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 "internal/goexperiment"
10 "runtime/internal/atomic"
11 _ "unsafe" // for go:linkname
15 // gcGoalUtilization is the goal CPU utilization for
16 // marking as a fraction of GOMAXPROCS.
18 // Increasing the goal utilization will shorten GC cycles as the GC
19 // has more resources behind it, lessening costs from the write barrier,
20 // but comes at the cost of increasing mutator latency.
21 gcGoalUtilization = gcBackgroundUtilization
23 // gcBackgroundUtilization is the fixed CPU utilization for background
24 // marking. It must be <= gcGoalUtilization. The difference between
25 // gcGoalUtilization and gcBackgroundUtilization will be made up by
26 // mark assists. The scheduler will aim to use within 50% of this
29 // As a general rule, there's little reason to set gcBackgroundUtilization
30 // < gcGoalUtilization. One reason might be in mostly idle applications,
31 // where goroutines are unlikely to assist at all, so the actual
32 // utilization will be lower than the goal. But this is moot point
33 // because the idle mark workers already soak up idle CPU resources.
34 // These two values are still kept separate however because they are
35 // distinct conceptually, and in previous iterations of the pacer the
36 // distinction was more important.
37 gcBackgroundUtilization = 0.25
39 // gcCreditSlack is the amount of scan work credit that can
40 // accumulate locally before updating gcController.heapScanWork and,
41 // optionally, gcController.bgScanCredit. Lower values give a more
42 // accurate assist ratio and make it more likely that assists will
43 // successfully steal background credit. Higher values reduce memory
47 // gcAssistTimeSlack is the nanoseconds of mutator assist time that
48 // can accumulate on a P before updating gcController.assistTime.
49 gcAssistTimeSlack = 5000
51 // gcOverAssistWork determines how many extra units of scan work a GC
52 // assist does when an assist happens. This amortizes the cost of an
53 // assist by pre-paying for this many bytes of future allocations.
54 gcOverAssistWork = 64 << 10
56 // defaultHeapMinimum is the value of heapMinimum for GOGC==100.
57 defaultHeapMinimum = (goexperiment.HeapMinimum512KiBInt)*(512<<10) +
58 (1-goexperiment.HeapMinimum512KiBInt)*(4<<20)
60 // maxStackScanSlack is the bytes of stack space allocated or freed
61 // that can accumulate on a P before updating gcController.stackSize.
62 maxStackScanSlack = 8 << 10
64 // memoryLimitMinHeapGoalHeadroom is the minimum amount of headroom the
65 // pacer gives to the heap goal when operating in the memory-limited regime.
66 // That is, it'll reduce the heap goal by this many extra bytes off of the
67 // base calculation, at minimum.
68 memoryLimitMinHeapGoalHeadroom = 1 << 20
70 // memoryLimitHeapGoalHeadroomPercent is how headroom the memory-limit-based
71 // heap goal should have as a percent of the maximum possible heap goal allowed
72 // to maintain the memory limit.
73 memoryLimitHeapGoalHeadroomPercent = 3
76 // gcController implements the GC pacing controller that determines
77 // when to trigger concurrent garbage collection and how much marking
78 // work to do in mutator assists and background marking.
80 // It calculates the ratio between the allocation rate (in terms of CPU
81 // time) and the GC scan throughput to determine the heap size at which to
82 // trigger a GC cycle such that no GC assists are required to finish on time.
83 // This algorithm thus optimizes GC CPU utilization to the dedicated background
84 // mark utilization of 25% of GOMAXPROCS by minimizing GC assists.
85 // GOMAXPROCS. The high-level design of this algorithm is documented
86 // at https://github.com/golang/proposal/blob/master/design/44167-gc-pacer-redesign.md.
87 // See https://golang.org/s/go15gcpacing for additional historical context.
88 var gcController gcControllerState
90 type gcControllerState struct {
91 // Initialized from GOGC. GOGC=off means no GC.
92 gcPercent atomic.Int32
94 // memoryLimit is the soft memory limit in bytes.
96 // Initialized from GOMEMLIMIT. GOMEMLIMIT=off is equivalent to MaxInt64
97 // which means no soft memory limit in practice.
99 // This is an int64 instead of a uint64 to more easily maintain parity with
100 // the SetMemoryLimit API, which sets a maximum at MaxInt64. This value
101 // should never be negative.
102 memoryLimit atomic.Int64
104 // heapMinimum is the minimum heap size at which to trigger GC.
105 // For small heaps, this overrides the usual GOGC*live set rule.
107 // When there is a very small live set but a lot of allocation, simply
108 // collecting when the heap reaches GOGC*live results in many GC
109 // cycles and high total per-GC overhead. This minimum amortizes this
110 // per-GC overhead while keeping the heap reasonably small.
112 // During initialization this is set to 4MB*GOGC/100. In the case of
113 // GOGC==0, this will set heapMinimum to 0, resulting in constant
114 // collection even when the heap size is small, which is useful for
118 // runway is the amount of runway in heap bytes allocated by the
119 // application that we want to give the GC once it starts.
121 // This is computed from consMark during mark termination.
124 // consMark is the estimated per-CPU consMark ratio for the application.
126 // It represents the ratio between the application's allocation
127 // rate, as bytes allocated per CPU-time, and the GC's scan rate,
128 // as bytes scanned per CPU-time.
129 // The units of this ratio are (B / cpu-ns) / (B / cpu-ns).
131 // At a high level, this value is computed as the bytes of memory
132 // allocated (cons) per unit of scan work completed (mark) in a GC
133 // cycle, divided by the CPU time spent on each activity.
135 // Updated at the end of each GC cycle, in endCycle.
138 // lastConsMark is the computed cons/mark value for the previous 4 GC
139 // cycles. Note that this is *not* the last value of consMark, but the
140 // measured cons/mark value in endCycle.
141 lastConsMark [4]float64
143 // gcPercentHeapGoal is the goal heapLive for when next GC ends derived
146 // Set to ^uint64(0) if gcPercent is disabled.
147 gcPercentHeapGoal atomic.Uint64
149 // sweepDistMinTrigger is the minimum trigger to ensure a minimum
152 // This bound is also special because it applies to both the trigger
153 // *and* the goal (all other trigger bounds must be based *on* the goal).
155 // It is computed ahead of time, at commit time. The theory is that,
156 // absent a sudden change to a parameter like gcPercent, the trigger
157 // will be chosen to always give the sweeper enough headroom. However,
158 // such a change might dramatically and suddenly move up the trigger,
159 // in which case we need to ensure the sweeper still has enough headroom.
160 sweepDistMinTrigger atomic.Uint64
162 // triggered is the point at which the current GC cycle actually triggered.
163 // Only valid during the mark phase of a GC cycle, otherwise set to ^uint64(0).
165 // Updated while the world is stopped.
168 // lastHeapGoal is the value of heapGoal at the moment the last GC
169 // ended. Note that this is distinct from the last value heapGoal had,
170 // because it could change if e.g. gcPercent changes.
172 // Read and written with the world stopped or with mheap_.lock held.
