
if debug.gcstoptheworld > 0 {
c.dedicatedMarkWorkersNeeded = int64(gomaxprocs)
c.fractionalUtilizationGoal = 0
}
// Clear per-P state
for _, p := range allp {
p.gcAssistTime = 0
p.gcFractionalMarkTime = 0
}
// Compute initial values for controls that are updated
// throughout the cycle.
c.revise()
if debug.gcpacertrace > 0 {
assistRatio := float64frombits(atomic.Load64(&c.assistWorkPerByte))
print("pacer: assist ratio=", assistRatio,
" (scan ", memstats.heap_scan>>20, " MB in ",
work.initialHeapLive>>20, "->",
memstats.next_gc>>20, " MB)",
" workers=", c.dedicatedMarkWorkersNeeded,
"+", c.fractionalUtilizationGoal, "\n")
}
}
// revise updates the assist ratio during the GC cycle to account for
// improved estimates. This should be called whenever memstats.heap_scan,
// memstats.heap_live, or memstats.next_gc is updated. It is safe to
// call concurrently, but it may race with other calls to revise.
//
// The result of this race is that the two assist ratio values may not line
// up or may be stale. In practice this is OK because the assist ratio
// moves slowly throughout a GC cycle, and the assist ratio is a best-effort
// heuristic anyway. Furthermore, no part of the heuristic depends on
// the two assist ratio values being exact reciprocals of one another, since
// the two values are used to convert values from different sources.
//
// The worst case result of this raciness is that we may miss a larger shift
// in the ratio (say, if we decide to pace more aggressively against the
// hard heap goal) but even this "hard goal" is best-effort (see #40460).
// The dedicated GC should ensure we don't exceed the hard goal by too much
// in the rare case we do exceed it.
//
// It should only be called when gcBlackenEnabled != 0 (because this
// is when assists are enabled and the necessary statistics are
// available).
func (c *gcControllerState) revise() {
gcpercent := gcpercent
if gcpercent < 0 {
// If GC is disabled but we're running a forced GC,
// act like GOGC is huge for the below calculations.
gcpercent = 100000
}
live := atomic.Load64(&memstats.heap_live)
scan := atomic.Load64(&memstats.heap_scan)
work := atomic.Loadint64(&c.scanWork)
// Assume we're under the soft goal. Pace GC to complete at
// next_gc assuming the heap is in steady-state.
heapGoal := int64(atomic.Load64(&memstats.next_gc))
// Compute the expected scan work remaining.
//
// This is estimated based on the expected
// steady-state scannable heap. For example, with
// GOGC=100, only half of the scannable heap is
// expected to be live, so that's what we target.
//
// (This is a float calculation to avoid overflowing on
// 100*heap_scan.)
scanWorkExpected := int64(float64(scan) * 100 / float64(100+gcpercent))
if int64(live) > heapGoal || work > scanWorkExpected {
// We're past the soft goal, or we've already done more scan
// work than we expected. Pace GC so that in the worst case it
// will complete by the hard goal.
const maxOvershoot = 1.1
heapGoal = int64(float64(heapGoal) * maxOvershoot)
// Compute the upper bound on the scan work remaining.
scanWorkExpected = int64(scan)
}
// Compute the remaining scan work estimate.
//
// Note that we currently count allocations during GC as both
// scannable heap (heap_scan) and scan work completed
// (scanWork), so allocation will change this difference
// slowly in the soft regime and not at all in the hard
// regime.
scanWorkRemaining := scanWorkExpected - work
if scanWorkRemaining < 1000 {
// We set a somewhat arbitrary lower bound on
// remaining scan work since if we aim a little high,
// we can miss by a little.
//
// We *do* need to enforce that this is at least 1,
// since marking is racy and double-scanning objects
// may legitimately make the remaining scan work
// negative, even in the hard goal regime.
scanWorkRemaining = 1000
}
// Compute the heap distance remaining.
heapRemaining := heapGoal - int64(live)
if heapRemaining <= 0 {
// This shouldn't happen, but if it does, avoid
// dividing by zero or setting the assist negative.
heapRemaining = 1
}
// Compute the mutator assist ratio so by the time the mutator
// allocates the remaining heap bytes up to next_gc, it will
// have done (or stolen) the remaining amount of scan work.
