golang mgcpacer 代码
文件路径：/src/runtime/mgcpacer.go
/ Copyright 2021 The Go Authors. All rights reserved.
/ Use of this source code is governed by a BSD-style
/ license that can be found in the LICENSE file.
package runtime
import (
"internal/cpu"
"internal/goexperiment"
"runtime/internal/atomic"
"unsafe"
)
/ go119MemoryLimitSupport is a feature flag for a number of changes
/ related to the memory limit feature (#48409). Disabling this flag
/ disables those features, as well as the memory limit mechanism,
/ which becomes a no-op.
const go119MemoryLimitSupport = true
const (
/ gcGoalUtilization is the goal CPU utilization for
/ marking as a fraction of GOMAXPROCS.
/
/ Increasing the goal utilization will shorten GC cycles as the GC
/ has more resources behind it, lessening costs from the write barrier,
/ but comes at the cost of increasing mutator latency.
gcGoalUtilization = gcBackgroundUtilization
/ gcBackgroundUtilization is the fixed CPU utilization for background
/ marking. It must be <= gcGoalUtilization. The difference between
/ gcGoalUtilization and gcBackgroundUtilization will be made up by
/ mark assists. The scheduler will aim to use within 50% of this
/ goal.
/
/ As a general rule, there's little reason to set gcBackgroundUtilization
/ < gcGoalUtilization. One reason might be in mostly idle applications,
/ where goroutines are unlikely to assist at all, so the actual
/ utilization will be lower than the goal. But this is moot point
/ because the idle mark workers already soak up idle CPU resources.
/ These two values are still kept separate however because they are
/ distinct conceptually, and in previous iterations of the pacer the
/ distinction was more important.
gcBackgroundUtilization = 0.25
/ gcCreditSlack is the amount of scan work credit that can
/ accumulate locally before updating gcController.heapScanWork and,
/ optionally, gcController.bgScanCredit. Lower values give a more
/ accurate assist ratio and make it more likely that assists will
/ successfully steal background credit. Higher values reduce memory
/ contention.
gcCreditSlack = 2000
/ gcAssistTimeSlack is the nanoseconds of mutator assist time that
/ can accumulate on a P before updating gcController.assistTime.
gcAssistTimeSlack = 5000
/ gcOverAssistWork determines how many extra units of scan work a GC
/ assist does when an assist happens. This amortizes the cost of an
/ assist by pre-paying for this many bytes of future allocations.
gcOverAssistWork = 64 << 10
/ defaultHeapMinimum is the value of heapMinimum for GOGC==100.
defaultHeapMinimum = (goexperiment.HeapMinimum512KiBInt)*(512<<10) +
(1-goexperiment.HeapMinimum512KiBInt)*(4<<20)
/ maxStackScanSlack is the bytes of stack space allocated or freed
/ that can accumulate on a P before updating gcController.stackSize.
maxStackScanSlack = 8 << 10
/ memoryLimitHeapGoalHeadroom is the amount of headroom the pacer gives to
/ the heap goal when operating in the memory-limited regime. That is,
/ it'll reduce the heap goal by this many extra bytes off of the base
/ calculation.
memoryLimitHeapGoalHeadroom = 1 << 20
)
func init() {
if offset := unsafe.Offsetof(gcController.heapLive); offset%8 != 0 {
println(offset)
throw("gcController.heapLive not aligned to 8 bytes")
}
}
/ gcController implements the GC pacing controller that determines
/ when to trigger concurrent garbage collection and how much marking
/ work to do in mutator assists and background marking.
/
/ It calculates the ratio between the allocation rate (in terms of CPU
/ time) and the GC scan throughput to determine the heap size at which to
/ trigger a GC cycle such that no GC assists are required to finish on time.
/ This algorithm thus optimizes GC CPU utilization to the dedicated background
/ mark utilization of 25% of GOMAXPROCS by minimizing GC assists.
/ GOMAXPROCS. The high-level design of this algorithm is documented
/ at https://github.com/golang/proposal/blob/master/design/44167-gc-pacer-redesign.md.
/ See https://golang.org/s/go15gcpacing for additional historical context.
var gcController gcControllerState
type gcControllerState struct {
/ Initialized from GOGC. GOGC=off means no GC.
gcPercent atomic.Int32
_ uint32 / padding so following 64-bit values are 8-byte aligned
/ memoryLimit is the soft memory limit in bytes.
/
/ Initialized from GOMEMLIMIT. GOMEMLIMIT=off is equivalent to MaxInt64
/ which means no soft memory limit in practice.
/
/ This is an int64 instead of a uint64 to more easily maintain parity with
/ the SetMemoryLimit API, which sets a maximum at MaxInt64. This value
/ should never be negative.
memoryLimit atomic.Int64
/ heapMinimum is the minimum heap size at which to trigger GC.
/ For small heaps, this overrides the usual GOGC*live set rule.