175 // heapLive is the number of bytes considered live by the GC.
176 // That is: retained by the most recent GC plus allocated
177 // since then. heapLive ≤ memstats.totalAlloc-memstats.totalFree, since
178 // heapAlloc includes unmarked objects that have not yet been swept (and
179 // hence goes up as we allocate and down as we sweep) while heapLive
180 // excludes these objects (and hence only goes up between GCs).
182 // To reduce contention, this is updated only when obtaining a span
183 // from an mcentral and at this point it counts all of the unallocated
184 // slots in that span (which will be allocated before that mcache
185 // obtains another span from that mcentral). Hence, it slightly
186 // overestimates the "true" live heap size. It's better to overestimate
187 // than to underestimate because 1) this triggers the GC earlier than
188 // necessary rather than potentially too late and 2) this leads to a
189 // conservative GC rate rather than a GC rate that is potentially too
192 // Whenever this is updated, call traceHeapAlloc() and
193 // this gcControllerState's revise() method.
194 heapLive atomic.Uint64
196 // heapScan is the number of bytes of "scannable" heap. This is the
197 // live heap (as counted by heapLive), but omitting no-scan objects and
198 // no-scan tails of objects.
200 // This value is fixed at the start of a GC cycle. It represents the
201 // maximum scannable heap.
202 heapScan atomic.Uint64
204 // lastHeapScan is the number of bytes of heap that were scanned
205 // last GC cycle. It is the same as heapMarked, but only
206 // includes the "scannable" parts of objects.
208 // Updated when the world is stopped.
211 // lastStackScan is the number of bytes of stack that were scanned
213 lastStackScan atomic.Uint64
215 // maxStackScan is the amount of allocated goroutine stack space in
216 // use by goroutines.
218 // This number tracks allocated goroutine stack space rather than used
219 // goroutine stack space (i.e. what is actually scanned) because used
220 // goroutine stack space is much harder to measure cheaply. By using
221 // allocated space, we make an overestimate; this is OK, it's better
222 // to conservatively overcount than undercount.
223 maxStackScan atomic.Uint64
225 // globalsScan is the total amount of global variable space
226 // that is scannable.
227 globalsScan atomic.Uint64
229 // heapMarked is the number of bytes marked by the previous
230 // GC. After mark termination, heapLive == heapMarked, but
231 // unlike heapLive, heapMarked does not change until the
232 // next mark termination.
235 // heapScanWork is the total heap scan work performed this cycle.
236 // stackScanWork is the total stack scan work performed this cycle.
237 // globalsScanWork is the total globals scan work performed this cycle.
239 // These are updated atomically during the cycle. Updates occur in
240 // bounded batches, since they are both written and read
241 // throughout the cycle. At the end of the cycle, heapScanWork is how
242 // much of the retained heap is scannable.
244 // Currently these are measured in bytes. For most uses, this is an
245 // opaque unit of work, but for estimation the definition is important.
247 // Note that stackScanWork includes only stack space scanned, not all
248 // of the allocated stack.
249 heapScanWork atomic.Int64
250 stackScanWork atomic.Int64
251 globalsScanWork atomic.Int64
253 // bgScanCredit is the scan work credit accumulated by the concurrent
254 // background scan. This credit is accumulated by the background scan
255 // and stolen by mutator assists. Updates occur in bounded batches,
256 // since it is both written and read throughout the cycle.
257 bgScanCredit atomic.Int64
259 // assistTime is the nanoseconds spent in mutator assists
260 // during this cycle. This is updated atomically, and must also
261 // be updated atomically even during a STW, because it is read
262 // by sysmon. Updates occur in bounded batches, since it is both
263 // written and read throughout the cycle.
264 assistTime atomic.Int64
266 // dedicatedMarkTime is the nanoseconds spent in dedicated mark workers
267 // during this cycle. This is updated at the end of the concurrent mark
269 dedicatedMarkTime atomic.Int64
271 // fractionalMarkTime is the nanoseconds spent in the fractional mark
272 // worker during this cycle. This is updated throughout the cycle and
273 // will be up-to-date if the fractional mark worker is not currently
275 fractionalMarkTime atomic.Int64
277 // idleMarkTime is the nanoseconds spent in idle marking during this
278 // cycle. This is updated throughout the cycle.
279 idleMarkTime atomic.Int64
281 // markStartTime is the absolute start time in nanoseconds
282 // that assists and background mark workers started.
285 // dedicatedMarkWorkersNeeded is the number of dedicated mark workers
286 // that need to be started. This is computed at the beginning of each
287 // cycle and decremented as dedicated mark workers get started.
288 dedicatedMarkWorkersNeeded atomic.Int64
290 // idleMarkWorkers is two packed int32 values in a single uint64.
291 // These two values are always updated simultaneously.
293 // The bottom int32 is the current number of idle mark workers executing.
295 // The top int32 is the maximum number of idle mark workers allowed to
296 // execute concurrently. Normally, this number is just gomaxprocs. However,
297 // during periodic GC cycles it is set to 0 because the system is idle
298 // anyway; there's no need to go full blast on all of GOMAXPROCS.
300 // The maximum number of idle mark workers is used to prevent new workers
301 // from starting, but it is not a hard maximum. It is possible (but
302 // exceedingly rare) for the current number of idle mark workers to
303 // transiently exceed the maximum. This could happen if the maximum changes
304 // just after a GC ends, and an M with no P.
306 // Note that if we have no dedicated mark workers, we set this value to
307 // 1 in this case we only have fractional GC workers which aren't scheduled
308 // strictly enough to ensure GC progress. As a result, idle-priority mark
309 // workers are vital to GC progress in these situations.
311 // For example, consider a situation in which goroutines block on the GC
312 // (such as via runtime.GOMAXPROCS) and only fractional mark workers are
313 // scheduled (e.g. GOMAXPROCS=1). Without idle-priority mark workers, the
314 // last running M might skip scheduling a fractional mark worker if its
315 // utilization goal is met, such that once it goes to sleep (because there's
316 // nothing to do), there will be nothing else to spin up a new M for the
317 // fractional worker in the future, stalling GC progress and causing a
318 // deadlock. However, idle-priority workers will *always* run when there is
319 // nothing left to do, ensuring the GC makes progress.
321 // See github.com/golang/go/issues/44163 for more details.
322 idleMarkWorkers atomic.Uint64
324 // assistWorkPerByte is the ratio of scan work to allocated
325 // bytes that should be performed by mutator assists. This is
326 // computed at the beginning of each cycle and updated every
327 // time heapScan is updated.
328 assistWorkPerByte atomic.Float64
330 // assistBytesPerWork is 1/assistWorkPerByte.
332 // Note that because this is read and written independently
333 // from assistWorkPerByte users may notice a skew between
334 // the two values, and such a state should be safe.
335 assistBytesPerWork atomic.Float64
337 // fractionalUtilizationGoal is the fraction of wall clock
338 // time that should be spent in the fractional mark worker on
339 // each P that isn't running a dedicated worker.