// Note that the assist ratio values are updated atomically
// but not together. This means there may be some degree of
// skew between the two values. This is generally OK as the
// values shift relatively slowly over the course of a GC
// cycle.
assistWorkPerByte := float64(scanWorkRemaining) / float64(heapRemaining)
assistBytesPerWork := float64(heapRemaining) / float64(scanWorkRemaining)
atomic.Store64(&c.assistWorkPerByte, float64bits(assistWorkPerByte))
atomic.Store64(&c.assistBytesPerWork, float64bits(assistBytesPerWork))
}
// endCycle computes the trigger ratio for the next cycle.
func (c *gcControllerState) endCycle() float64 {
if work.userForced {
// Forced GC means this cycle didn't start at the
// trigger, so where it finished isn't good
// information about how to adjust the trigger.
// Just leave it where it is.
return memstats.triggerRatio
}
// Proportional response gain for the trigger controller. Must
// be in [0, 1]. Lower values smooth out transient effects but
// take longer to respond to phase changes. Higher values
// react to phase changes quickly, but are more affected by
// transient changes. Values near 1 may be unstable.
const triggerGain = 0.5
// Compute next cycle trigger ratio. First, this computes the
// "error" for this cycle; that is, how far off the trigger
// was from what it should have been, accounting for both heap
// growth and GC CPU utilization. We compute the actual heap
// growth during this cycle and scale that by how far off from
// the goal CPU utilization we were (to estimate the heap
// growth if we had the desired CPU utilization). The
// difference between this estimate and the GOGC-based goal
// heap growth is the error.
goalGrowthRatio := gcEffectiveGrowthRatio()
actualGrowthRatio := float64(memstats.heap_live)/float64(memstats.heap_marked) - 1
assistDuration := nanotime() - c.markStartTime
// Assume background mark hit its utilization goal.
utilization := gcBackgroundUtilization
// Add assist utilization; avoid divide by zero.
if assistDuration > 0 {
utilization += float64(c.assistTime) / float64(assistDuration*int64(gomaxprocs))
}
triggerError := goalGrowthRatio - memstats.triggerRatio - utilization/gcGoalUtilization*(actualGrowthRatio-memstats.triggerRatio)
// Finally, we adjust the trigger for next time by this error,
// damped by the proportional gain.
triggerRatio := memstats.triggerRatio + triggerGain*triggerError
if debug.gcpacertrace > 0 {
// Print controller state in terms of the design
// document.
H_m_prev := memstats.heap_marked
h_t := memstats.triggerRatio
H_T := memstats.gc_trigger
h_a := actualGrowthRatio
H_a := memstats.heap_live
h_g := goalGrowthRatio
H_g := int64(float64(H_m_prev) * (1 + h_g))
u_a := utilization
u_g := gcGoalUtilization
W_a := c.scanWork
print("pacer: H_m_prev=", H_m_prev,
" h_t=", h_t, " H_T=", H_T,
" h_a=", h_a, " H_a=", H_a,
" h_g=", h_g, " H_g=", H_g,
" u_a=", u_a, " u_g=", u_g,
" W_a=", W_a,
" goalΔ=", goalGrowthRatio-h_t,
" actualΔ=", h_a-h_t,
" u_a/u_g=", u_a/u_g,
"\n")
}
return triggerRatio
}
// enlistWorker encourages another dedicated mark worker to start on
// another P if there are spare worker slots. It is used by putfull
// when more work is made available.
//
//go:nowritebarrier
func (c *gcControllerState) enlistWorker() {
// If there are idle Ps, wake one so it will run an idle worker.
// NOTE: This is suspected of causing deadlocks. See golang.org/issue/19112.
//
// if atomic.Load(&sched.npidle) != 0 && atomic.Load(&sched.nmspinning) == 0 {
// wakep()
// return
// }
// There are no idle Ps. If we need more dedicated workers,
// try to preempt a running P so it will switch to a worker.
if c.dedicatedMarkWorkersNeeded <= 0 {
return
}
// Pick a random other P to preempt.
if gomaxprocs <= 1 {
return
}
gp := getg()
if gp == nil || gp.m == nil || gp.m.p == 0 {
return
}
myID := gp.m.p.ptr().id
for tries := 0; tries < 5; tries++ {
id := int32(fastrandn(uint32(gomaxprocs - 1)))
if id >= myID {
id++
}
p := allp[id]
if p.status != _Prunning {
continue
}
if preemptone(p) {
return
}
}
}
// findRunnableGCWorker returns a background mark worker for _p_ if it
// should be run. This must only be called when gcBlackenEnabled != 0.
func (c *gcControllerState) findRunnableGCWorker(_p_ *p) *g {
if gcBlackenEnabled == 0 {
throw("gcControllerState.findRunnable: blackening not enabled")
}
if !gcMarkWorkAvailable(_p_) {
// No work to be done right now. This can happen at
// the end of the mark phase when there are still
// assists tapering off. Don't bother running a worker
// now because it'll just return immediately.
return nil
}
// Grab a worker before we commit to running below.
node := (*gcBgMarkWorkerNode)(gcBgMarkWorkerPool.pop())
if node == nil {
// There is at least one worker per P, so normally there are
// enough workers to run on all Ps, if necessary. However, once
// a worker enters gcMarkDone it may park without rejoining the
// pool, thus freeing a P with no corresponding worker.
