1 // Copyright 2021 The Go Authors. All rights reserved.
2 // Use of this source code is governed by a BSD-style
3 // license that can be found in the LICENSE file.
9 "runtime/internal/atomic"
10 _ "unsafe" // for linkname
14 // gcGoalUtilization is the goal CPU utilization for
15 // marking as a fraction of GOMAXPROCS.
16 gcGoalUtilization = 0.30
18 // gcBackgroundUtilization is the fixed CPU utilization for background
19 // marking. It must be <= gcGoalUtilization. The difference between
20 // gcGoalUtilization and gcBackgroundUtilization will be made up by
21 // mark assists. The scheduler will aim to use within 50% of this
24 // Setting this to < gcGoalUtilization avoids saturating the trigger
25 // feedback controller when there are no assists, which allows it to
26 // better control CPU and heap growth. However, the larger the gap,
27 // the more mutator assists are expected to happen, which impact
29 gcBackgroundUtilization = 0.25
31 // gcCreditSlack is the amount of scan work credit that can
32 // accumulate locally before updating gcController.scanWork and,
33 // optionally, gcController.bgScanCredit. Lower values give a more
34 // accurate assist ratio and make it more likely that assists will
35 // successfully steal background credit. Higher values reduce memory
39 // gcAssistTimeSlack is the nanoseconds of mutator assist time that
40 // can accumulate on a P before updating gcController.assistTime.
41 gcAssistTimeSlack = 5000
43 // gcOverAssistWork determines how many extra units of scan work a GC
44 // assist does when an assist happens. This amortizes the cost of an
45 // assist by pre-paying for this many bytes of future allocations.
46 gcOverAssistWork = 64 << 10
48 // defaultHeapMinimum is the value of heapMinimum for GOGC==100.
49 defaultHeapMinimum = 4 << 20
53 // heapMinimum is the minimum heap size at which to trigger GC.
54 // For small heaps, this overrides the usual GOGC*live set rule.
56 // When there is a very small live set but a lot of allocation, simply
57 // collecting when the heap reaches GOGC*live results in many GC
58 // cycles and high total per-GC overhead. This minimum amortizes this
59 // per-GC overhead while keeping the heap reasonably small.
61 // During initialization this is set to 4MB*GOGC/100. In the case of
62 // GOGC==0, this will set heapMinimum to 0, resulting in constant
63 // collection even when the heap size is small, which is useful for
65 heapMinimum uint64 = defaultHeapMinimum
67 // Initialized from $GOGC. GOGC=off means no GC.
71 // gcController implements the GC pacing controller that determines
72 // when to trigger concurrent garbage collection and how much marking
73 // work to do in mutator assists and background marking.
75 // It uses a feedback control algorithm to adjust the memstats.gc_trigger
76 // trigger based on the heap growth and GC CPU utilization each cycle.
77 // This algorithm optimizes for heap growth to match GOGC and for CPU
78 // utilization between assist and background marking to be 25% of
79 // GOMAXPROCS. The high-level design of this algorithm is documented
80 // at https://golang.org/s/go15gcpacing.
82 // All fields of gcController are used only during a single mark
84 var gcController gcControllerState
86 type gcControllerState struct {
87 // scanWork is the total scan work performed this cycle. This
88 // is updated atomically during the cycle. Updates occur in
89 // bounded batches, since it is both written and read
90 // throughout the cycle. At the end of the cycle, this is how
91 // much of the retained heap is scannable.
93 // Currently this is the bytes of heap scanned. For most uses,
94 // this is an opaque unit of work, but for estimation the
95 // definition is important.
98 // bgScanCredit is the scan work credit accumulated by the
99 // concurrent background scan. This credit is accumulated by
100 // the background scan and stolen by mutator assists. This is
101 // updated atomically. Updates occur in bounded batches, since
102 // it is both written and read throughout the cycle.
105 // assistTime is the nanoseconds spent in mutator assists
106 // during this cycle. This is updated atomically. Updates
107 // occur in bounded batches, since it is both written and read
108 // throughout the cycle.
111 // dedicatedMarkTime is the nanoseconds spent in dedicated
112 // mark workers during this cycle. This is updated atomically
113 // at the end of the concurrent mark phase.
114 dedicatedMarkTime int64
116 // fractionalMarkTime is the nanoseconds spent in the
117 // fractional mark worker during this cycle. This is updated
118 // atomically throughout the cycle and will be up-to-date if
119 // the fractional mark worker is not currently running.