341 // For example, if the utilization goal is 25% and there are
342 // no dedicated workers, this will be 0.25. If the goal is
343 // 25%, there is one dedicated worker, and GOMAXPROCS is 5,
344 // this will be 0.05 to make up the missing 5%.
346 // If this is zero, no fractional workers are needed.
347 fractionalUtilizationGoal float64
349 // These memory stats are effectively duplicates of fields from
350 // memstats.heapStats but are updated atomically or with the world
351 // stopped and don't provide the same consistency guarantees.
353 // Because the runtime is responsible for managing a memory limit, it's
354 // useful to couple these stats more tightly to the gcController, which
355 // is intimately connected to how that memory limit is maintained.
356 heapInUse sysMemStat // bytes in mSpanInUse spans
357 heapReleased sysMemStat // bytes released to the OS
358 heapFree sysMemStat // bytes not in any span, but not released to the OS
359 totalAlloc atomic.Uint64 // total bytes allocated
360 totalFree atomic.Uint64 // total bytes freed
361 mappedReady atomic.Uint64 // total virtual memory in the Ready state (see mem.go).
363 // test indicates that this is a test-only copy of gcControllerState.
369 func (c *gcControllerState) init(gcPercent int32, memoryLimit int64) {
370 c.heapMinimum = defaultHeapMinimum
371 c.triggered = ^uint64(0)
372 c.setGCPercent(gcPercent)
373 c.setMemoryLimit(memoryLimit)
374 c.commit(true) // No sweep phase in the first GC cycle.
375 // N.B. Don't bother calling traceHeapGoal. Tracing is never enabled at
376 // initialization time.
377 // N.B. No need to call revise; there's no GC enabled during
381 // startCycle resets the GC controller's state and computes estimates
382 // for a new GC cycle. The caller must hold worldsema and the world
384 func (c *gcControllerState) startCycle(markStartTime int64, procs int, trigger gcTrigger) {
385 c.heapScanWork.Store(0)
386 c.stackScanWork.Store(0)
387 c.globalsScanWork.Store(0)
388 c.bgScanCredit.Store(0)
389 c.assistTime.Store(0)
390 c.dedicatedMarkTime.Store(0)
391 c.fractionalMarkTime.Store(0)
392 c.idleMarkTime.Store(0)
393 c.markStartTime = markStartTime
394 c.triggered = c.heapLive.Load()
396 // Compute the background mark utilization goal. In general,
397 // this may not come out exactly. We round the number of
398 // dedicated workers so that the utilization is closest to
399 // 25%. For small GOMAXPROCS, this would introduce too much
400 // error, so we add fractional workers in that case.
401 totalUtilizationGoal := float64(procs) * gcBackgroundUtilization
402 dedicatedMarkWorkersNeeded := int64(totalUtilizationGoal + 0.5)
403 utilError := float64(dedicatedMarkWorkersNeeded)/totalUtilizationGoal - 1
404 const maxUtilError = 0.3
405 if utilError < -maxUtilError || utilError > maxUtilError {
406 // Rounding put us more than 30% off our goal. With
407 // gcBackgroundUtilization of 25%, this happens for
408 // GOMAXPROCS<=3 or GOMAXPROCS=6. Enable fractional
409 // workers to compensate.
410 if float64(dedicatedMarkWorkersNeeded) > totalUtilizationGoal {
411 // Too many dedicated workers.
412 dedicatedMarkWorkersNeeded--
414 c.fractionalUtilizationGoal = (totalUtilizationGoal - float64(dedicatedMarkWorkersNeeded)) / float64(procs)
416 c.fractionalUtilizationGoal = 0
419 // In STW mode, we just want dedicated workers.
420 if debug.gcstoptheworld > 0 {
421 dedicatedMarkWorkersNeeded = int64(procs)
422 c.fractionalUtilizationGoal = 0
426 for _, p := range allp {
428 p.gcFractionalMarkTime = 0
431 if trigger.kind == gcTriggerTime {
432 // During a periodic GC cycle, reduce the number of idle mark workers
433 // required. However, we need at least one dedicated mark worker or
434 // idle GC worker to ensure GC progress in some scenarios (see comment
435 // on maxIdleMarkWorkers).
436 if dedicatedMarkWorkersNeeded > 0 {
437 c.setMaxIdleMarkWorkers(0)
439 // TODO(mknyszek): The fundamental reason why we need this is because
440 // we can't count on the fractional mark worker to get scheduled.
441 // Fix that by ensuring it gets scheduled according to its quota even
442 // if the rest of the application is idle.
443 c.setMaxIdleMarkWorkers(1)
446 // N.B. gomaxprocs and dedicatedMarkWorkersNeeded are guaranteed not to
447 // change during a GC cycle.
448 c.setMaxIdleMarkWorkers(int32(procs) - int32(dedicatedMarkWorkersNeeded))
451 // Compute initial values for controls that are updated
452 // throughout the cycle.
453 c.dedicatedMarkWorkersNeeded.Store(dedicatedMarkWorkersNeeded)
456 if debug.gcpacertrace > 0 {
457 heapGoal := c.heapGoal()
458 assistRatio := c.assistWorkPerByte.Load()
459 print("pacer: assist ratio=", assistRatio,
460 " (scan ", gcController.heapScan.Load()>>20, " MB in ",
461 work.initialHeapLive>>20, "->",
462 heapGoal>>20, " MB)",
463 " workers=", dedicatedMarkWorkersNeeded,
464 "+", c.fractionalUtilizationGoal, "\n")
468 // revise updates the assist ratio during the GC cycle to account for
469 // improved estimates. This should be called whenever gcController.heapScan,
470 // gcController.heapLive, or if any inputs to gcController.heapGoal are
471 // updated. It is safe to call concurrently, but it may race with other
474 // The result of this race is that the two assist ratio values may not line
475 // up or may be stale. In practice this is OK because the assist ratio
476 // moves slowly throughout a GC cycle, and the assist ratio is a best-effort
477 // heuristic anyway. Furthermore, no part of the heuristic depends on
478 // the two assist ratio values being exact reciprocals of one another, since
479 // the two values are used to convert values from different sources.
481 // The worst case result of this raciness is that we may miss a larger shift
482 // in the ratio (say, if we decide to pace more aggressively against the
483 // hard heap goal) but even this "hard goal" is best-effort (see #40460).
484 // The dedicated GC should ensure we don't exceed the hard goal by too much
485 // in the rare case we do exceed it.
487 // It should only be called when gcBlackenEnabled != 0 (because this
488 // is when assists are enabled and the necessary statistics are
490 func (c *gcControllerState) revise() {
491 gcPercent := c.gcPercent.Load()
493 // If GC is disabled but we're running a forced GC,
494 // act like GOGC is huge for the below calculations.
497 live := c.heapLive.Load()
498 scan := c.heapScan.Load()
499 work := c.heapScanWork.Load() + c.stackScanWork.Load() + c.globalsScanWork.Load()
501 // Assume we're under the soft goal. Pace GC to complete at
502 // heapGoal assuming the heap is in steady-state.
503 heapGoal := int64(c.heapGoal())
505 // The expected scan work is computed as the amount of bytes scanned last
506 // GC cycle (both heap and stack), plus our estimate of globals work for this cycle.
