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"
14 // go119MemoryLimitSupport is a feature flag for a number of changes
15 // related to the memory limit feature (#48409). Disabling this flag
16 // disables those features, as well as the memory limit mechanism,
17 // which becomes a no-op.
18 const go119MemoryLimitSupport = true
21 // gcGoalUtilization is the goal CPU utilization for
22 // marking as a fraction of GOMAXPROCS.
24 // Increasing the goal utilization will shorten GC cycles as the GC
25 // has more resources behind it, lessening costs from the write barrier,
26 // but comes at the cost of increasing mutator latency.
27 gcGoalUtilization = gcBackgroundUtilization
29 // gcBackgroundUtilization is the fixed CPU utilization for background
30 // marking. It must be <= gcGoalUtilization. The difference between
31 // gcGoalUtilization and gcBackgroundUtilization will be made up by
32 // mark assists. The scheduler will aim to use within 50% of this
35 // As a general rule, there's little reason to set gcBackgroundUtilization
36 // < gcGoalUtilization. One reason might be in mostly idle applications,
37 // where goroutines are unlikely to assist at all, so the actual
38 // utilization will be lower than the goal. But this is moot point
39 // because the idle mark workers already soak up idle CPU resources.
40 // These two values are still kept separate however because they are
41 // distinct conceptually, and in previous iterations of the pacer the
42 // distinction was more important.
43 gcBackgroundUtilization = 0.25
45 // gcCreditSlack is the amount of scan work credit that can
46 // accumulate locally before updating gcController.heapScanWork and,
47 // optionally, gcController.bgScanCredit. Lower values give a more
48 // accurate assist ratio and make it more likely that assists will
49 // successfully steal background credit. Higher values reduce memory
53 // gcAssistTimeSlack is the nanoseconds of mutator assist time that
54 // can accumulate on a P before updating gcController.assistTime.
55 gcAssistTimeSlack = 5000
57 // gcOverAssistWork determines how many extra units of scan work a GC
58 // assist does when an assist happens. This amortizes the cost of an
59 // assist by pre-paying for this many bytes of future allocations.
60 gcOverAssistWork = 64 << 10
62 // defaultHeapMinimum is the value of heapMinimum for GOGC==100.
63 defaultHeapMinimum = (goexperiment.HeapMinimum512KiBInt)*(512<<10) +
64 (1-goexperiment.HeapMinimum512KiBInt)*(4<<20)
66 // maxStackScanSlack is the bytes of stack space allocated or freed
67 // that can accumulate on a P before updating gcController.stackSize.
68 maxStackScanSlack = 8 << 10
70 // memoryLimitHeapGoalHeadroom is the amount of headroom the pacer gives to
71 // the heap goal when operating in the memory-limited regime. That is,
72 // it'll reduce the heap goal by this many extra bytes off of the base
74 memoryLimitHeapGoalHeadroom = 1 << 20
78 if offset := unsafe.Offsetof(gcController.heapLive); offset%8 != 0 {
80 throw("gcController.heapLive not aligned to 8 bytes")
84 // gcController implements the GC pacing controller that determines
85 // when to trigger concurrent garbage collection and how much marking
86 // work to do in mutator assists and background marking.
88 // It calculates the ratio between the allocation rate (in terms of CPU
89 // time) and the GC scan throughput to determine the heap size at which to
90 // trigger a GC cycle such that no GC assists are required to finish on time.
91 // This algorithm thus optimizes GC CPU utilization to the dedicated background
92 // mark utilization of 25% of GOMAXPROCS by minimizing GC assists.
93 // GOMAXPROCS. The high-level design of this algorithm is documented
94 // at https://github.com/golang/proposal/blob/master/design/44167-gc-pacer-redesign.md.
95 // See https://golang.org/s/go15gcpacing for additional historical context.
96 var gcController gcControllerState
98 type gcControllerState struct {
99 // Initialized from GOGC. GOGC=off means no GC.
100 gcPercent atomic.Int32
102 _ uint32 // padding so following 64-bit values are 8-byte aligned
104 // memoryLimit is the soft memory limit in bytes.
106 // Initialized from GOMEMLIMIT. GOMEMLIMIT=off is equivalent to MaxInt64
107 // which means no soft memory limit in practice.
109 // This is an int64 instead of a uint64 to more easily maintain parity with
110 // the SetMemoryLimit API, which sets a maximum at MaxInt64. This value
111 // should never be negative.
112 memoryLimit atomic.Int64
114 // heapMinimum is the minimum heap size at which to trigger GC.
115 // For small heaps, this overrides the usual GOGC*live set rule.
117 // When there is a very small live set but a lot of allocation, simply
118 // collecting when the heap reaches GOGC*live results in many GC
119 // cycles and high total per-GC overhead. This minimum amortizes this
120 // per-GC overhead while keeping the heap reasonably small.
122 // During initialization this is set to 4MB*GOGC/100. In the case of
123 // GOGC==0, this will set heapMinimum to 0, resulting in constant
124 // collection even when the heap size is small, which is useful for
128 // runway is the amount of runway in heap bytes allocated by the
129 // application that we want to give the GC once it starts.
131 // This is computed from consMark during mark termination.
134 // consMark is the estimated per-CPU consMark ratio for the application.
136 // It represents the ratio between the application's allocation
137 // rate, as bytes allocated per CPU-time, and the GC's scan rate,
138 // as bytes scanned per CPU-time.
139 // The units of this ratio are (B / cpu-ns) / (B / cpu-ns).
141 // At a high level, this value is computed as the bytes of memory
142 // allocated (cons) per unit of scan work completed (mark) in a GC
143 // cycle, divided by the CPU time spent on each activity.
145 // Updated at the end of each GC cycle, in endCycle.
148 // consMarkController holds the state for the mark-cons ratio
149 // estimation over time.
151 // Its purpose is to smooth out noisiness in the computation of
152 // consMark; see consMark for details.
153 consMarkController piController
155 _ uint32 // Padding for atomics on 32-bit platforms.
157 // gcPercentHeapGoal is the goal heapLive for when next GC ends derived
160 // Set to ^uint64(0) if gcPercent is disabled.
161 gcPercentHeapGoal atomic.Uint64
163 // sweepDistMinTrigger is the minimum trigger to ensure a minimum
166 // This bound is also special because it applies to both the trigger
167 // *and* the goal (all other trigger bounds must be based *on* the goal).
169 // It is computed ahead of time, at commit time. The theory is that,
170 // absent a sudden change to a parameter like gcPercent, the trigger
171 // will be chosen to always give the sweeper enough headroom. However,
172 // such a change might dramatically and suddenly move up the trigger,
173 // in which case we need to ensure the sweeper still has enough headroom.
174 sweepDistMinTrigger atomic.Uint64
176 // triggered is the point at which the current GC cycle actually triggered.
177 // Only valid during the mark phase of a GC cycle, otherwise set to ^uint64(0).
179 // Updated while the world is stopped.
182 // lastHeapGoal is the value of heapGoal at the moment the last GC
183 // ended. Note that this is distinct from the last value heapGoal had,
184 // because it could change if e.g. gcPercent changes.
186 // Read and written with the world stopped or with mheap_.lock held.
189 // heapLive is the number of bytes considered live by the GC.
190 // That is: retained by the most recent GC plus allocated
191 // since then. heapLive ≤ memstats.totalAlloc-memstats.totalFree, since
192 // heapAlloc includes unmarked objects that have not yet been swept (and
193 // hence goes up as we allocate and down as we sweep) while heapLive
194 // excludes these objects (and hence only goes up between GCs).