120 fractionalMarkTime int64
122 // idleMarkTime is the nanoseconds spent in idle marking
123 // during this cycle. This is updated atomically throughout
127 // markStartTime is the absolute start time in nanoseconds
128 // that assists and background mark workers started.
131 // dedicatedMarkWorkersNeeded is the number of dedicated mark
132 // workers that need to be started. This is computed at the
133 // beginning of each cycle and decremented atomically as
134 // dedicated mark workers get started.
135 dedicatedMarkWorkersNeeded int64
137 // assistWorkPerByte is the ratio of scan work to allocated
138 // bytes that should be performed by mutator assists. This is
139 // computed at the beginning of each cycle and updated every
140 // time heap_scan is updated.
142 // Stored as a uint64, but it's actually a float64. Use
143 // float64frombits to get the value.
145 // Read and written atomically.
146 assistWorkPerByte uint64
148 // assistBytesPerWork is 1/assistWorkPerByte.
150 // Stored as a uint64, but it's actually a float64. Use
151 // float64frombits to get the value.
153 // Read and written atomically.
155 // Note that because this is read and written independently
156 // from assistWorkPerByte users may notice a skew between
157 // the two values, and such a state should be safe.
158 assistBytesPerWork uint64
160 // fractionalUtilizationGoal is the fraction of wall clock
161 // time that should be spent in the fractional mark worker on
162 // each P that isn't running a dedicated worker.
164 // For example, if the utilization goal is 25% and there are
165 // no dedicated workers, this will be 0.25. If the goal is
166 // 25%, there is one dedicated worker, and GOMAXPROCS is 5,
167 // this will be 0.05 to make up the missing 5%.
169 // If this is zero, no fractional workers are needed.
170 fractionalUtilizationGoal float64
175 // startCycle resets the GC controller's state and computes estimates
176 // for a new GC cycle. The caller must hold worldsema and the world
178 func (c *gcControllerState) startCycle() {
182 c.dedicatedMarkTime = 0
183 c.fractionalMarkTime = 0
186 // Ensure that the heap goal is at least a little larger than
187 // the current live heap size. This may not be the case if GC
188 // start is delayed or if the allocation that pushed heap_live
189 // over gc_trigger is large or if the trigger is really close to
190 // GOGC. Assist is proportional to this distance, so enforce a
191 // minimum distance, even if it means going over the GOGC goal
193 if memstats.next_gc < memstats.heap_live+1024*1024 {
194 memstats.next_gc = memstats.heap_live + 1024*1024
197 // Compute the background mark utilization goal. In general,
198 // this may not come out exactly. We round the number of
199 // dedicated workers so that the utilization is closest to
200 // 25%. For small GOMAXPROCS, this would introduce too much
201 // error, so we add fractional workers in that case.
202 totalUtilizationGoal := float64(gomaxprocs) * gcBackgroundUtilization
203 c.dedicatedMarkWorkersNeeded = int64(totalUtilizationGoal + 0.5)
204 utilError := float64(c.dedicatedMarkWorkersNeeded)/totalUtilizationGoal - 1
205 const maxUtilError = 0.3
206 if utilError < -maxUtilError || utilError > maxUtilError {
207 // Rounding put us more than 30% off our goal. With
208 // gcBackgroundUtilization of 25%, this happens for
209 // GOMAXPROCS<=3 or GOMAXPROCS=6. Enable fractional
210 // workers to compensate.
211 if float64(c.dedicatedMarkWorkersNeeded) > totalUtilizationGoal {
212 // Too many dedicated workers.
213 c.dedicatedMarkWorkersNeeded--
215 c.fractionalUtilizationGoal = (totalUtilizationGoal - float64(c.dedicatedMarkWorkersNeeded)) / float64(gomaxprocs)
217 c.fractionalUtilizationGoal = 0
220 // In STW mode, we just want dedicated workers.
221 if debug.gcstoptheworld > 0 {
222 c.dedicatedMarkWorkersNeeded = int64(gomaxprocs)
223 c.fractionalUtilizationGoal = 0
227 for _, p := range allp {
229 p.gcFractionalMarkTime = 0
232 // Compute initial values for controls that are updated
233 // throughout the cycle.