507 scanWorkExpected := int64(c.lastHeapScan + c.lastStackScan.Load() + c.globalsScan.Load())
509 // maxScanWork is a worst-case estimate of the amount of scan work that
510 // needs to be performed in this GC cycle. Specifically, it represents
511 // the case where *all* scannable memory turns out to be live, and
512 // *all* allocated stack space is scannable.
513 maxStackScan := c.maxStackScan.Load()
514 maxScanWork := int64(scan + maxStackScan + c.globalsScan.Load())
515 if work > scanWorkExpected {
516 // We've already done more scan work than expected. Because our expectation
517 // is based on a steady-state scannable heap size, we assume this means our
518 // heap is growing. Compute a new heap goal that takes our existing runway
519 // computed for scanWorkExpected and extrapolates it to maxScanWork, the worst-case
520 // scan work. This keeps our assist ratio stable if the heap continues to grow.
522 // The effect of this mechanism is that assists stay flat in the face of heap
523 // growths. It's OK to use more memory this cycle to scan all the live heap,
524 // because the next GC cycle is inevitably going to use *at least* that much
526 extHeapGoal := int64(float64(heapGoal-int64(c.triggered))/float64(scanWorkExpected)*float64(maxScanWork)) + int64(c.triggered)
527 scanWorkExpected = maxScanWork
529 // hardGoal is a hard limit on the amount that we're willing to push back the
530 // heap goal, and that's twice the heap goal (i.e. if GOGC=100 and the heap and/or
531 // stacks and/or globals grow to twice their size, this limits the current GC cycle's
532 // growth to 4x the original live heap's size).
534 // This maintains the invariant that we use no more memory than the next GC cycle
536 hardGoal := int64((1.0 + float64(gcPercent)/100.0) * float64(heapGoal))
537 if extHeapGoal > hardGoal {
538 extHeapGoal = hardGoal
540 heapGoal = extHeapGoal
542 if int64(live) > heapGoal {
543 // We're already past our heap goal, even the extrapolated one.
544 // Leave ourselves some extra runway, so in the worst case we
545 // finish by that point.
546 const maxOvershoot = 1.1
547 heapGoal = int64(float64(heapGoal) * maxOvershoot)
549 // Compute the upper bound on the scan work remaining.
550 scanWorkExpected = maxScanWork
553 // Compute the remaining scan work estimate.
555 // Note that we currently count allocations during GC as both
556 // scannable heap (heapScan) and scan work completed
557 // (scanWork), so allocation will change this difference
558 // slowly in the soft regime and not at all in the hard
560 scanWorkRemaining := scanWorkExpected - work
561 if scanWorkRemaining < 1000 {
562 // We set a somewhat arbitrary lower bound on
563 // remaining scan work since if we aim a little high,
564 // we can miss by a little.
566 // We *do* need to enforce that this is at least 1,
567 // since marking is racy and double-scanning objects
568 // may legitimately make the remaining scan work
569 // negative, even in the hard goal regime.
570 scanWorkRemaining = 1000
573 // Compute the heap distance remaining.
574 heapRemaining := heapGoal - int64(live)
575 if heapRemaining <= 0 {
576 // This shouldn't happen, but if it does, avoid
577 // dividing by zero or setting the assist negative.
581 // Compute the mutator assist ratio so by the time the mutator
582 // allocates the remaining heap bytes up to heapGoal, it will
583 // have done (or stolen) the remaining amount of scan work.
584 // Note that the assist ratio values are updated atomically
585 // but not together. This means there may be some degree of
586 // skew between the two values. This is generally OK as the
587 // values shift relatively slowly over the course of a GC
589 assistWorkPerByte := float64(scanWorkRemaining) / float64(heapRemaining)
590 assistBytesPerWork := float64(heapRemaining) / float64(scanWorkRemaining)
591 c.assistWorkPerByte.Store(assistWorkPerByte)
592 c.assistBytesPerWork.Store(assistBytesPerWork)
595 // endCycle computes the consMark estimate for the next cycle.
596 // userForced indicates whether the current GC cycle was forced
597 // by the application.
598 func (c *gcControllerState) endCycle(now int64, procs int, userForced bool) {
599 // Record last heap goal for the scavenger.
600 // We'll be updating the heap goal soon.
601 gcController.lastHeapGoal = c.heapGoal()
603 // Compute the duration of time for which assists were turned on.
604 assistDuration := now - c.markStartTime
606 // Assume background mark hit its utilization goal.
607 utilization := gcBackgroundUtilization
608 // Add assist utilization; avoid divide by zero.
609 if assistDuration > 0 {
610 utilization += float64(c.assistTime.Load()) / float64(assistDuration*int64(procs))
613 if c.heapLive.Load() <= c.triggered {
614 // Shouldn't happen, but let's be very safe about this in case the
615 // GC is somehow extremely short.
617 // In this case though, the only reasonable value for c.heapLive-c.triggered
618 // would be 0, which isn't really all that useful, i.e. the GC was so short
619 // that it didn't matter.
621 // Ignore this case and don't update anything.
624 idleUtilization := 0.0
625 if assistDuration > 0 {
626 idleUtilization = float64(c.idleMarkTime.Load()) / float64(assistDuration*int64(procs))
628 // Determine the cons/mark ratio.
630 // The units we want for the numerator and denominator are both B / cpu-ns.
631 // We get this by taking the bytes allocated or scanned, and divide by the amount of
632 // CPU time it took for those operations. For allocations, that CPU time is
634 // assistDuration * procs * (1 - utilization)
636 // Where utilization includes just background GC workers and assists. It does *not*
637 // include idle GC work time, because in theory the mutator is free to take that at
640 // For scanning, that CPU time is
642 // assistDuration * procs * (utilization + idleUtilization)
644 // In this case, we *include* idle utilization, because that is additional CPU time that
645 // the GC had available to it.
647 // In effect, idle GC time is sort of double-counted here, but it's very weird compared
648 // to other kinds of GC work, because of how fluid it is. Namely, because the mutator is
649 // *always* free to take it.
651 // So this calculation is really:
652 // (heapLive-trigger) / (assistDuration * procs * (1-utilization)) /
653 // (scanWork) / (assistDuration * procs * (utilization+idleUtilization))
655 // Note that because we only care about the ratio, assistDuration and procs cancel out.