/
/ When there is a very small live set but a lot of allocation, simply
/ collecting when the heap reaches GOGC*live results in many GC
/ cycles and high total per-GC overhead. This minimum amortizes this
/ per-GC overhead while keeping the heap reasonably small.
/
/ During initialization this is set to 4MB*GOGC/100. In the case of
/ GOGC==0, this will set heapMinimum to 0, resulting in constant
/ collection even when the heap size is small, which is useful for
/ debugging.
heapMinimum uint64
/ runway is the amount of runway in heap bytes allocated by the
/ application that we want to give the GC once it starts.
/
/ This is computed from consMark during mark termination.
runway atomic.Uint64
/ consMark is the estimated per-CPU consMark ratio for the application.
/
/ It represents the ratio between the application's allocation
/ rate, as bytes allocated per CPU-time, and the GC's scan rate,
/ as bytes scanned per CPU-time.
/ The units of this ratio are (B / cpu-ns) / (B / cpu-ns).
/
/ At a high level, this value is computed as the bytes of memory
/ allocated (cons) per unit of scan work completed (mark) in a GC
/ cycle, divided by the CPU time spent on each activity.
/
/ Updated at the end of each GC cycle, in endCycle.
consMark float64
/ consMarkController holds the state for the mark-cons ratio
/ estimation over time.
/
/ Its purpose is to smooth out noisiness in the computation of
/ consMark; see consMark for details.
consMarkController piController
_ uint32 / Padding for atomics on 32-bit platforms.
/ gcPercentHeapGoal is the goal heapLive for when next GC ends derived
/ from gcPercent.
/
/ Set to ^uint64(0) if gcPercent is disabled.
gcPercentHeapGoal atomic.Uint64
/ sweepDistMinTrigger is the minimum trigger to ensure a minimum
/ sweep distance.
/
/ This bound is also special because it applies to both the trigger
/ *and* the goal (all other trigger bounds must be based *on* the goal).
/
/ It is computed ahead of time, at commit time. The theory is that,
/ absent a sudden change to a parameter like gcPercent, the trigger
/ will be chosen to always give the sweeper enough headroom. However,
/ such a change might dramatically and suddenly move up the trigger,
/ in which case we need to ensure the sweeper still has enough headroom.
sweepDistMinTrigger atomic.Uint64
/ triggered is the point at which the current GC cycle actually triggered.
/ Only valid during the mark phase of a GC cycle, otherwise set to ^uint64(0).
/
/ Updated while the world is stopped.
triggered uint64
/ lastHeapGoal is the value of heapGoal at the moment the last GC
/ ended. Note that this is distinct from the last value heapGoal had,
/ because it could change if e.g. gcPercent changes.
/
/ Read and written with the world stopped or with mheap_.lock held.
lastHeapGoal uint64
/ heapLive is the number of bytes considered live by the GC.
/ That is: retained by the most recent GC plus allocated
/ since then. heapLive ≤ memstats.totalAlloc-memstats.totalFree, since
/ heapAlloc includes unmarked objects that have not yet been swept (and
/ hence goes up as we allocate and down as we sweep) while heapLive
/ excludes these objects (and hence only goes up between GCs).
/
/ This is updated atomically without locking. To reduce
/ contention, this is updated only when obtaining a span from
/ an mcentral and at this point it counts all of the
/ unallocated slots in that span (which will be allocated
/ before that mcache obtains another span from that
/ mcentral). Hence, it slightly overestimates the "true" live
/ heap size. It's better to overestimate than to
/ underestimate because 1) this triggers the GC earlier than
/ necessary rather than potentially too late and 2) this
/ leads to a conservative GC rate rather than a GC rate that
/ is potentially too low.
/
/ Reads should likewise be atomic (or during STW).
/
/ Whenever this is updated, call traceHeapAlloc() and
/ this gcControllerState's revise() method.
heapLive uint64
/ heapScan is the number of bytes of "scannable" heap. This
/ is the live heap (as counted by heapLive), but omitting
/ no-scan objects and no-scan tails of objects.
/
/ This value is fixed at the start of a GC cycle, so during a
/ GC cycle it is safe to read without atomics, and it represents
/ the maximum scannable heap.
heapScan uint64
/ lastHeapScan is the number of bytes of heap that were scanned
/ last GC cycle. It is the same as heapMarked, but only
/ includes the "scannable" parts of objects.
/
/ Updated when the world is stopped.
lastHeapScan uint64
/ lastStackScan is the number of bytes of stack that were scanned
/ last GC cycle.
lastStackScan uint64
/ maxStackScan is the amount of allocated goroutine stack space in
/ use by goroutines.
/
/ This number tracks allocated goroutine stack space rather than used
/ goroutine stack space (i.e. what is actually scanned) because used
/ goroutine stack space is much harder to measure cheaply. By using
/ allocated space, we make an overestimate; this is OK, it's better
/ to conservatively overcount than undercount.