// gcMarkDone never depends on another worker doing work, so it
// is safe to simply do nothing here.
//
// If gcMarkDone bails out without completing the mark phase,
// it will always do so with queued global work. Thus, that P
// will be immediately eligible to re-run the worker G it was
// just using, ensuring work can complete.
return nil
}
decIfPositive := func(ptr *int64) bool {
for {
v := atomic.Loadint64(ptr)
if v <= 0 {
return false
}
if atomic.Casint64(ptr, v, v-1) {
return true
}
}
}
if decIfPositive(&c.dedicatedMarkWorkersNeeded) {
// This P is now dedicated to marking until the end of
// the concurrent mark phase.
_p_.gcMarkWorkerMode = gcMarkWorkerDedicatedMode
} else if c.fractionalUtilizationGoal == 0 {
// No need for fractional workers.
gcBgMarkWorkerPool.push(&node.node)
return nil
} else {
// Is this P behind on the fractional utilization
// goal?
//
// This should be kept in sync with pollFractionalWorkerExit.
delta := nanotime() - gcController.markStartTime
if delta > 0 && float64(_p_.gcFractionalMarkTime)/float64(delta) > c.fractionalUtilizationGoal {
// Nope. No need to run a fractional worker.
gcBgMarkWorkerPool.push(&node.node)
return nil
}
// Run a fractional worker.
_p_.gcMarkWorkerMode = gcMarkWorkerFractionalMode
}
// Run the background mark worker.
gp := node.gp.ptr()
casgstatus(gp, _Gwaiting, _Grunnable)
if trace.enabled {
traceGoUnpark(gp, 0)
}
return gp
}
// gcSetTriggerRatio sets the trigger ratio and updates everything
// derived from it: the absolute trigger, the heap goal, mark pacing,
// and sweep pacing.
//
// This can be called any time. If GC is the in the middle of a
// concurrent phase, it will adjust the pacing of that phase.
//
// This depends on gcpercent, memstats.heap_marked, and
// memstats.heap_live. These must be up to date.
//
// mheap_.lock must be held or the world must be stopped.
func gcSetTriggerRatio(triggerRatio float64) {
assertWorldStoppedOrLockHeld(&mheap_.lock)
// Compute the next GC goal, which is when the allocated heap
// has grown by GOGC/100 over the heap marked by the last
// cycle.
goal := ^uint64(0)
if gcpercent >= 0 {
goal = memstats.heap_marked + memstats.heap_marked*uint64(gcpercent)/100
}
// Set the trigger ratio, capped to reasonable bounds.
if gcpercent >= 0 {
scalingFactor := float64(gcpercent) / 100
// Ensure there's always a little margin so that the
// mutator assist ratio isn't infinity.
maxTriggerRatio := 0.95 * scalingFactor
if triggerRatio > maxTriggerRatio {
triggerRatio = maxTriggerRatio
}
// If we let triggerRatio go too low, then if the application
// is allocating very rapidly we might end up in a situation
// where we're allocating black during a nearly always-on GC.
// The result of this is a growing heap and ultimately an
// increase in RSS. By capping us at a point >0, we're essentially
// saying that we're OK using more CPU during the GC to prevent
// this growth in RSS.
//
// The current constant was chosen empirically: given a sufficiently
// fast/scalable allocator with 48 Ps that could drive the trigger ratio
// to <0.05, this constant causes applications to retain the same peak
// RSS compared to not having this allocator.
minTriggerRatio := 0.6 * scalingFactor
if triggerRatio < minTriggerRatio {
triggerRatio = minTriggerRatio
}
} else if triggerRatio < 0 {
// gcpercent < 0, so just make sure we're not getting a negative
// triggerRatio. This case isn't expected to happen in practice,
// and doesn't really matter because if gcpercent < 0 then we won't
// ever consume triggerRatio further on in this function, but let's
// just be defensive here; the triggerRatio being negative is almost
// certainly undesirable.
triggerRatio = 0
}
memstats.triggerRatio = triggerRatio
// Compute the absolute GC trigger from the trigger ratio.