196 // This is updated atomically without locking. To reduce
197 // contention, this is updated only when obtaining a span from
198 // an mcentral and at this point it counts all of the
199 // unallocated slots in that span (which will be allocated
200 // before that mcache obtains another span from that
201 // mcentral). Hence, it slightly overestimates the "true" live
202 // heap size. It's better to overestimate than to
203 // underestimate because 1) this triggers the GC earlier than
204 // necessary rather than potentially too late and 2) this
205 // leads to a conservative GC rate rather than a GC rate that
206 // is potentially too low.
208 // Reads should likewise be atomic (or during STW).
210 // Whenever this is updated, call traceHeapAlloc() and
211 // this gcControllerState's revise() method.
214 // heapScan is the number of bytes of "scannable" heap. This
215 // is the live heap (as counted by heapLive), but omitting
216 // no-scan objects and no-scan tails of objects.
218 // This value is fixed at the start of a GC cycle, so during a
219 // GC cycle it is safe to read without atomics, and it represents
220 // the maximum scannable heap.
223 // lastHeapScan is the number of bytes of heap that were scanned
224 // last GC cycle. It is the same as heapMarked, but only
225 // includes the "scannable" parts of objects.
227 // Updated when the world is stopped.
230 // lastStackScan is the number of bytes of stack that were scanned
234 // maxStackScan is the amount of allocated goroutine stack space in
235 // use by goroutines.
237 // This number tracks allocated goroutine stack space rather than used
238 // goroutine stack space (i.e. what is actually scanned) because used
239 // goroutine stack space is much harder to measure cheaply. By using
240 // allocated space, we make an overestimate; this is OK, it's better
241 // to conservatively overcount than undercount.
243 // Read and updated atomically.
246 // globalsScan is the total amount of global variable space
247 // that is scannable.
249 // Read and updated atomically.
252 // heapMarked is the number of bytes marked by the previous
253 // GC. After mark termination, heapLive == heapMarked, but
254 // unlike heapLive, heapMarked does not change until the
255 // next mark termination.
258 // heapScanWork is the total heap scan work performed this cycle.
259 // stackScanWork is the total stack scan work performed this cycle.
260 // globalsScanWork is the total globals scan work performed this cycle.
262 // These are updated atomically during the cycle. Updates occur in
263 // bounded batches, since they are both written and read
264 // throughout the cycle. At the end of the cycle, heapScanWork is how
265 // much of the retained heap is scannable.
267 // Currently these are measured in bytes. For most uses, this is an
268 // opaque unit of work, but for estimation the definition is important.
270 // Note that stackScanWork includes only stack space scanned, not all
271 // of the allocated stack.
272 heapScanWork atomic.Int64
273 stackScanWork atomic.Int64
274 globalsScanWork atomic.Int64
276 // bgScanCredit is the scan work credit accumulated by the
277 // concurrent background scan. This credit is accumulated by
278 // the background scan and stolen by mutator assists. This is
279 // updated atomically. Updates occur in bounded batches, since
280 // it is both written and read throughout the cycle.
283 // assistTime is the nanoseconds spent in mutator assists
284 // during this cycle. This is updated atomically, and must also
285 // be updated atomically even during a STW, because it is read
286 // by sysmon. Updates occur in bounded batches, since it is both
287 // written and read throughout the cycle.
288 assistTime atomic.Int64
290 // dedicatedMarkTime is the nanoseconds spent in dedicated
291 // mark workers during this cycle. This is updated atomically
292 // at the end of the concurrent mark phase.
293 dedicatedMarkTime int64
295 // fractionalMarkTime is the nanoseconds spent in the
296 // fractional mark worker during this cycle. This is updated
297 // atomically throughout the cycle and will be up-to-date if
298 // the fractional mark worker is not currently running.
299 fractionalMarkTime int64
301 // idleMarkTime is the nanoseconds spent in idle marking
302 // during this cycle. This is updated atomically throughout
306 // markStartTime is the absolute start time in nanoseconds
307 // that assists and background mark workers started.
310 // dedicatedMarkWorkersNeeded is the number of dedicated mark
311 // workers that need to be started. This is computed at the
312 // beginning of each cycle and decremented atomically as
313 // dedicated mark workers get started.
314 dedicatedMarkWorkersNeeded int64
316 // idleMarkWorkers is two packed int32 values in a single uint64.
317 // These two values are always updated simultaneously.
319 // The bottom int32 is the current number of idle mark workers executing.
321 // The top int32 is the maximum number of idle mark workers allowed to
322 // execute concurrently. Normally, this number is just gomaxprocs. However,
323 // during periodic GC cycles it is set to 0 because the system is idle
324 // anyway; there's no need to go full blast on all of GOMAXPROCS.
326 // The maximum number of idle mark workers is used to prevent new workers
327 // from starting, but it is not a hard maximum. It is possible (but
328 // exceedingly rare) for the current number of idle mark workers to
329 // transiently exceed the maximum. This could happen if the maximum changes
330 // just after a GC ends, and an M with no P.
332 // Note that if we have no dedicated mark workers, we set this value to
333 // 1 in this case we only have fractional GC workers which aren't scheduled
334 // strictly enough to ensure GC progress. As a result, idle-priority mark
335 // workers are vital to GC progress in these situations.
337 // For example, consider a situation in which goroutines block on the GC
338 // (such as via runtime.GOMAXPROCS) and only fractional mark workers are
339 // scheduled (e.g. GOMAXPROCS=1). Without idle-priority mark workers, the
340 // last running M might skip scheduling a fractional mark worker if its
341 // utilization goal is met, such that once it goes to sleep (because there's
342 // nothing to do), there will be nothing else to spin up a new M for the
343 // fractional worker in the future, stalling GC progress and causing a
344 // deadlock. However, idle-priority workers will *always* run when there is
345 // nothing left to do, ensuring the GC makes progress.
347 // See github.com/golang/go/issues/44163 for more details.
348 idleMarkWorkers atomic.Uint64
350 // assistWorkPerByte is the ratio of scan work to allocated
351 // bytes that should be performed by mutator assists. This is
352 // computed at the beginning of each cycle and updated every
353 // time heapScan is updated.
354 assistWorkPerByte atomic.Float64
356 // assistBytesPerWork is 1/assistWorkPerByte.
358 // Note that because this is read and written independently
359 // from assistWorkPerByte users may notice a skew between
360 // the two values, and such a state should be safe.
361 assistBytesPerWork atomic.Float64
363 // fractionalUtilizationGoal is the fraction of wall clock
364 // time that should be spent in the fractional mark worker on
365 // each P that isn't running a dedicated worker.
367 // For example, if the utilization goal is 25% and there are
368 // no dedicated workers, this will be 0.25. If the goal is
369 // 25%, there is one dedicated worker, and GOMAXPROCS is 5,
370 // this will be 0.05 to make up the missing 5%.
372 // If this is zero, no fractional workers are needed.
373 fractionalUtilizationGoal float64
375 // These memory stats are effectively duplicates of fields from
376 // memstats.heapStats but are updated atomically or with the world
377 // stopped and don't provide the same consistency guarantees.
379 // Because the runtime is responsible for managing a memory limit, it's
380 // useful to couple these stats more tightly to the gcController, which
381 // is intimately connected to how that memory limit is maintained.
382 heapInUse sysMemStat // bytes in mSpanInUse spans
383 heapReleased sysMemStat // bytes released to the OS
384 heapFree sysMemStat // bytes not in any span, but not released to the OS
385 totalAlloc atomic.Uint64 // total bytes allocated
386 totalFree atomic.Uint64 // total bytes freed
387 mappedReady atomic.Uint64 // total virtual memory in the Ready state (see mem.go).