236 if debug.gcpacertrace > 0 {
237 assistRatio := float64frombits(atomic.Load64(&c.assistWorkPerByte))
238 print("pacer: assist ratio=", assistRatio,
239 " (scan ", memstats.heap_scan>>20, " MB in ",
240 work.initialHeapLive>>20, "->",
241 memstats.next_gc>>20, " MB)",
242 " workers=", c.dedicatedMarkWorkersNeeded,
243 "+", c.fractionalUtilizationGoal, "\n")
247 // revise updates the assist ratio during the GC cycle to account for
248 // improved estimates. This should be called whenever memstats.heap_scan,
249 // memstats.heap_live, or memstats.next_gc is updated. It is safe to
250 // call concurrently, but it may race with other calls to revise.
252 // The result of this race is that the two assist ratio values may not line
253 // up or may be stale. In practice this is OK because the assist ratio
254 // moves slowly throughout a GC cycle, and the assist ratio is a best-effort
255 // heuristic anyway. Furthermore, no part of the heuristic depends on
256 // the two assist ratio values being exact reciprocals of one another, since
257 // the two values are used to convert values from different sources.
259 // The worst case result of this raciness is that we may miss a larger shift
260 // in the ratio (say, if we decide to pace more aggressively against the
261 // hard heap goal) but even this "hard goal" is best-effort (see #40460).
262 // The dedicated GC should ensure we don't exceed the hard goal by too much
263 // in the rare case we do exceed it.
265 // It should only be called when gcBlackenEnabled != 0 (because this
266 // is when assists are enabled and the necessary statistics are
268 func (c *gcControllerState) revise() {
269 gcPercent := gcPercent
271 // If GC is disabled but we're running a forced GC,
272 // act like GOGC is huge for the below calculations.
275 live := atomic.Load64(&memstats.heap_live)
276 scan := atomic.Load64(&memstats.heap_scan)
277 work := atomic.Loadint64(&c.scanWork)
279 // Assume we're under the soft goal. Pace GC to complete at
280 // next_gc assuming the heap is in steady-state.
281 heapGoal := int64(atomic.Load64(&memstats.next_gc))
283 // Compute the expected scan work remaining.
285 // This is estimated based on the expected
286 // steady-state scannable heap. For example, with
287 // GOGC=100, only half of the scannable heap is
288 // expected to be live, so that's what we target.
290 // (This is a float calculation to avoid overflowing on
292 scanWorkExpected := int64(float64(scan) * 100 / float64(100+gcPercent))
294 if int64(live) > heapGoal || work > scanWorkExpected {
295 // We're past the soft goal, or we've already done more scan
296 // work than we expected. Pace GC so that in the worst case it
297 // will complete by the hard goal.
298 const maxOvershoot = 1.1
299 heapGoal = int64(float64(heapGoal) * maxOvershoot)
301 // Compute the upper bound on the scan work remaining.
302 scanWorkExpected = int64(scan)
305 // Compute the remaining scan work estimate.
307 // Note that we currently count allocations during GC as both
308 // scannable heap (heap_scan) and scan work completed
309 // (scanWork), so allocation will change this difference
310 // slowly in the soft regime and not at all in the hard
312 scanWorkRemaining := scanWorkExpected - work
313 if scanWorkRemaining < 1000 {
314 // We set a somewhat arbitrary lower bound on
315 // remaining scan work since if we aim a little high,
316 // we can miss by a little.
318 // We *do* need to enforce that this is at least 1,
319 // since marking is racy and double-scanning objects
320 // may legitimately make the remaining scan work
321 // negative, even in the hard goal regime.
322 scanWorkRemaining = 1000
325 // Compute the heap distance remaining.
326 heapRemaining := heapGoal - int64(live)
327 if heapRemaining <= 0 {
328 // This shouldn't happen, but if it does, avoid
329 // dividing by zero or setting the assist negative.
333 // Compute the mutator assist ratio so by the time the mutator
334 // allocates the remaining heap bytes up to next_gc, it will
335 // have done (or stolen) the remaining amount of scan work.
336 // Note that the assist ratio values are updated atomically
337 // but not together. This means there may be some degree of
338 // skew between the two values. This is generally OK as the
339 // values shift relatively slowly over the course of a GC
341 assistWorkPerByte := float64(scanWorkRemaining) / float64(heapRemaining)
342 assistBytesPerWork := float64(heapRemaining) / float64(scanWorkRemaining)
343 atomic.Store64(&c.assistWorkPerByte, float64bits(assistWorkPerByte))
344 atomic.Store64(&c.assistBytesPerWork, float64bits(assistBytesPerWork))
347 // endCycle computes the trigger ratio for the next cycle.