656 scanWork := c.heapScanWork.Load() + c.stackScanWork.Load() + c.globalsScanWork.Load()
657 currentConsMark := (float64(c.heapLive.Load()-c.triggered) * (utilization + idleUtilization)) /
658 (float64(scanWork) * (1 - utilization))
660 // Update our cons/mark estimate. This is the maximum of the value we just computed and the last
661 // 4 cons/mark values we measured. The reason we take the maximum here is to bias a noisy
662 // cons/mark measurement toward fewer assists at the expense of additional GC cycles (starting
664 oldConsMark := c.consMark
665 c.consMark = currentConsMark
666 for i := range c.lastConsMark {
667 if c.lastConsMark[i] > c.consMark {
668 c.consMark = c.lastConsMark[i]
671 copy(c.lastConsMark[:], c.lastConsMark[1:])
672 c.lastConsMark[len(c.lastConsMark)-1] = currentConsMark
674 if debug.gcpacertrace > 0 {
676 goal := gcGoalUtilization * 100
677 print("pacer: ", int(utilization*100), "% CPU (", int(goal), " exp.) for ")
678 print(c.heapScanWork.Load(), "+", c.stackScanWork.Load(), "+", c.globalsScanWork.Load(), " B work (", c.lastHeapScan+c.lastStackScan.Load()+c.globalsScan.Load(), " B exp.) ")
679 live := c.heapLive.Load()
680 print("in ", c.triggered, " B -> ", live, " B (∆goal ", int64(live)-int64(c.lastHeapGoal), ", cons/mark ", oldConsMark, ")")
686 // enlistWorker encourages another dedicated mark worker to start on
687 // another P if there are spare worker slots. It is used by putfull
688 // when more work is made available.
691 func (c *gcControllerState) enlistWorker() {
692 // If there are idle Ps, wake one so it will run an idle worker.
693 // NOTE: This is suspected of causing deadlocks. See golang.org/issue/19112.
695 // if sched.npidle.Load() != 0 && sched.nmspinning.Load() == 0 {
700 // There are no idle Ps. If we need more dedicated workers,
701 // try to preempt a running P so it will switch to a worker.
702 if c.dedicatedMarkWorkersNeeded.Load() <= 0 {
705 // Pick a random other P to preempt.
710 if gp == nil || gp.m == nil || gp.m.p == 0 {
713 myID := gp.m.p.ptr().id
714 for tries := 0; tries < 5; tries++ {
715 id := int32(fastrandn(uint32(gomaxprocs - 1)))
720 if p.status != _Prunning {
729 // findRunnableGCWorker returns a background mark worker for pp if it
730 // should be run. This must only be called when gcBlackenEnabled != 0.
731 func (c *gcControllerState) findRunnableGCWorker(pp *p, now int64) (*g, int64) {
732 if gcBlackenEnabled == 0 {
733 throw("gcControllerState.findRunnable: blackening not enabled")
736 // Since we have the current time, check if the GC CPU limiter
737 // hasn't had an update in a while. This check is necessary in
738 // case the limiter is on but hasn't been checked in a while and
739 // so may have left sufficient headroom to turn off again.
743 if gcCPULimiter.needUpdate(now) {
744 gcCPULimiter.update(now)
747 if !gcMarkWorkAvailable(pp) {
748 // No work to be done right now. This can happen at
749 // the end of the mark phase when there are still
750 // assists tapering off. Don't bother running a worker
751 // now because it'll just return immediately.
755 // Grab a worker before we commit to running below.
756 node := (*gcBgMarkWorkerNode)(gcBgMarkWorkerPool.pop())
758 // There is at least one worker per P, so normally there are
759 // enough workers to run on all Ps, if necessary. However, once
760 // a worker enters gcMarkDone it may park without rejoining the
761 // pool, thus freeing a P with no corresponding worker.
762 // gcMarkDone never depends on another worker doing work, so it
763 // is safe to simply do nothing here.
765 // If gcMarkDone bails out without completing the mark phase,
766 // it will always do so with queued global work. Thus, that P
767 // will be immediately eligible to re-run the worker G it was
768 // just using, ensuring work can complete.
772 decIfPositive := func(val *atomic.Int64) bool {
779 if val.CompareAndSwap(v, v-1) {
785 if decIfPositive(&c.dedicatedMarkWorkersNeeded) {
786 // This P is now dedicated to marking until the end of
787 // the concurrent mark phase.
788 pp.gcMarkWorkerMode = gcMarkWorkerDedicatedMode
789 } else if c.fractionalUtilizationGoal == 0 {
790 // No need for fractional workers.
791 gcBgMarkWorkerPool.push(&node.node)
794 // Is this P behind on the fractional utilization
797 // This should be kept in sync with pollFractionalWorkerExit.
798 delta := now - c.markStartTime
799 if delta > 0 && float64(pp.gcFractionalMarkTime)/float64(delta) > c.fractionalUtilizationGoal {
800 // Nope. No need to run a fractional worker.
801 gcBgMarkWorkerPool.push(&node.node)
804 // Run a fractional worker.
805 pp.gcMarkWorkerMode = gcMarkWorkerFractionalMode
808 // Run the background mark worker.
810 trace := traceAcquire()
811 casgstatus(gp, _Gwaiting, _Grunnable)
813 trace.GoUnpark(gp, 0)
819 // resetLive sets up the controller state for the next mark phase after the end
820 // of the previous one. Must be called after endCycle and before commit, before
821 // the world is started.
823 // The world must be stopped.
824 func (c *gcControllerState) resetLive(bytesMarked uint64) {
825 c.heapMarked = bytesMarked
826 c.heapLive.Store(bytesMarked)
827 c.heapScan.Store(uint64(c.heapScanWork.Load()))
828 c.lastHeapScan = uint64(c.heapScanWork.Load())
829 c.lastStackScan.Store(uint64(c.stackScanWork.Load()))
830 c.triggered = ^uint64(0) // Reset triggered.
832 // heapLive was updated, so emit a trace event.
833 trace := traceAcquire()
835 trace.HeapAlloc(bytesMarked)
840 // markWorkerStop must be called whenever a mark worker stops executing.
842 // It updates mark work accounting in the controller by a duration of
843 // work in nanoseconds and other bookkeeping.
845 // Safe to execute at any time.
846 func (c *gcControllerState) markWorkerStop(mode gcMarkWorkerMode, duration int64) {
848 case gcMarkWorkerDedicatedMode:
849 c.dedicatedMarkTime.Add(duration)
850 c.dedicatedMarkWorkersNeeded.Add(1)
851 case gcMarkWorkerFractionalMode:
852 c.fractionalMarkTime.Add(duration)
853 case gcMarkWorkerIdleMode:
854 c.idleMarkTime.Add(duration)
855 c.removeIdleMarkWorker()
857 throw("markWorkerStop: unknown mark worker mode")
861 func (c *gcControllerState) update(dHeapLive, dHeapScan int64) {
863 trace := traceAcquire()
864 live := gcController.heapLive.Add(dHeapLive)
866 // gcController.heapLive changed.
867 trace.HeapAlloc(live)
871 if gcBlackenEnabled == 0 {
872 // Update heapScan when we're not in a current GC. It is fixed
873 // at the beginning of a cycle.
875 gcController.heapScan.Add(dHeapScan)
878 // gcController.heapLive changed.
883 func (c *gcControllerState) addScannableStack(pp *p, amount int64) {
885 c.maxStackScan.Add(amount)
888 pp.maxStackScanDelta += amount
889 if pp.maxStackScanDelta >= maxStackScanSlack || pp.maxStackScanDelta <= -maxStackScanSlack {
890 c.maxStackScan.Add(pp.maxStackScanDelta)
891 pp.maxStackScanDelta = 0
895 func (c *gcControllerState) addGlobals(amount int64) {
896 c.globalsScan.Add(amount)
899 // heapGoal returns the current heap goal.