389 // test indicates that this is a test-only copy of gcControllerState.
395 func (c *gcControllerState) init(gcPercent int32, memoryLimit int64) {
396 c.heapMinimum = defaultHeapMinimum
397 c.triggered = ^uint64(0)
399 c.consMarkController = piController{
400 // Tuned first via the Ziegler-Nichols process in simulation,
401 // then the integral time was manually tuned against real-world
402 // applications to deal with noisiness in the measured cons/mark
407 // Set a high reset time in GC cycles.
408 // This is inversely proportional to the rate at which we
409 // accumulate error from clipping. By making this very high
410 // we make the accumulation slow. In general, clipping is
411 // OK in our situation, hence the choice.
413 // Tune this if we get unintended effects from clipping for
420 c.setGCPercent(gcPercent)
421 c.setMemoryLimit(memoryLimit)
422 c.commit(true) // No sweep phase in the first GC cycle.
423 // N.B. Don't bother calling traceHeapGoal. Tracing is never enabled at
424 // initialization time.
425 // N.B. No need to call revise; there's no GC enabled during
429 // startCycle resets the GC controller's state and computes estimates
430 // for a new GC cycle. The caller must hold worldsema and the world
432 func (c *gcControllerState) startCycle(markStartTime int64, procs int, trigger gcTrigger) {
433 c.heapScanWork.Store(0)
434 c.stackScanWork.Store(0)
435 c.globalsScanWork.Store(0)
437 c.assistTime.Store(0)
438 c.dedicatedMarkTime = 0
439 c.fractionalMarkTime = 0
441 c.markStartTime = markStartTime
443 // TODO(mknyszek): This is supposed to be the actual trigger point for the heap, but
444 // causes regressions in memory use. The cause is that the PI controller used to smooth
445 // the cons/mark ratio measurements tends to flail when using the less accurate precomputed
446 // trigger for the cons/mark calculation, and this results in the controller being more
447 // conservative about steady-states it tries to find in the future.
449 // This conservatism is transient, but these transient states tend to matter for short-lived
450 // programs, especially because the PI controller is overdamped, partially because it is
451 // configured with a relatively large time constant.
453 // Ultimately, I think this is just two mistakes piled on one another: the choice of a swingy
454 // smoothing function that recalls a fairly long history (due to its overdamped time constant)
455 // coupled with an inaccurate cons/mark calculation. It just so happens this works better
456 // today, and it makes it harder to change things in the future.
458 // This is described in #53738. Fix this for #53892 by changing back to the actual trigger
459 // point and simplifying the smoothing function.
460 heapTrigger, heapGoal := c.trigger()
461 c.triggered = heapTrigger
463 // Compute the background mark utilization goal. In general,
464 // this may not come out exactly. We round the number of
465 // dedicated workers so that the utilization is closest to
466 // 25%. For small GOMAXPROCS, this would introduce too much
467 // error, so we add fractional workers in that case.
468 totalUtilizationGoal := float64(procs) * gcBackgroundUtilization
469 c.dedicatedMarkWorkersNeeded = int64(totalUtilizationGoal + 0.5)
470 utilError := float64(c.dedicatedMarkWorkersNeeded)/totalUtilizationGoal - 1
471 const maxUtilError = 0.3
472 if utilError < -maxUtilError || utilError > maxUtilError {
473 // Rounding put us more than 30% off our goal. With
474 // gcBackgroundUtilization of 25%, this happens for
475 // GOMAXPROCS<=3 or GOMAXPROCS=6. Enable fractional
476 // workers to compensate.
477 if float64(c.dedicatedMarkWorkersNeeded) > totalUtilizationGoal {
478 // Too many dedicated workers.
479 c.dedicatedMarkWorkersNeeded--
481 c.fractionalUtilizationGoal = (totalUtilizationGoal - float64(c.dedicatedMarkWorkersNeeded)) / float64(procs)
483 c.fractionalUtilizationGoal = 0
486 // In STW mode, we just want dedicated workers.
487 if debug.gcstoptheworld > 0 {
488 c.dedicatedMarkWorkersNeeded = int64(procs)
489 c.fractionalUtilizationGoal = 0
493 for _, p := range allp {
495 p.gcFractionalMarkTime = 0
498 if trigger.kind == gcTriggerTime {
499 // During a periodic GC cycle, reduce the number of idle mark workers
500 // required. However, we need at least one dedicated mark worker or
501 // idle GC worker to ensure GC progress in some scenarios (see comment
502 // on maxIdleMarkWorkers).
503 if c.dedicatedMarkWorkersNeeded > 0 {
504 c.setMaxIdleMarkWorkers(0)
506 // TODO(mknyszek): The fundamental reason why we need this is because
507 // we can't count on the fractional mark worker to get scheduled.
508 // Fix that by ensuring it gets scheduled according to its quota even
509 // if the rest of the application is idle.
510 c.setMaxIdleMarkWorkers(1)
513 // N.B. gomaxprocs and dedicatedMarkWorkersNeeded is guaranteed not to
514 // change during a GC cycle.
515 c.setMaxIdleMarkWorkers(int32(procs) - int32(c.dedicatedMarkWorkersNeeded))
518 // Compute initial values for controls that are updated
519 // throughout the cycle.
522 if debug.gcpacertrace > 0 {
523 assistRatio := c.assistWorkPerByte.Load()
524 print("pacer: assist ratio=", assistRatio,
525 " (scan ", gcController.heapScan>>20, " MB in ",
526 work.initialHeapLive>>20, "->",
527 heapGoal>>20, " MB)",
528 " workers=", c.dedicatedMarkWorkersNeeded,
529 "+", c.fractionalUtilizationGoal, "\n")
533 // revise updates the assist ratio during the GC cycle to account for
534 // improved estimates. This should be called whenever gcController.heapScan,
535 // gcController.heapLive, or if any inputs to gcController.heapGoal are
536 // updated. It is safe to call concurrently, but it may race with other
539 // The result of this race is that the two assist ratio values may not line
540 // up or may be stale. In practice this is OK because the assist ratio
541 // moves slowly throughout a GC cycle, and the assist ratio is a best-effort
542 // heuristic anyway. Furthermore, no part of the heuristic depends on
543 // the two assist ratio values being exact reciprocals of one another, since
544 // the two values are used to convert values from different sources.
546 // The worst case result of this raciness is that we may miss a larger shift
547 // in the ratio (say, if we decide to pace more aggressively against the
548 // hard heap goal) but even this "hard goal" is best-effort (see #40460).
549 // The dedicated GC should ensure we don't exceed the hard goal by too much
550 // in the rare case we do exceed it.
552 // It should only be called when gcBlackenEnabled != 0 (because this
553 // is when assists are enabled and the necessary statistics are
555 func (c *gcControllerState) revise() {
556 gcPercent := c.gcPercent.Load()
558 // If GC is disabled but we're running a forced GC,
559 // act like GOGC is huge for the below calculations.
562 live := atomic.Load64(&c.heapLive)
563 scan := atomic.Load64(&c.heapScan)
564 work := c.heapScanWork.Load() + c.stackScanWork.Load() + c.globalsScanWork.Load()
566 // Assume we're under the soft goal. Pace GC to complete at
567 // heapGoal assuming the heap is in steady-state.