//
// We trigger the next GC cycle when the allocated heap has
// grown by the trigger ratio over the marked heap size.
trigger := ^uint64(0)
if gcpercent >= 0 {
trigger = uint64(float64(memstats.heap_marked) * (1 + triggerRatio))
// Don't trigger below the minimum heap size.
minTrigger := heapminimum
if !isSweepDone() {
// Concurrent sweep happens in the heap growth
// from heap_live to gc_trigger, so ensure
// that concurrent sweep has some heap growth
// in which to perform sweeping before we
// start the next GC cycle.
sweepMin := atomic.Load64(&memstats.heap_live) + sweepMinHeapDistance
if sweepMin > minTrigger {
minTrigger = sweepMin
}
}
if trigger < minTrigger {
trigger = minTrigger
}
if int64(trigger) < 0 {
print("runtime: next_gc=", memstats.next_gc, " heap_marked=", memstats.heap_marked, " heap_live=", memstats.heap_live, " initialHeapLive=", work.initialHeapLive, "triggerRatio=", triggerRatio, " minTrigger=", minTrigger, "\n")
throw("gc_trigger underflow")
}
if trigger > goal {
// The trigger ratio is always less than GOGC/100, but
// other bounds on the trigger may have raised it.
// Push up the goal, too.
goal = trigger
}
}
// Commit to the trigger and goal.
memstats.gc_trigger = trigger
atomic.Store64(&memstats.next_gc, goal)
if trace.enabled {
traceNextGC()
}
// Update mark pacing.
if gcphase != _GCoff {
gcController.revise()
}
// Update sweep pacing.
if isSweepDone() {
mheap_.sweepPagesPerByte = 0
} else {
// Concurrent sweep needs to sweep all of the in-use
// pages by the time the allocated heap reaches the GC
// trigger. Compute the ratio of in-use pages to sweep
// per byte allocated, accounting for the fact that
// some might already be swept.
heapLiveBasis := atomic.Load64(&memstats.heap_live)
heapDistance := int64(trigger) - int64(heapLiveBasis)
// Add a little margin so rounding errors and
// concurrent sweep are less likely to leave pages
// unswept when GC starts.
heapDistance -= 1024 * 1024
if heapDistance < _PageSize {
// Avoid setting the sweep ratio extremely high
heapDistance = _PageSize
}
pagesSwept := atomic.Load64(&mheap_.pagesSwept)
pagesInUse := atomic.Load64(&mheap_.pagesInUse)
sweepDistancePages := int64(pagesInUse) - int64(pagesSwept)
if sweepDistancePages <= 0 {
mheap_.sweepPagesPerByte = 0
} else {
mheap_.sweepPagesPerByte = float64(sweepDistancePages) / float64(heapDistance)
mheap_.sweepHeapLiveBasis = heapLiveBasis
// Write pagesSweptBasis last, since this
// signals concurrent sweeps to recompute
// their debt.
atomic.Store64(&mheap_.pagesSweptBasis, pagesSwept)
}
}
gcPaceScavenger()
}
// gcEffectiveGrowthRatio returns the current effective heap growth
// ratio (GOGC/100) based on heap_marked from the previous GC and
// next_gc for the current GC.
//
// This may differ from gcpercent/100 because of various upper and
// lower bounds on gcpercent. For example, if the heap is smaller than
// heapminimum, this can be higher than gcpercent/100.
//
// mheap_.lock must be held or the world must be stopped.
func gcEffectiveGrowthRatio() float64 {
assertWorldStoppedOrLockHeld(&mheap_.lock)
egogc := float64(atomic.Load64(&memstats.next_gc)-memstats.heap_marked) / float64(memstats.heap_marked)
if egogc < 0 {
// Shouldn't happen, but just in case.
egogc = 0
}
return egogc
}
//go:linkname setGCPercent runtime/debug.setGCPercent
func setGCPercent(in int32) (out int32) {
// Run on the system stack since we grab the heap lock.
systemstack(func() {
lock(&mheap_.lock)
out = gcpercent
if in < 0 {
in = -1
}
gcpercent = in
heapminimum = defaultHeapMinimum * uint64(gcpercent) / 100
// Update pacing in response to gcpercent change.
gcSetTriggerRatio(memstats.triggerRatio)
unlock(&mheap_.lock)
})
// If we just disabled GC, wait for any concurrent GC mark to
// finish so we always return with no GC running.
if in < 0 {
gcWaitOnMark(atomic.Load(&work.cycles))
}
return out
}
func readgogc() int32 {
p := gogetenv("GOGC")
if p == "off" {
return -1
}
if n, ok := atoi32(p); ok {
return n
}
return 100
}