900 func (c *gcControllerState) heapGoal() uint64 {
901 goal, _ := c.heapGoalInternal()
905 // heapGoalInternal is the implementation of heapGoal which returns additional
906 // information that is necessary for computing the trigger.
908 // The returned minTrigger is always <= goal.
909 func (c *gcControllerState) heapGoalInternal() (goal, minTrigger uint64) {
910 // Start with the goal calculated for gcPercent.
911 goal = c.gcPercentHeapGoal.Load()
913 // Check if the memory-limit-based goal is smaller, and if so, pick that.
914 if newGoal := c.memoryLimitHeapGoal(); newGoal < goal {
917 // We're not limited by the memory limit goal, so perform a series of
918 // adjustments that might move the goal forward in a variety of circumstances.
920 sweepDistTrigger := c.sweepDistMinTrigger.Load()
921 if sweepDistTrigger > goal {
922 // Set the goal to maintain a minimum sweep distance since
923 // the last call to commit. Note that we never want to do this
924 // if we're in the memory limit regime, because it could push
926 goal = sweepDistTrigger
928 // Since we ignore the sweep distance trigger in the memory
929 // limit regime, we need to ensure we don't propagate it to
930 // the trigger, because it could cause a violation of the
931 // invariant that the trigger < goal.
932 minTrigger = sweepDistTrigger
934 // Ensure that the heap goal is at least a little larger than
935 // the point at which we triggered. This may not be the case if GC
936 // start is delayed or if the allocation that pushed gcController.heapLive
937 // over trigger is large or if the trigger is really close to
938 // GOGC. Assist is proportional to this distance, so enforce a
939 // minimum distance, even if it means going over the GOGC goal
942 // Ignore this if we're in the memory limit regime: we'd prefer to
943 // have the GC respond hard about how close we are to the goal than to
944 // push the goal back in such a manner that it could cause us to exceed
946 const minRunway = 64 << 10
947 if c.triggered != ^uint64(0) && goal < c.triggered+minRunway {
948 goal = c.triggered + minRunway
954 // memoryLimitHeapGoal returns a heap goal derived from memoryLimit.
955 func (c *gcControllerState) memoryLimitHeapGoal() uint64 {
956 // Start by pulling out some values we'll need. Be careful about overflow.
957 var heapFree, heapAlloc, mappedReady uint64
959 heapFree = c.heapFree.load() // Free and unscavenged memory.
960 heapAlloc = c.totalAlloc.Load() - c.totalFree.Load() // Heap object bytes in use.
961 mappedReady = c.mappedReady.Load() // Total unreleased mapped memory.
962 if heapFree+heapAlloc <= mappedReady {
965 // It is impossible for total unreleased mapped memory to exceed heap memory, but
966 // because these stats are updated independently, we may observe a partial update
967 // including only some values. Thus, we appear to break the invariant. However,
968 // this condition is necessarily transient, so just try again. In the case of a
969 // persistent accounting error, we'll deadlock here.
972 // Below we compute a goal from memoryLimit. There are a few things to be aware of.
973 // Firstly, the memoryLimit does not easily compare to the heap goal: the former
974 // is total mapped memory by the runtime that hasn't been released, while the latter is
975 // only heap object memory. Intuitively, the way we convert from one to the other is to
976 // subtract everything from memoryLimit that both contributes to the memory limit (so,
977 // ignore scavenged memory) and doesn't contain heap objects. This isn't quite what
978 // lines up with reality, but it's a good starting point.
980 // In practice this computation looks like the following:
982 // goal := memoryLimit - ((mappedReady - heapFree - heapAlloc) + max(mappedReady - memoryLimit, 0))
984 // goal -= goal / 100 * memoryLimitHeapGoalHeadroomPercent
987 // Let's break this down.
989 // The first term (marker 1) is everything that contributes to the memory limit and isn't
990 // or couldn't become heap objects. It represents, broadly speaking, non-heap overheads.
991 // One oddity you may have noticed is that we also subtract out heapFree, i.e. unscavenged
992 // memory that may contain heap objects in the future.
994 // Let's take a step back. In an ideal world, this term would look something like just
995 // the heap goal. That is, we "reserve" enough space for the heap to grow to the heap
996 // goal, and subtract out everything else. This is of course impossible; the definition
997 // is circular! However, this impossible definition contains a key insight: the amount
998 // we're *going* to use matters just as much as whatever we're currently using.
1000 // Consider if the heap shrinks to 1/10th its size, leaving behind lots of free and
1001 // unscavenged memory. mappedReady - heapAlloc will be quite large, because of that free
1002 // and unscavenged memory, pushing the goal down significantly.
1004 // heapFree is also safe to exclude from the memory limit because in the steady-state, it's
1005 // just a pool of memory for future heap allocations, and making new allocations from heapFree
1006 // memory doesn't increase overall memory use. In transient states, the scavenger and the
1007 // allocator actively manage the pool of heapFree memory to maintain the memory limit.
1009 // The second term (marker 2) is the amount of memory we've exceeded the limit by, and is
1010 // intended to help recover from such a situation. By pushing the heap goal down, we also
1011 // push the trigger down, triggering and finishing a GC sooner in order to make room for
1012 // other memory sources. Note that since we're effectively reducing the heap goal by X bytes,
1013 // we're actually giving more than X bytes of headroom back, because the heap goal is in
1014 // terms of heap objects, but it takes more than X bytes (e.g. due to fragmentation) to store
1015 // X bytes worth of objects.
1017 // The final adjustment (marker 3) reduces the maximum possible memory limit heap goal by
1018 // memoryLimitHeapGoalPercent. As the name implies, this is to provide additional headroom in
1019 // the face of pacing inaccuracies, and also to leave a buffer of unscavenged memory so the
1020 // allocator isn't constantly scavenging. The reduction amount also has a fixed minimum
1021 // (memoryLimitMinHeapGoalHeadroom, not pictured) because the aforementioned pacing inaccuracies
1022 // disproportionately affect small heaps: as heaps get smaller, the pacer's inputs get fuzzier.
1023 // Shorter GC cycles and less GC work means noisy external factors like the OS scheduler have a
1026 memoryLimit := uint64(c.memoryLimit.Load())
1029 nonHeapMemory := mappedReady - heapFree - heapAlloc
1033 if mappedReady > memoryLimit {
1034 overage = mappedReady - memoryLimit
1037 if nonHeapMemory+overage >= memoryLimit {
1038 // We're at a point where non-heap memory exceeds the memory limit on its own.
1039 // There's honestly not much we can do here but just trigger GCs continuously
1040 // and let the CPU limiter reign that in. Something has to give at this point.
1041 // Set it to heapMarked, the lowest possible goal.
1045 // Compute the goal.
1046 goal := memoryLimit - (nonHeapMemory + overage)
1048 // Apply some headroom to the goal to account for pacing inaccuracies and to reduce
1049 // the impact of scavenging at allocation time in response to a high allocation rate
1050 // when GOGC=off. See issue #57069. Also, be careful about small limits.