568 heapGoal := int64(c.heapGoal())
570 // The expected scan work is computed as the amount of bytes scanned last
571 // GC cycle (both heap and stack), plus our estimate of globals work for this cycle.
572 scanWorkExpected := int64(c.lastHeapScan + c.lastStackScan + c.globalsScan)
574 // maxScanWork is a worst-case estimate of the amount of scan work that
575 // needs to be performed in this GC cycle. Specifically, it represents
576 // the case where *all* scannable memory turns out to be live, and
577 // *all* allocated stack space is scannable.
578 maxStackScan := atomic.Load64(&c.maxStackScan)
579 maxScanWork := int64(scan + maxStackScan + c.globalsScan)
580 if work > scanWorkExpected {
581 // We've already done more scan work than expected. Because our expectation
582 // is based on a steady-state scannable heap size, we assume this means our
583 // heap is growing. Compute a new heap goal that takes our existing runway
584 // computed for scanWorkExpected and extrapolates it to maxScanWork, the worst-case
585 // scan work. This keeps our assist ratio stable if the heap continues to grow.
587 // The effect of this mechanism is that assists stay flat in the face of heap
588 // growths. It's OK to use more memory this cycle to scan all the live heap,
589 // because the next GC cycle is inevitably going to use *at least* that much
591 extHeapGoal := int64(float64(heapGoal-int64(c.triggered))/float64(scanWorkExpected)*float64(maxScanWork)) + int64(c.triggered)
592 scanWorkExpected = maxScanWork
594 // hardGoal is a hard limit on the amount that we're willing to push back the
595 // heap goal, and that's twice the heap goal (i.e. if GOGC=100 and the heap and/or
596 // stacks and/or globals grow to twice their size, this limits the current GC cycle's
597 // growth to 4x the original live heap's size).
599 // This maintains the invariant that we use no more memory than the next GC cycle
601 hardGoal := int64((1.0 + float64(gcPercent)/100.0) * float64(heapGoal))
602 if extHeapGoal > hardGoal {
603 extHeapGoal = hardGoal
605 heapGoal = extHeapGoal
607 if int64(live) > heapGoal {
608 // We're already past our heap goal, even the extrapolated one.
609 // Leave ourselves some extra runway, so in the worst case we
610 // finish by that point.
611 const maxOvershoot = 1.1
612 heapGoal = int64(float64(heapGoal) * maxOvershoot)
614 // Compute the upper bound on the scan work remaining.
615 scanWorkExpected = maxScanWork
618 // Compute the remaining scan work estimate.
620 // Note that we currently count allocations during GC as both
621 // scannable heap (heapScan) and scan work completed
622 // (scanWork), so allocation will change this difference
623 // slowly in the soft regime and not at all in the hard
625 scanWorkRemaining := scanWorkExpected - work
626 if scanWorkRemaining < 1000 {
627 // We set a somewhat arbitrary lower bound on
628 // remaining scan work since if we aim a little high,
629 // we can miss by a little.
631 // We *do* need to enforce that this is at least 1,
632 // since marking is racy and double-scanning objects
633 // may legitimately make the remaining scan work
634 // negative, even in the hard goal regime.
635 scanWorkRemaining = 1000
638 // Compute the heap distance remaining.
639 heapRemaining := heapGoal - int64(live)
640 if heapRemaining <= 0 {
641 // This shouldn't happen, but if it does, avoid
642 // dividing by zero or setting the assist negative.
646 // Compute the mutator assist ratio so by the time the mutator
647 // allocates the remaining heap bytes up to heapGoal, it will
648 // have done (or stolen) the remaining amount of scan work.
649 // Note that the assist ratio values are updated atomically
650 // but not together. This means there may be some degree of
651 // skew between the two values. This is generally OK as the
652 // values shift relatively slowly over the course of a GC
654 assistWorkPerByte := float64(scanWorkRemaining) / float64(heapRemaining)
655 assistBytesPerWork := float64(heapRemaining) / float64(scanWorkRemaining)
656 c.assistWorkPerByte.Store(assistWorkPerByte)
657 c.assistBytesPerWork.Store(assistBytesPerWork)
660 // endCycle computes the consMark estimate for the next cycle.
661 // userForced indicates whether the current GC cycle was forced
662 // by the application.
663 func (c *gcControllerState) endCycle(now int64, procs int, userForced bool) {
664 // Record last heap goal for the scavenger.
665 // We'll be updating the heap goal soon.
666 gcController.lastHeapGoal = c.heapGoal()
668 // Compute the duration of time for which assists were turned on.
669 assistDuration := now - c.markStartTime
671 // Assume background mark hit its utilization goal.
672 utilization := gcBackgroundUtilization
673 // Add assist utilization; avoid divide by zero.
674 if assistDuration > 0 {
675 utilization += float64(c.assistTime.Load()) / float64(assistDuration*int64(procs))
678 if c.heapLive <= c.triggered {
679 // Shouldn't happen, but let's be very safe about this in case the
680 // GC is somehow extremely short.
682 // In this case though, the only reasonable value for c.heapLive-c.triggered
683 // would be 0, which isn't really all that useful, i.e. the GC was so short
684 // that it didn't matter.
686 // Ignore this case and don't update anything.
689 idleUtilization := 0.0
690 if assistDuration > 0 {
691 idleUtilization = float64(c.idleMarkTime) / float64(assistDuration*int64(procs))
693 // Determine the cons/mark ratio.
695 // The units we want for the numerator and denominator are both B / cpu-ns.
696 // We get this by taking the bytes allocated or scanned, and divide by the amount of
697 // CPU time it took for those operations. For allocations, that CPU time is
699 // assistDuration * procs * (1 - utilization)
701 // Where utilization includes just background GC workers and assists. It does *not*
702 // include idle GC work time, because in theory the mutator is free to take that at
705 // For scanning, that CPU time is
707 // assistDuration * procs * (utilization + idleUtilization)
709 // In this case, we *include* idle utilization, because that is additional CPU time that the
710 // the GC had available to it.
712 // In effect, idle GC time is sort of double-counted here, but it's very weird compared
713 // to other kinds of GC work, because of how fluid it is. Namely, because the mutator is
714 // *always* free to take it.
716 // So this calculation is really:
717 // (heapLive-trigger) / (assistDuration * procs * (1-utilization)) /
718 // (scanWork) / (assistDuration * procs * (utilization+idleUtilization)
720 // Note that because we only care about the ratio, assistDuration and procs cancel out.
721 scanWork := c.heapScanWork.Load() + c.stackScanWork.Load() + c.globalsScanWork.Load()
722 currentConsMark := (float64(c.heapLive-c.triggered) * (utilization + idleUtilization)) /
723 (float64(scanWork) * (1 - utilization))
725 // Update cons/mark controller. The time period for this is 1 GC cycle.
727 // This use of a PI controller might seem strange. So, here's an explanation:
729 // currentConsMark represents the consMark we *should've* had to be perfectly
730 // on-target for this cycle. Given that we assume the next GC will be like this
731 // one in the steady-state, it stands to reason that we should just pick that
732 // as our next consMark. In practice, however, currentConsMark is too noisy:
733 // we're going to be wildly off-target in each GC cycle if we do that.
735 // What we do instead is make a long-term assumption: there is some steady-state
736 // consMark value, but it's obscured by noise. By constantly shooting for this
737 // noisy-but-perfect consMark value, the controller will bounce around a bit,
738 // but its average behavior, in aggregate, should be less noisy and closer to
739 // the true long-term consMark value, provided its tuned to be slightly overdamped.