348 func (c *gcControllerState) endCycle() float64 {
350 // Forced GC means this cycle didn't start at the
351 // trigger, so where it finished isn't good
352 // information about how to adjust the trigger.
353 // Just leave it where it is.
354 return memstats.triggerRatio
357 // Proportional response gain for the trigger controller. Must
358 // be in [0, 1]. Lower values smooth out transient effects but
359 // take longer to respond to phase changes. Higher values
360 // react to phase changes quickly, but are more affected by
361 // transient changes. Values near 1 may be unstable.
362 const triggerGain = 0.5
364 // Compute next cycle trigger ratio. First, this computes the
365 // "error" for this cycle; that is, how far off the trigger
366 // was from what it should have been, accounting for both heap
367 // growth and GC CPU utilization. We compute the actual heap
368 // growth during this cycle and scale that by how far off from
369 // the goal CPU utilization we were (to estimate the heap
370 // growth if we had the desired CPU utilization). The
371 // difference between this estimate and the GOGC-based goal
372 // heap growth is the error.
373 goalGrowthRatio := gcEffectiveGrowthRatio()
374 actualGrowthRatio := float64(memstats.heap_live)/float64(memstats.heap_marked) - 1
375 assistDuration := nanotime() - c.markStartTime
377 // Assume background mark hit its utilization goal.
378 utilization := gcBackgroundUtilization
379 // Add assist utilization; avoid divide by zero.
380 if assistDuration > 0 {
381 utilization += float64(c.assistTime) / float64(assistDuration*int64(gomaxprocs))
384 triggerError := goalGrowthRatio - memstats.triggerRatio - utilization/gcGoalUtilization*(actualGrowthRatio-memstats.triggerRatio)
386 // Finally, we adjust the trigger for next time by this error,
387 // damped by the proportional gain.
388 triggerRatio := memstats.triggerRatio + triggerGain*triggerError
390 if debug.gcpacertrace > 0 {
391 // Print controller state in terms of the design
393 H_m_prev := memstats.heap_marked
394 h_t := memstats.triggerRatio
395 H_T := memstats.gc_trigger
396 h_a := actualGrowthRatio
397 H_a := memstats.heap_live
398 h_g := goalGrowthRatio
399 H_g := int64(float64(H_m_prev) * (1 + h_g))
401 u_g := gcGoalUtilization
403 print("pacer: H_m_prev=", H_m_prev,
404 " h_t=", h_t, " H_T=", H_T,
405 " h_a=", h_a, " H_a=", H_a,
406 " h_g=", h_g, " H_g=", H_g,
407 " u_a=", u_a, " u_g=", u_g,
409 " goalΔ=", goalGrowthRatio-h_t,
410 " actualΔ=", h_a-h_t,
411 " u_a/u_g=", u_a/u_g,
418 // enlistWorker encourages another dedicated mark worker to start on
419 // another P if there are spare worker slots. It is used by putfull
420 // when more work is made available.
423 func (c *gcControllerState) enlistWorker() {
424 // If there are idle Ps, wake one so it will run an idle worker.
425 // NOTE: This is suspected of causing deadlocks. See golang.org/issue/19112.
427 // if atomic.Load(&sched.npidle) != 0 && atomic.Load(&sched.nmspinning) == 0 {
432 // There are no idle Ps. If we need more dedicated workers,
433 // try to preempt a running P so it will switch to a worker.
434 if c.dedicatedMarkWorkersNeeded <= 0 {
437 // Pick a random other P to preempt.
442 if gp == nil || gp.m == nil || gp.m.p == 0 {
445 myID := gp.m.p.ptr().id
446 for tries := 0; tries < 5; tries++ {
447 id := int32(fastrandn(uint32(gomaxprocs - 1)))
452 if p.status != _Prunning {
461 // findRunnableGCWorker returns a background mark worker for _p_ if it
462 // should be run. This must only be called when gcBlackenEnabled != 0.