1051 headroom := goal / 100 * memoryLimitHeapGoalHeadroomPercent
1052 if headroom < memoryLimitMinHeapGoalHeadroom {
1053 // Set a fixed minimum to deal with the particularly large effect pacing inaccuracies
1054 // have for smaller heaps.
1055 headroom = memoryLimitMinHeapGoalHeadroom
1057 if goal < headroom || goal-headroom < headroom {
1060 goal = goal - headroom
1062 // Don't let us go below the live heap. A heap goal below the live heap doesn't make sense.
1063 if goal < c.heapMarked {
1070 // These constants determine the bounds on the GC trigger as a fraction
1071 // of heap bytes allocated between the start of a GC (heapLive == heapMarked)
1072 // and the end of a GC (heapLive == heapGoal).
1074 // The constants are obscured in this way for efficiency. The denominator
1075 // of the fraction is always a power-of-two for a quick division, so that
1076 // the numerator is a single constant integer multiplication.
1077 triggerRatioDen = 64
1079 // The minimum trigger constant was chosen empirically: given a sufficiently
1080 // fast/scalable allocator with 48 Ps that could drive the trigger ratio
1081 // to <0.05, this constant causes applications to retain the same peak
1082 // RSS compared to not having this allocator.
1083 minTriggerRatioNum = 45 // ~0.7
1085 // The maximum trigger constant is chosen somewhat arbitrarily, but the
1086 // current constant has served us well over the years.
1087 maxTriggerRatioNum = 61 // ~0.95
1090 // trigger returns the current point at which a GC should trigger along with
1093 // The returned value may be compared against heapLive to determine whether
1094 // the GC should trigger. Thus, the GC trigger condition should be (but may
1095 // not be, in the case of small movements for efficiency) checked whenever
1096 // the heap goal may change.
1097 func (c *gcControllerState) trigger() (uint64, uint64) {
1098 goal, minTrigger := c.heapGoalInternal()
1100 // Invariant: the trigger must always be less than the heap goal.
1102 // Note that the memory limit sets a hard maximum on our heap goal,
1103 // but the live heap may grow beyond it.
1105 if c.heapMarked >= goal {
1106 // The goal should never be smaller than heapMarked, but let's be
1107 // defensive about it. The only reasonable trigger here is one that
1108 // causes a continuous GC cycle at heapMarked, but respect the goal
1109 // if it came out as smaller than that.
1113 // Below this point, c.heapMarked < goal.
1115 // heapMarked is our absolute minimum, and it's possible the trigger
1116 // bound we get from heapGoalinternal is less than that.
1117 if minTrigger < c.heapMarked {
1118 minTrigger = c.heapMarked
1121 // If we let the trigger go too low, then if the application
1122 // is allocating very rapidly we might end up in a situation
1123 // where we're allocating black during a nearly always-on GC.
1124 // The result of this is a growing heap and ultimately an
1125 // increase in RSS. By capping us at a point >0, we're essentially
1126 // saying that we're OK using more CPU during the GC to prevent
1127 // this growth in RSS.
1128 triggerLowerBound := ((goal-c.heapMarked)/triggerRatioDen)*minTriggerRatioNum + c.heapMarked
1129 if minTrigger < triggerLowerBound {
1130 minTrigger = triggerLowerBound
1133 // For small heaps, set the max trigger point at maxTriggerRatio of the way
1134 // from the live heap to the heap goal. This ensures we always have *some*
1135 // headroom when the GC actually starts. For larger heaps, set the max trigger
1136 // point at the goal, minus the minimum heap size.
1138 // This choice follows from the fact that the minimum heap size is chosen
1139 // to reflect the costs of a GC with no work to do. With a large heap but
1140 // very little scan work to perform, this gives us exactly as much runway
1141 // as we would need, in the worst case.
1142 maxTrigger := ((goal-c.heapMarked)/triggerRatioDen)*maxTriggerRatioNum + c.heapMarked
1143 if goal > defaultHeapMinimum && goal-defaultHeapMinimum > maxTrigger {
1144 maxTrigger = goal - defaultHeapMinimum
1146 maxTrigger = max(maxTrigger, minTrigger)
1148 // Compute the trigger from our bounds and the runway stored by commit.
1150 runway := c.runway.Load()
1152 trigger = minTrigger
1154 trigger = goal - runway
1156 trigger = max(trigger, minTrigger)
1157 trigger = min(trigger, maxTrigger)
1159 print("trigger=", trigger, " heapGoal=", goal, "\n")
1160 print("minTrigger=", minTrigger, " maxTrigger=", maxTrigger, "\n")
1161 throw("produced a trigger greater than the heap goal")
1163 return trigger, goal
1166 // commit recomputes all pacing parameters needed to derive the
1167 // trigger and the heap goal. Namely, the gcPercent-based heap goal,
1168 // and the amount of runway we want to give the GC this cycle.
1170 // This can be called any time. If GC is the in the middle of a
1171 // concurrent phase, it will adjust the pacing of that phase.
1173 // isSweepDone should be the result of calling isSweepDone(),
1174 // unless we're testing or we know we're executing during a GC cycle.
1176 // This depends on gcPercent, gcController.heapMarked, and
1177 // gcController.heapLive. These must be up to date.
1179 // Callers must call gcControllerState.revise after calling this
1180 // function if the GC is enabled.
1182 // mheap_.lock must be held or the world must be stopped.
1183 func (c *gcControllerState) commit(isSweepDone bool) {
1185 assertWorldStoppedOrLockHeld(&mheap_.lock)
1189 // The sweep is done, so there aren't any restrictions on the trigger
1190 // we need to think about.
1191 c.sweepDistMinTrigger.Store(0)
1193 // Concurrent sweep happens in the heap growth
1194 // from gcController.heapLive to trigger. Make sure we
1195 // give the sweeper some runway if it doesn't have enough.
1196 c.sweepDistMinTrigger.Store(c.heapLive.Load() + sweepMinHeapDistance)
1199 // Compute the next GC goal, which is when the allocated heap
1200 // has grown by GOGC/100 over where it started the last cycle,
1201 // plus additional runway for non-heap sources of GC work.
1202 gcPercentHeapGoal := ^uint64(0)
1203 if gcPercent := c.gcPercent.Load(); gcPercent >= 0 {
1204 gcPercentHeapGoal = c.heapMarked + (c.heapMarked+c.lastStackScan.Load()+c.globalsScan.Load())*uint64(gcPercent)/100
1206 // Apply the minimum heap size here. It's defined in terms of gcPercent
1207 // and is only updated by functions that call commit.
1208 if gcPercentHeapGoal < c.heapMinimum {
1209 gcPercentHeapGoal = c.heapMinimum
1211 c.gcPercentHeapGoal.Store(gcPercentHeapGoal)
1213 // Compute the amount of runway we want the GC to have by using our
1214 // estimate of the cons/mark ratio.