741 oldConsMark := c.consMark
742 c.consMark, ok = c.consMarkController.next(c.consMark, currentConsMark, 1.0)
744 // The error spiraled out of control. This is incredibly unlikely seeing
745 // as this controller is essentially just a smoothing function, but it might
746 // mean that something went very wrong with how currentConsMark was calculated.
747 // Just reset consMark and keep going.
751 if debug.gcpacertrace > 0 {
753 goal := gcGoalUtilization * 100
754 print("pacer: ", int(utilization*100), "% CPU (", int(goal), " exp.) for ")
755 print(c.heapScanWork.Load(), "+", c.stackScanWork.Load(), "+", c.globalsScanWork.Load(), " B work (", c.lastHeapScan+c.lastStackScan+c.globalsScan, " B exp.) ")
756 print("in ", c.triggered, " B -> ", c.heapLive, " B (∆goal ", int64(c.heapLive)-int64(c.lastHeapGoal), ", cons/mark ", oldConsMark, ")")
758 print("[controller reset]")
765 // enlistWorker encourages another dedicated mark worker to start on
766 // another P if there are spare worker slots. It is used by putfull
767 // when more work is made available.
770 func (c *gcControllerState) enlistWorker() {
771 // If there are idle Ps, wake one so it will run an idle worker.
772 // NOTE: This is suspected of causing deadlocks. See golang.org/issue/19112.
774 // if atomic.Load(&sched.npidle) != 0 && atomic.Load(&sched.nmspinning) == 0 {
779 // There are no idle Ps. If we need more dedicated workers,
780 // try to preempt a running P so it will switch to a worker.
781 if c.dedicatedMarkWorkersNeeded <= 0 {
784 // Pick a random other P to preempt.
789 if gp == nil || gp.m == nil || gp.m.p == 0 {
792 myID := gp.m.p.ptr().id
793 for tries := 0; tries < 5; tries++ {
794 id := int32(fastrandn(uint32(gomaxprocs - 1)))
799 if p.status != _Prunning {
808 // findRunnableGCWorker returns a background mark worker for pp if it
809 // should be run. This must only be called when gcBlackenEnabled != 0.
810 func (c *gcControllerState) findRunnableGCWorker(pp *p, now int64) (*g, int64) {
811 if gcBlackenEnabled == 0 {
812 throw("gcControllerState.findRunnable: blackening not enabled")
815 // Since we have the current time, check if the GC CPU limiter
816 // hasn't had an update in a while. This check is necessary in
817 // case the limiter is on but hasn't been checked in a while and
818 // so may have left sufficient headroom to turn off again.
822 if gcCPULimiter.needUpdate(now) {
823 gcCPULimiter.update(now)
826 if !gcMarkWorkAvailable(pp) {
827 // No work to be done right now. This can happen at
828 // the end of the mark phase when there are still
829 // assists tapering off. Don't bother running a worker
830 // now because it'll just return immediately.
834 // Grab a worker before we commit to running below.
835 node := (*gcBgMarkWorkerNode)(gcBgMarkWorkerPool.pop())
837 // There is at least one worker per P, so normally there are
838 // enough workers to run on all Ps, if necessary. However, once
839 // a worker enters gcMarkDone it may park without rejoining the
840 // pool, thus freeing a P with no corresponding worker.
841 // gcMarkDone never depends on another worker doing work, so it
842 // is safe to simply do nothing here.
844 // If gcMarkDone bails out without completing the mark phase,
845 // it will always do so with queued global work. Thus, that P
846 // will be immediately eligible to re-run the worker G it was
847 // just using, ensuring work can complete.
851 decIfPositive := func(ptr *int64) bool {
853 v := atomic.Loadint64(ptr)
858 if atomic.Casint64(ptr, v, v-1) {
864 if decIfPositive(&c.dedicatedMarkWorkersNeeded) {
865 // This P is now dedicated to marking until the end of
866 // the concurrent mark phase.
867 pp.gcMarkWorkerMode = gcMarkWorkerDedicatedMode
868 } else if c.fractionalUtilizationGoal == 0 {
869 // No need for fractional workers.
870 gcBgMarkWorkerPool.push(&node.node)
873 // Is this P behind on the fractional utilization
876 // This should be kept in sync with pollFractionalWorkerExit.
877 delta := now - c.markStartTime
878 if delta > 0 && float64(pp.gcFractionalMarkTime)/float64(delta) > c.fractionalUtilizationGoal {
879 // Nope. No need to run a fractional worker.
880 gcBgMarkWorkerPool.push(&node.node)
883 // Run a fractional worker.
884 pp.gcMarkWorkerMode = gcMarkWorkerFractionalMode
887 // Run the background mark worker.
889 casgstatus(gp, _Gwaiting, _Grunnable)
896 // resetLive sets up the controller state for the next mark phase after the end
897 // of the previous one. Must be called after endCycle and before commit, before
898 // the world is started.
900 // The world must be stopped.
901 func (c *gcControllerState) resetLive(bytesMarked uint64) {
902 c.heapMarked = bytesMarked
903 c.heapLive = bytesMarked
904 c.heapScan = uint64(c.heapScanWork.Load())
905 c.lastHeapScan = uint64(c.heapScanWork.Load())
906 c.lastStackScan = uint64(c.stackScanWork.Load())
907 c.triggered = ^uint64(0) // Reset triggered.
909 // heapLive was updated, so emit a trace event.
915 // markWorkerStop must be called whenever a mark worker stops executing.
917 // It updates mark work accounting in the controller by a duration of
918 // work in nanoseconds and other bookkeeping.
920 // Safe to execute at any time.
921 func (c *gcControllerState) markWorkerStop(mode gcMarkWorkerMode, duration int64) {
923 case gcMarkWorkerDedicatedMode:
924 atomic.Xaddint64(&c.dedicatedMarkTime, duration)
925 atomic.Xaddint64(&c.dedicatedMarkWorkersNeeded, 1)
926 case gcMarkWorkerFractionalMode:
927 atomic.Xaddint64(&c.fractionalMarkTime, duration)
928 case gcMarkWorkerIdleMode:
929 atomic.Xaddint64(&c.idleMarkTime, duration)
930 c.removeIdleMarkWorker()
932 throw("markWorkerStop: unknown mark worker mode")
936 func (c *gcControllerState) update(dHeapLive, dHeapScan int64) {
938 atomic.Xadd64(&gcController.heapLive, dHeapLive)
940 // gcController.heapLive changed.
944 if gcBlackenEnabled == 0 {
945 // Update heapScan when we're not in a current GC. It is fixed
946 // at the beginning of a cycle.
948 atomic.Xadd64(&gcController.heapScan, dHeapScan)
951 // gcController.heapLive changed.
956 func (c *gcControllerState) addScannableStack(pp *p, amount int64) {
958 atomic.Xadd64(&c.maxStackScan, amount)
961 pp.maxStackScanDelta += amount
962 if pp.maxStackScanDelta >= maxStackScanSlack || pp.maxStackScanDelta <= -maxStackScanSlack {
963 atomic.Xadd64(&c.maxStackScan, pp.maxStackScanDelta)
964 pp.maxStackScanDelta = 0
968 func (c *gcControllerState) addGlobals(amount int64) {
969 atomic.Xadd64(&c.globalsScan, amount)
972 // heapGoal returns the current heap goal.
973 func (c *gcControllerState) heapGoal() uint64 {
974 goal, _ := c.heapGoalInternal()
978 // heapGoalInternal is the implementation of heapGoal which returns additional
979 // information that is necessary for computing the trigger.
981 // The returned minTrigger is always <= goal.