463 func (c *gcControllerState) findRunnableGCWorker(_p_ *p) *g {
464 if gcBlackenEnabled == 0 {
465 throw("gcControllerState.findRunnable: blackening not enabled")
468 if !gcMarkWorkAvailable(_p_) {
469 // No work to be done right now. This can happen at
470 // the end of the mark phase when there are still
471 // assists tapering off. Don't bother running a worker
472 // now because it'll just return immediately.
476 // Grab a worker before we commit to running below.
477 node := (*gcBgMarkWorkerNode)(gcBgMarkWorkerPool.pop())
479 // There is at least one worker per P, so normally there are
480 // enough workers to run on all Ps, if necessary. However, once
481 // a worker enters gcMarkDone it may park without rejoining the
482 // pool, thus freeing a P with no corresponding worker.
483 // gcMarkDone never depends on another worker doing work, so it
484 // is safe to simply do nothing here.
486 // If gcMarkDone bails out without completing the mark phase,
487 // it will always do so with queued global work. Thus, that P
488 // will be immediately eligible to re-run the worker G it was
489 // just using, ensuring work can complete.
493 decIfPositive := func(ptr *int64) bool {
495 v := atomic.Loadint64(ptr)
500 if atomic.Casint64(ptr, v, v-1) {
506 if decIfPositive(&c.dedicatedMarkWorkersNeeded) {
507 // This P is now dedicated to marking until the end of
508 // the concurrent mark phase.
509 _p_.gcMarkWorkerMode = gcMarkWorkerDedicatedMode
510 } else if c.fractionalUtilizationGoal == 0 {
511 // No need for fractional workers.
512 gcBgMarkWorkerPool.push(&node.node)
515 // Is this P behind on the fractional utilization
518 // This should be kept in sync with pollFractionalWorkerExit.
519 delta := nanotime() - gcController.markStartTime
520 if delta > 0 && float64(_p_.gcFractionalMarkTime)/float64(delta) > c.fractionalUtilizationGoal {
521 // Nope. No need to run a fractional worker.
522 gcBgMarkWorkerPool.push(&node.node)
525 // Run a fractional worker.
526 _p_.gcMarkWorkerMode = gcMarkWorkerFractionalMode
529 // Run the background mark worker.
531 casgstatus(gp, _Gwaiting, _Grunnable)
538 // gcSetTriggerRatio sets the trigger ratio and updates everything
539 // derived from it: the absolute trigger, the heap goal, mark pacing,
542 // This can be called any time. If GC is the in the middle of a
543 // concurrent phase, it will adjust the pacing of that phase.
545 // This depends on gcPercent, memstats.heap_marked, and
546 // memstats.heap_live. These must be up to date.
548 // mheap_.lock must be held or the world must be stopped.
549 func gcSetTriggerRatio(triggerRatio float64) {
550 assertWorldStoppedOrLockHeld(&mheap_.lock)
552 // Compute the next GC goal, which is when the allocated heap
553 // has grown by GOGC/100 over the heap marked by the last
557 goal = memstats.heap_marked + memstats.heap_marked*uint64(gcPercent)/100
560 // Set the trigger ratio, capped to reasonable bounds.
562 scalingFactor := float64(gcPercent) / 100
563 // Ensure there's always a little margin so that the
564 // mutator assist ratio isn't infinity.
565 maxTriggerRatio := 0.95 * scalingFactor
566 if triggerRatio > maxTriggerRatio {
567 triggerRatio = maxTriggerRatio
570 // If we let triggerRatio go too low, then if the application
571 // is allocating very rapidly we might end up in a situation
572 // where we're allocating black during a nearly always-on GC.
573 // The result of this is a growing heap and ultimately an
574 // increase in RSS. By capping us at a point >0, we're essentially
575 // saying that we're OK using more CPU during the GC to prevent
576 // this growth in RSS.
578 // The current constant was chosen empirically: given a sufficiently
579 // fast/scalable allocator with 48 Ps that could drive the trigger ratio
580 // to <0.05, this constant causes applications to retain the same peak
581 // RSS compared to not having this allocator.
582 minTriggerRatio := 0.6 * scalingFactor
583 if triggerRatio < minTriggerRatio {
584 triggerRatio = minTriggerRatio
586 } else if triggerRatio < 0 {
587 // gcPercent < 0, so just make sure we're not getting a negative
588 // triggerRatio. This case isn't expected to happen in practice,
589 // and doesn't really matter because if gcPercent < 0 then we won't
590 // ever consume triggerRatio further on in this function, but let's
591 // just be defensive here; the triggerRatio being negative is almost
592 // certainly undesirable.