1216 // The idea is to take our expected scan work, and multiply it by
1217 // the cons/mark ratio to determine how long it'll take to complete
1218 // that scan work in terms of bytes allocated. This gives us our GC's
1221 // However, the cons/mark ratio is a ratio of rates per CPU-second, but
1222 // here we care about the relative rates for some division of CPU
1223 // resources among the mutator and the GC.
1225 // To summarize, we have B / cpu-ns, and we want B / ns. We get that
1226 // by multiplying by our desired division of CPU resources. We choose
1227 // to express CPU resources as GOMAPROCS*fraction. Note that because
1228 // we're working with a ratio here, we can omit the number of CPU cores,
1229 // because they'll appear in the numerator and denominator and cancel out.
1230 // As a result, this is basically just "weighing" the cons/mark ratio by
1231 // our desired division of resources.
1233 // Furthermore, by setting the runway so that CPU resources are divided
1234 // this way, assuming that the cons/mark ratio is correct, we make that
1235 // division a reality.
1236 c.runway.Store(uint64((c.consMark * (1 - gcGoalUtilization) / (gcGoalUtilization)) * float64(c.lastHeapScan+c.lastStackScan.Load()+c.globalsScan.Load())))
1239 // setGCPercent updates gcPercent. commit must be called after.
1240 // Returns the old value of gcPercent.
1242 // The world must be stopped, or mheap_.lock must be held.
1243 func (c *gcControllerState) setGCPercent(in int32) int32 {
1245 assertWorldStoppedOrLockHeld(&mheap_.lock)
1248 out := c.gcPercent.Load()
1252 c.heapMinimum = defaultHeapMinimum * uint64(in) / 100
1253 c.gcPercent.Store(in)
1258 //go:linkname setGCPercent runtime/debug.setGCPercent
1259 func setGCPercent(in int32) (out int32) {
1260 // Run on the system stack since we grab the heap lock.
1261 systemstack(func() {
1263 out = gcController.setGCPercent(in)
1264 gcControllerCommit()
1265 unlock(&mheap_.lock)
1268 // If we just disabled GC, wait for any concurrent GC mark to
1269 // finish so we always return with no GC running.
1271 gcWaitOnMark(work.cycles.Load())
1277 func readGOGC() int32 {
1278 p := gogetenv("GOGC")
1282 if n, ok := atoi32(p); ok {
1288 // setMemoryLimit updates memoryLimit. commit must be called after
1289 // Returns the old value of memoryLimit.
1291 // The world must be stopped, or mheap_.lock must be held.
1292 func (c *gcControllerState) setMemoryLimit(in int64) int64 {
1294 assertWorldStoppedOrLockHeld(&mheap_.lock)
1297 out := c.memoryLimit.Load()
1299 c.memoryLimit.Store(in)
1305 //go:linkname setMemoryLimit runtime/debug.setMemoryLimit
1306 func setMemoryLimit(in int64) (out int64) {
1307 // Run on the system stack since we grab the heap lock.
1308 systemstack(func() {
1310 out = gcController.setMemoryLimit(in)
1311 if in < 0 || out == in {
1312 // If we're just checking the value or not changing
1313 // it, there's no point in doing the rest.
1314 unlock(&mheap_.lock)
1317 gcControllerCommit()
1318 unlock(&mheap_.lock)
1323 func readGOMEMLIMIT() int64 {
1324 p := gogetenv("GOMEMLIMIT")
1325 if p == "" || p == "off" {
1328 n, ok := parseByteCount(p)
1330 print("GOMEMLIMIT=", p, "\n")
1331 throw("malformed GOMEMLIMIT; see `go doc runtime/debug.SetMemoryLimit`")
1336 // addIdleMarkWorker attempts to add a new idle mark worker.
1338 // If this returns true, the caller must become an idle mark worker unless
1339 // there's no background mark worker goroutines in the pool. This case is
1340 // harmless because there are already background mark workers running.
1341 // If this returns false, the caller must NOT become an idle mark worker.
1343 // nosplit because it may be called without a P.
1346 func (c *gcControllerState) addIdleMarkWorker() bool {
1348 old := c.idleMarkWorkers.Load()
1349 n, max := int32(old&uint64(^uint32(0))), int32(old>>32)
1351 // See the comment on idleMarkWorkers for why
1352 // n > max is tolerated.
1356 print("n=", n, " max=", max, "\n")
1357 throw("negative idle mark workers")
1359 new := uint64(uint32(n+1)) | (uint64(max) << 32)
1360 if c.idleMarkWorkers.CompareAndSwap(old, new) {
1366 // needIdleMarkWorker is a hint as to whether another idle mark worker is needed.
1368 // The caller must still call addIdleMarkWorker to become one. This is mainly
1369 // useful for a quick check before an expensive operation.
1371 // nosplit because it may be called without a P.
1374 func (c *gcControllerState) needIdleMarkWorker() bool {
1375 p := c.idleMarkWorkers.Load()
1376 n, max := int32(p&uint64(^uint32(0))), int32(p>>32)
1380 // removeIdleMarkWorker must be called when an new idle mark worker stops executing.
1381 func (c *gcControllerState) removeIdleMarkWorker() {
1383 old := c.idleMarkWorkers.Load()
1384 n, max := int32(old&uint64(^uint32(0))), int32(old>>32)
1386 print("n=", n, " max=", max, "\n")
1387 throw("negative idle mark workers")
1389 new := uint64(uint32(n-1)) | (uint64(max) << 32)
1390 if c.idleMarkWorkers.CompareAndSwap(old, new) {
1396 // setMaxIdleMarkWorkers sets the maximum number of idle mark workers allowed.
1398 // This method is optimistic in that it does not wait for the number of
1399 // idle mark workers to reduce to max before returning; it assumes the workers
1400 // will deschedule themselves.
1401 func (c *gcControllerState) setMaxIdleMarkWorkers(max int32) {
1403 old := c.idleMarkWorkers.Load()
1404 n := int32(old & uint64(^uint32(0)))
1406 print("n=", n, " max=", max, "\n")
1407 throw("negative idle mark workers")
1409 new := uint64(uint32(n)) | (uint64(max) << 32)
1410 if c.idleMarkWorkers.CompareAndSwap(old, new) {
1416 // gcControllerCommit is gcController.commit, but passes arguments from live
1417 // (non-test) data. It also updates any consumers of the GC pacing, such as
1418 // sweep pacing and the background scavenger.
1420 // Calls gcController.commit.
1422 // The heap lock must be held, so this must be executed on the system stack.
1425 func gcControllerCommit() {
1426 assertWorldStoppedOrLockHeld(&mheap_.lock)
1428 gcController.commit(isSweepDone())
1430 // Update mark pacing.
1431 if gcphase != _GCoff {
1432 gcController.revise()
1435 // TODO(mknyszek): This isn't really accurate any longer because the heap
1436 // goal is computed dynamically. Still useful to snapshot, but not as useful.
1437 trace := traceAcquire()
1443 trigger, heapGoal := gcController.trigger()
1444 gcPaceSweeper(trigger)
1445 gcPaceScavenger(gcController.memoryLimit.Load(), heapGoal, gcController.lastHeapGoal)