982 func (c *gcControllerState) heapGoalInternal() (goal, minTrigger uint64) {
983 // Start with the goal calculated for gcPercent.
984 goal = c.gcPercentHeapGoal.Load()
986 // Check if the memory-limit-based goal is smaller, and if so, pick that.
987 if newGoal := c.memoryLimitHeapGoal(); go119MemoryLimitSupport && newGoal < goal {
990 // We're not limited by the memory limit goal, so perform a series of
991 // adjustments that might move the goal forward in a variety of circumstances.
993 sweepDistTrigger := c.sweepDistMinTrigger.Load()
994 if sweepDistTrigger > goal {
995 // Set the goal to maintain a minimum sweep distance since
996 // the last call to commit. Note that we never want to do this
997 // if we're in the memory limit regime, because it could push
999 goal = sweepDistTrigger
1001 // Since we ignore the sweep distance trigger in the memory
1002 // limit regime, we need to ensure we don't propagate it to
1003 // the trigger, because it could cause a violation of the
1004 // invariant that the trigger < goal.
1005 minTrigger = sweepDistTrigger
1007 // Ensure that the heap goal is at least a little larger than
1008 // the point at which we triggered. This may not be the case if GC
1009 // start is delayed or if the allocation that pushed gcController.heapLive
1010 // over trigger is large or if the trigger is really close to
1011 // GOGC. Assist is proportional to this distance, so enforce a
1012 // minimum distance, even if it means going over the GOGC goal
1015 // Ignore this if we're in the memory limit regime: we'd prefer to
1016 // have the GC respond hard about how close we are to the goal than to
1017 // push the goal back in such a manner that it could cause us to exceed
1018 // the memory limit.
1019 const minRunway = 64 << 10
1020 if c.triggered != ^uint64(0) && goal < c.triggered+minRunway {
1021 goal = c.triggered + minRunway
1027 // memoryLimitHeapGoal returns a heap goal derived from memoryLimit.
1028 func (c *gcControllerState) memoryLimitHeapGoal() uint64 {
1029 // Start by pulling out some values we'll need. Be careful about overflow.
1030 var heapFree, heapAlloc, mappedReady uint64
1032 heapFree = c.heapFree.load() // Free and unscavenged memory.
1033 heapAlloc = c.totalAlloc.Load() - c.totalFree.Load() // Heap object bytes in use.
1034 mappedReady = c.mappedReady.Load() // Total unreleased mapped memory.
1035 if heapFree+heapAlloc <= mappedReady {
1038 // It is impossible for total unreleased mapped memory to exceed heap memory, but
1039 // because these stats are updated independently, we may observe a partial update
1040 // including only some values. Thus, we appear to break the invariant. However,
1041 // this condition is necessarily transient, so just try again. In the case of a
1042 // persistent accounting error, we'll deadlock here.
1045 // Below we compute a goal from memoryLimit. There are a few things to be aware of.
1046 // Firstly, the memoryLimit does not easily compare to the heap goal: the former
1047 // is total mapped memory by the runtime that hasn't been released, while the latter is
1048 // only heap object memory. Intuitively, the way we convert from one to the other is to
1049 // subtract everything from memoryLimit that both contributes to the memory limit (so,
1050 // ignore scavenged memory) and doesn't contain heap objects. This isn't quite what
1051 // lines up with reality, but it's a good starting point.
1053 // In practice this computation looks like the following:
1055 // memoryLimit - ((mappedReady - heapFree - heapAlloc) + max(mappedReady - memoryLimit, 0)) - memoryLimitHeapGoalHeadroom
1058 // Let's break this down.
1060 // The first term (marker 1) is everything that contributes to the memory limit and isn't
1061 // or couldn't become heap objects. It represents, broadly speaking, non-heap overheads.
1062 // One oddity you may have noticed is that we also subtract out heapFree, i.e. unscavenged
1063 // memory that may contain heap objects in the future.
1065 // Let's take a step back. In an ideal world, this term would look something like just
1066 // the heap goal. That is, we "reserve" enough space for the heap to grow to the heap
1067 // goal, and subtract out everything else. This is of course impossible; the definition
1068 // is circular! However, this impossible definition contains a key insight: the amount
1069 // we're *going* to use matters just as much as whatever we're currently using.
1071 // Consider if the heap shrinks to 1/10th its size, leaving behind lots of free and
1072 // unscavenged memory. mappedReady - heapAlloc will be quite large, because of that free
1073 // and unscavenged memory, pushing the goal down significantly.
1075 // heapFree is also safe to exclude from the memory limit because in the steady-state, it's
1076 // just a pool of memory for future heap allocations, and making new allocations from heapFree
1077 // memory doesn't increase overall memory use. In transient states, the scavenger and the
1078 // allocator actively manage the pool of heapFree memory to maintain the memory limit.
1080 // The second term (marker 2) is the amount of memory we've exceeded the limit by, and is
1081 // intended to help recover from such a situation. By pushing the heap goal down, we also
1082 // push the trigger down, triggering and finishing a GC sooner in order to make room for
1083 // other memory sources. Note that since we're effectively reducing the heap goal by X bytes,
1084 // we're actually giving more than X bytes of headroom back, because the heap goal is in
1085 // terms of heap objects, but it takes more than X bytes (e.g. due to fragmentation) to store
1086 // X bytes worth of objects.
1088 // The third term (marker 3) subtracts an additional memoryLimitHeapGoalHeadroom bytes from the
1089 // heap goal. As the name implies, this is to provide additional headroom in the face of pacing
1090 // inaccuracies. This is a fixed number of bytes because these inaccuracies disproportionately
1091 // affect small heaps: as heaps get smaller, the pacer's inputs get fuzzier. Shorter GC cycles
1092 // and less GC work means noisy external factors like the OS scheduler have a greater impact.
1094 memoryLimit := uint64(c.memoryLimit.Load())
1097 nonHeapMemory := mappedReady - heapFree - heapAlloc
1101 if mappedReady > memoryLimit {
1102 overage = mappedReady - memoryLimit
1105 if nonHeapMemory+overage >= memoryLimit {
1106 // We're at a point where non-heap memory exceeds the memory limit on its own.
1107 // There's honestly not much we can do here but just trigger GCs continuously
1108 // and let the CPU limiter reign that in. Something has to give at this point.
1109 // Set it to heapMarked, the lowest possible goal.
1113 // Compute the goal.
1114 goal := memoryLimit - (nonHeapMemory + overage)
1116 // Apply some headroom to the goal to account for pacing inaccuracies.
1117 // Be careful about small limits.
1118 if goal < memoryLimitHeapGoalHeadroom || goal-memoryLimitHeapGoalHeadroom < memoryLimitHeapGoalHeadroom {
1119 goal = memoryLimitHeapGoalHeadroom
1121 goal = goal - memoryLimitHeapGoalHeadroom
1123 // Don't let us go below the live heap. A heap goal below the live heap doesn't make sense.
1124 if goal < c.heapMarked {
1131 // These constants determine the bounds on the GC trigger as a fraction
1132 // of heap bytes allocated between the start of a GC (heapLive == heapMarked)
1133 // and the end of a GC (heapLive == heapGoal).
1135 // The constants are obscured in this way for efficiency. The denominator
1136 // of the fraction is always a power-of-two for a quick division, so that
1137 // the numerator is a single constant integer multiplication.
1138 triggerRatioDen = 64
1140 // The minimum trigger constant was chosen empirically: given a sufficiently
1141 // fast/scalable allocator with 48 Ps that could drive the trigger ratio
1142 // to <0.05, this constant causes applications to retain the same peak
1143 // RSS compared to not having this allocator.