595 memstats.triggerRatio = triggerRatio
597 // Compute the absolute GC trigger from the trigger ratio.
599 // We trigger the next GC cycle when the allocated heap has
600 // grown by the trigger ratio over the marked heap size.
601 trigger := ^uint64(0)
603 trigger = uint64(float64(memstats.heap_marked) * (1 + triggerRatio))
604 // Don't trigger below the minimum heap size.
605 minTrigger := heapMinimum
607 // Concurrent sweep happens in the heap growth
608 // from heap_live to gc_trigger, so ensure
609 // that concurrent sweep has some heap growth
610 // in which to perform sweeping before we
611 // start the next GC cycle.
612 sweepMin := atomic.Load64(&memstats.heap_live) + sweepMinHeapDistance
613 if sweepMin > minTrigger {
614 minTrigger = sweepMin
617 if trigger < minTrigger {
620 if int64(trigger) < 0 {
621 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")
622 throw("gc_trigger underflow")
625 // The trigger ratio is always less than GOGC/100, but
626 // other bounds on the trigger may have raised it.
627 // Push up the goal, too.
632 // Commit to the trigger and goal.
633 memstats.gc_trigger = trigger
634 atomic.Store64(&memstats.next_gc, goal)
639 // Update mark pacing.
640 if gcphase != _GCoff {
641 gcController.revise()
644 // Update sweep pacing.
646 mheap_.sweepPagesPerByte = 0
648 // Concurrent sweep needs to sweep all of the in-use
649 // pages by the time the allocated heap reaches the GC
650 // trigger. Compute the ratio of in-use pages to sweep
651 // per byte allocated, accounting for the fact that
652 // some might already be swept.
653 heapLiveBasis := atomic.Load64(&memstats.heap_live)
654 heapDistance := int64(trigger) - int64(heapLiveBasis)
655 // Add a little margin so rounding errors and
656 // concurrent sweep are less likely to leave pages
657 // unswept when GC starts.
658 heapDistance -= 1024 * 1024
659 if heapDistance < _PageSize {
660 // Avoid setting the sweep ratio extremely high
661 heapDistance = _PageSize
663 pagesSwept := atomic.Load64(&mheap_.pagesSwept)
664 pagesInUse := atomic.Load64(&mheap_.pagesInUse)
665 sweepDistancePages := int64(pagesInUse) - int64(pagesSwept)
666 if sweepDistancePages <= 0 {
667 mheap_.sweepPagesPerByte = 0
669 mheap_.sweepPagesPerByte = float64(sweepDistancePages) / float64(heapDistance)
670 mheap_.sweepHeapLiveBasis = heapLiveBasis
671 // Write pagesSweptBasis last, since this
672 // signals concurrent sweeps to recompute
674 atomic.Store64(&mheap_.pagesSweptBasis, pagesSwept)
681 // gcEffectiveGrowthRatio returns the current effective heap growth
682 // ratio (GOGC/100) based on heap_marked from the previous GC and
683 // next_gc for the current GC.
685 // This may differ from gcPercent/100 because of various upper and
686 // lower bounds on gcPercent. For example, if the heap is smaller than
687 // heapMinimum, this can be higher than gcPercent/100.
689 // mheap_.lock must be held or the world must be stopped.
690 func gcEffectiveGrowthRatio() float64 {
691 assertWorldStoppedOrLockHeld(&mheap_.lock)
693 egogc := float64(atomic.Load64(&memstats.next_gc)-memstats.heap_marked) / float64(memstats.heap_marked)
695 // Shouldn't happen, but just in case.
701 //go:linkname setGCPercent runtime/debug.setGCPercent
702 func setGCPercent(in int32) (out int32) {
703 // Run on the system stack since we grab the heap lock.
711 heapMinimum = defaultHeapMinimum * uint64(gcPercent) / 100
712 // Update pacing in response to gcPercent change.
713 gcSetTriggerRatio(memstats.triggerRatio)
717 // If we just disabled GC, wait for any concurrent GC mark to
718 // finish so we always return with no GC running.
720 gcWaitOnMark(atomic.Load(&work.cycles))
726 func readGOGC() int32 {
727 p := gogetenv("GOGC")
731 if n, ok := atoi32(p); ok {