1144 minTriggerRatioNum = 45 // ~0.7
1146 // The maximum trigger constant is chosen somewhat arbitrarily, but the
1147 // current constant has served us well over the years.
1148 maxTriggerRatioNum = 61 // ~0.95
1151 // trigger returns the current point at which a GC should trigger along with
1154 // The returned value may be compared against heapLive to determine whether
1155 // the GC should trigger. Thus, the GC trigger condition should be (but may
1156 // not be, in the case of small movements for efficiency) checked whenever
1157 // the heap goal may change.
1158 func (c *gcControllerState) trigger() (uint64, uint64) {
1159 goal, minTrigger := c.heapGoalInternal()
1161 // Invariant: the trigger must always be less than the heap goal.
1163 // Note that the memory limit sets a hard maximum on our heap goal,
1164 // but the live heap may grow beyond it.
1166 if c.heapMarked >= goal {
1167 // The goal should never be smaller than heapMarked, but let's be
1168 // defensive about it. The only reasonable trigger here is one that
1169 // causes a continuous GC cycle at heapMarked, but respect the goal
1170 // if it came out as smaller than that.
1174 // Below this point, c.heapMarked < goal.
1176 // heapMarked is our absolute minimum, and it's possible the trigger
1177 // bound we get from heapGoalinternal is less than that.
1178 if minTrigger < c.heapMarked {
1179 minTrigger = c.heapMarked
1182 // If we let the trigger go too low, then if the application
1183 // is allocating very rapidly we might end up in a situation
1184 // where we're allocating black during a nearly always-on GC.
1185 // The result of this is a growing heap and ultimately an
1186 // increase in RSS. By capping us at a point >0, we're essentially
1187 // saying that we're OK using more CPU during the GC to prevent
1188 // this growth in RSS.
1189 triggerLowerBound := uint64(((goal-c.heapMarked)/triggerRatioDen)*minTriggerRatioNum) + c.heapMarked
1190 if minTrigger < triggerLowerBound {
1191 minTrigger = triggerLowerBound
1194 // For small heaps, set the max trigger point at maxTriggerRatio of the way
1195 // from the live heap to the heap goal. This ensures we always have *some*
1196 // headroom when the GC actually starts. For larger heaps, set the max trigger
1197 // point at the goal, minus the minimum heap size.
1199 // This choice follows from the fact that the minimum heap size is chosen
1200 // to reflect the costs of a GC with no work to do. With a large heap but
1201 // very little scan work to perform, this gives us exactly as much runway
1202 // as we would need, in the worst case.
1203 maxTrigger := uint64(((goal-c.heapMarked)/triggerRatioDen)*maxTriggerRatioNum) + c.heapMarked
1204 if goal > defaultHeapMinimum && goal-defaultHeapMinimum > maxTrigger {
1205 maxTrigger = goal - defaultHeapMinimum
1207 if maxTrigger < minTrigger {
1208 maxTrigger = minTrigger
1211 // Compute the trigger from our bounds and the runway stored by commit.
1213 runway := c.runway.Load()
1215 trigger = minTrigger
1217 trigger = goal - runway
1219 if trigger < minTrigger {
1220 trigger = minTrigger
1222 if trigger > maxTrigger {
1223 trigger = maxTrigger
1226 print("trigger=", trigger, " heapGoal=", goal, "\n")
1227 print("minTrigger=", minTrigger, " maxTrigger=", maxTrigger, "\n")
1228 throw("produced a trigger greater than the heap goal")
1230 return trigger, goal
1233 // commit recomputes all pacing parameters needed to derive the
1234 // trigger and the heap goal. Namely, the gcPercent-based heap goal,
1235 // and the amount of runway we want to give the GC this cycle.
1237 // This can be called any time. If GC is the in the middle of a
1238 // concurrent phase, it will adjust the pacing of that phase.
1240 // isSweepDone should be the result of calling isSweepDone(),
1241 // unless we're testing or we know we're executing during a GC cycle.
1243 // This depends on gcPercent, gcController.heapMarked, and
1244 // gcController.heapLive. These must be up to date.
1246 // Callers must call gcControllerState.revise after calling this
1247 // function if the GC is enabled.
1249 // mheap_.lock must be held or the world must be stopped.
1250 func (c *gcControllerState) commit(isSweepDone bool) {
1252 assertWorldStoppedOrLockHeld(&mheap_.lock)
1256 // The sweep is done, so there aren't any restrictions on the trigger
1257 // we need to think about.
1258 c.sweepDistMinTrigger.Store(0)
1260 // Concurrent sweep happens in the heap growth
1261 // from gcController.heapLive to trigger. Make sure we
1262 // give the sweeper some runway if it doesn't have enough.
1263 c.sweepDistMinTrigger.Store(atomic.Load64(&c.heapLive) + sweepMinHeapDistance)
1266 // Compute the next GC goal, which is when the allocated heap
1267 // has grown by GOGC/100 over where it started the last cycle,
1268 // plus additional runway for non-heap sources of GC work.
1269 gcPercentHeapGoal := ^uint64(0)
1270 if gcPercent := c.gcPercent.Load(); gcPercent >= 0 {
1271 gcPercentHeapGoal = c.heapMarked + (c.heapMarked+atomic.Load64(&c.lastStackScan)+atomic.Load64(&c.globalsScan))*uint64(gcPercent)/100
1273 // Apply the minimum heap size here. It's defined in terms of gcPercent
1274 // and is only updated by functions that call commit.
1275 if gcPercentHeapGoal < c.heapMinimum {
1276 gcPercentHeapGoal = c.heapMinimum
1278 c.gcPercentHeapGoal.Store(gcPercentHeapGoal)
1280 // Compute the amount of runway we want the GC to have by using our
1281 // estimate of the cons/mark ratio.
1283 // The idea is to take our expected scan work, and multiply it by
1284 // the cons/mark ratio to determine how long it'll take to complete
1285 // that scan work in terms of bytes allocated. This gives us our GC's
1288 // However, the cons/mark ratio is a ratio of rates per CPU-second, but
1289 // here we care about the relative rates for some division of CPU
1290 // resources among the mutator and the GC.
1292 // To summarize, we have B / cpu-ns, and we want B / ns. We get that
1293 // by multiplying by our desired division of CPU resources. We choose
1294 // to express CPU resources as GOMAPROCS*fraction. Note that because
1295 // we're working with a ratio here, we can omit the number of CPU cores,
1296 // because they'll appear in the numerator and denominator and cancel out.
1297 // As a result, this is basically just "weighing" the cons/mark ratio by
1298 // our desired division of resources.
1300 // Furthermore, by setting the runway so that CPU resources are divided
1301 // this way, assuming that the cons/mark ratio is correct, we make that
1302 // division a reality.
1303 c.runway.Store(uint64((c.consMark * (1 - gcGoalUtilization) / (gcGoalUtilization)) * float64(c.lastHeapScan+c.lastStackScan+c.globalsScan)))
1306 // setGCPercent updates gcPercent. commit must be called after.
1307 // Returns the old value of gcPercent.
1309 // The world must be stopped, or mheap_.lock must be held.
1310 func (c *gcControllerState) setGCPercent(in int32) int32 {
1312 assertWorldStoppedOrLockHeld(&mheap_.lock)
1315 out := c.gcPercent.Load()
1319 c.heapMinimum = defaultHeapMinimum * uint64(in) / 100
1320 c.gcPercent.Store(in)
1325 //go:linkname setGCPercent runtime/debug.setGCPercent
1326 func setGCPercent(in int32) (out int32) {
1327 // Run on the system stack since we grab the heap lock.
1328 systemstack(func() {
1330 out = gcController.setGCPercent(in)
1331 gcControllerCommit()
1332 unlock(&mheap_.lock)
1335 // If we just disabled GC, wait for any concurrent GC mark to
1336 // finish so we always return with no GC running.
1338 gcWaitOnMark(atomic.Load(&work.cycles))
1344 func readGOGC() int32 {
1345 p := gogetenv("GOGC")
1349 if n, ok := atoi32(p); ok {
1355 // setMemoryLimit updates memoryLimit. commit must be called after
1356 // Returns the old value of memoryLimit.
1358 // The world must be stopped, or mheap_.lock must be held.
1359 func (c *gcControllerState) setMemoryLimit(in int64) int64 {
1361 assertWorldStoppedOrLockHeld(&mheap_.lock)
1364 out := c.memoryLimit.Load()
1366 c.memoryLimit.Store(in)
1372 //go:linkname setMemoryLimit runtime/debug.setMemoryLimit
1373 func setMemoryLimit(in int64) (out int64) {
1374 // Run on the system stack since we grab the heap lock.
1375 systemstack(func() {
1377 out = gcController.setMemoryLimit(in)
1378 if in < 0 || out == in {
1379 // If we're just checking the value or not changing
1380 // it, there's no point in doing the rest.
1381 unlock(&mheap_.lock)
1384 gcControllerCommit()
1385 unlock(&mheap_.lock)
1390 func readGOMEMLIMIT() int64 {
1391 p := gogetenv("GOMEMLIMIT")
1392 if p == "" || p == "off" {
1395 n, ok := parseByteCount(p)
1397 print("GOMEMLIMIT=", p, "\n")
1398 throw("malformed GOMEMLIMIT; see `go doc runtime/debug.SetMemoryLimit`")
1403 type piController struct {
1404 kp float64 // Proportional constant.
1405 ti float64 // Integral time constant.
1406 tt float64 // Reset time.
1408 min, max float64 // Output boundaries.
1410 // PI controller state.
1412 errIntegral float64 // Integral of the error from t=0 to now.
1415 errOverflow bool // Set if errIntegral ever overflowed.
1416 inputOverflow bool // Set if an operation with the input overflowed.
1419 // next provides a new sample to the controller.
1421 // input is the sample, setpoint is the desired point, and period is how much
1422 // time (in whatever unit makes the most sense) has passed since the last sample.
1424 // Returns a new value for the variable it's controlling, and whether the operation
1425 // completed successfully. One reason this might fail is if error has been growing
1426 // in an unbounded manner, to the point of overflow.
1428 // In the specific case of an error overflow occurs, the errOverflow field will be
1429 // set and the rest of the controller's internal state will be fully reset.
1430 func (c *piController) next(input, setpoint, period float64) (float64, bool) {
1431 // Compute the raw output value.
1432 prop := c.kp * (setpoint - input)
1433 rawOutput := prop + c.errIntegral
1435 // Clamp rawOutput into output.
1437 if isInf(output) || isNaN(output) {
1438 // The input had a large enough magnitude that either it was already
1439 // overflowed, or some operation with it overflowed.
1440 // Set a flag and reset. That's the safest thing to do.
1442 c.inputOverflow = true
1447 } else if output > c.max {
1451 // Update the controller's state.
1452 if c.ti != 0 && c.tt != 0 {
1453 c.errIntegral += (c.kp*period/c.ti)*(setpoint-input) + (period/c.tt)*(output-rawOutput)
1454 if isInf(c.errIntegral) || isNaN(c.errIntegral) {
1455 // So much error has accumulated that we managed to overflow.
1456 // The assumptions around the controller have likely broken down.
1457 // Set a flag and reset. That's the safest thing to do.
1459 c.errOverflow = true
1466 // reset resets the controller state, except for controller error flags.
1467 func (c *piController) reset() {
1471 // addIdleMarkWorker attempts to add a new idle mark worker.
1473 // If this returns true, the caller must become an idle mark worker unless
1474 // there's no background mark worker goroutines in the pool. This case is
1475 // harmless because there are already background mark workers running.
1476 // If this returns false, the caller must NOT become an idle mark worker.
1478 // nosplit because it may be called without a P.
1481 func (c *gcControllerState) addIdleMarkWorker() bool {
1483 old := c.idleMarkWorkers.Load()
1484 n, max := int32(old&uint64(^uint32(0))), int32(old>>32)
1486 // See the comment on idleMarkWorkers for why
1487 // n > max is tolerated.
1491 print("n=", n, " max=", max, "\n")
1492 throw("negative idle mark workers")
1494 new := uint64(uint32(n+1)) | (uint64(max) << 32)
1495 if c.idleMarkWorkers.CompareAndSwap(old, new) {
1501 // needIdleMarkWorker is a hint as to whether another idle mark worker is needed.
1503 // The caller must still call addIdleMarkWorker to become one. This is mainly
1504 // useful for a quick check before an expensive operation.
1506 // nosplit because it may be called without a P.
1509 func (c *gcControllerState) needIdleMarkWorker() bool {
1510 p := c.idleMarkWorkers.Load()
1511 n, max := int32(p&uint64(^uint32(0))), int32(p>>32)
1515 // removeIdleMarkWorker must be called when an new idle mark worker stops executing.
1516 func (c *gcControllerState) removeIdleMarkWorker() {
1518 old := c.idleMarkWorkers.Load()
1519 n, max := int32(old&uint64(^uint32(0))), int32(old>>32)
1521 print("n=", n, " max=", max, "\n")
1522 throw("negative idle mark workers")
1524 new := uint64(uint32(n-1)) | (uint64(max) << 32)
1525 if c.idleMarkWorkers.CompareAndSwap(old, new) {
1531 // setMaxIdleMarkWorkers sets the maximum number of idle mark workers allowed.
1533 // This method is optimistic in that it does not wait for the number of
1534 // idle mark workers to reduce to max before returning; it assumes the workers
1535 // will deschedule themselves.
1536 func (c *gcControllerState) setMaxIdleMarkWorkers(max int32) {
1538 old := c.idleMarkWorkers.Load()
1539 n := int32(old & uint64(^uint32(0)))
1541 print("n=", n, " max=", max, "\n")
1542 throw("negative idle mark workers")
1544 new := uint64(uint32(n)) | (uint64(max) << 32)
1545 if c.idleMarkWorkers.CompareAndSwap(old, new) {
1551 // gcControllerCommit is gcController.commit, but passes arguments from live
1552 // (non-test) data. It also updates any consumers of the GC pacing, such as
1553 // sweep pacing and the background scavenger.
1555 // Calls gcController.commit.
1557 // The heap lock must be held, so this must be executed on the system stack.
1560 func gcControllerCommit() {
1561 assertWorldStoppedOrLockHeld(&mheap_.lock)
1563 gcController.commit(isSweepDone())
1565 // Update mark pacing.
1566 if gcphase != _GCoff {
1567 gcController.revise()
1570 // TODO(mknyszek): This isn't really accurate any longer because the heap
1571 // goal is computed dynamically. Still useful to snapshot, but not as useful.
1576 trigger, heapGoal := gcController.trigger()
1577 gcPaceSweeper(trigger)
1578 gcPaceScavenger(gcController.memoryLimit.Load(), heapGoal, gcController.lastHeapGoal)