/
/ Read and updated atomically.
maxStackScan uint64
/ globalsScan is the total amount of global variable space
/ that is scannable.
/
/ Read and updated atomically.
globalsScan uint64
/ heapMarked is the number of bytes marked by the previous
/ GC. After mark termination, heapLive == heapMarked, but
/ unlike heapLive, heapMarked does not change until the
/ next mark termination.
heapMarked uint64
/ heapScanWork is the total heap scan work performed this cycle.
/ stackScanWork is the total stack scan work performed this cycle.
/ globalsScanWork is the total globals scan work performed this cycle.
/
/ These are updated atomically during the cycle. Updates occur in
/ bounded batches, since they are both written and read
/ throughout the cycle. At the end of the cycle, heapScanWork is how
/ much of the retained heap is scannable.
/
/ Currently these are measured in bytes. For most uses, this is an
/ opaque unit of work, but for estimation the definition is important.
/
/ Note that stackScanWork includes only stack space scanned, not all
/ of the allocated stack.
heapScanWork atomic.Int64
stackScanWork atomic.Int64
globalsScanWork atomic.Int64
/ bgScanCredit is the scan work credit accumulated by the
/ concurrent background scan. This credit is accumulated by
/ the background scan and stolen by mutator assists. This is
/ updated atomically. Updates occur in bounded batches, since
/ it is both written and read throughout the cycle.
bgScanCredit int64
/ assistTime is the nanoseconds spent in mutator assists
/ during this cycle. This is updated atomically, and must also
/ be updated atomically even during a STW, because it is read
/ by sysmon. Updates occur in bounded batches, since it is both
/ written and read throughout the cycle.
assistTime atomic.Int64
/ dedicatedMarkTime is the nanoseconds spent in dedicated
/ mark workers during this cycle. This is updated atomically
/ at the end of the concurrent mark phase.
dedicatedMarkTime int64
/ fractionalMarkTime is the nanoseconds spent in the
/ fractional mark worker during this cycle. This is updated
/ atomically throughout the cycle and will be up-to-date if
/ the fractional mark worker is not currently running.
fractionalMarkTime int64
/ idleMarkTime is the nanoseconds spent in idle marking
/ during this cycle. This is updated atomically throughout
/ the cycle.
idleMarkTime int64
/ markStartTime is the absolute start time in nanoseconds
/ that assists and background mark workers started.
markStartTime int64
/ dedicatedMarkWorkersNeeded is the number of dedicated mark
/ workers that need to be started. This is computed at the
/ beginning of each cycle and decremented atomically as
/ dedicated mark workers get started.
dedicatedMarkWorkersNeeded int64
/ idleMarkWorkers is two packed int32 values in a single uint64.
/ These two values are always updated simultaneously.
/
/ The bottom int32 is the current number of idle mark workers executing.
/
/ The top int32 is the maximum number of idle mark workers allowed to
/ execute concurrently. Normally, this number is just gomaxprocs. However,
/ during periodic GC cycles it is set to 0 because the system is idle
/ anyway; there's no need to go full blast on all of GOMAXPROCS.
/
/ The maximum number of idle mark workers is used to prevent new workers
/ from starting, but it is not a hard maximum. It is possible (but
/ exceedingly rare) for the current number of idle mark workers to
/ transiently exceed the maximum. This could happen if the maximum changes
/ just after a GC ends, and an M with no P.
/
/ Note that if we have no dedicated mark workers, we set this value to
/ 1 in this case we only have fractional GC workers which aren't scheduled
/ strictly enough to ensure GC progress. As a result, idle-priority mark
/ workers are vital to GC progress in these situations.
/
/ For example, consider a situation in which goroutines block on the GC
/ (such as via runtime.GOMAXPROCS) and only fractional mark workers are
/ scheduled (e.g. GOMAXPROCS=1). Without idle-priority mark workers, the
/ last running M might skip scheduling a fractional mark worker if its
/ utilization goal is met, such that once it goes to sleep (because there's
/ nothing to do), there will be nothing else to spin up a new M for the
/ fractional worker in the future, stalling GC progress and causing a
/ deadlock. However, idle-priority workers will *always* run when there is
/ nothing left to do, ensuring the GC makes progress.
/
/ See github.com/golang/go/issues/44163 for more details.
idleMarkWorkers atomic.Uint64
/ assistWorkPerByte is the ratio of scan work to allocated
/ bytes that should be performed by mutator assists. This is
/ computed at the beginning of each cycle and updated every
/ time heapScan is updated.
assistWorkPerByte atomic.Float64
/ assistBytesPerWork is 1/assistWorkPerByte.
/
/ Note that because this is read and written independently
/ from assistWorkPerByte users may notice a skew between
/ the two values, and such a state should be safe.
assistBytesPerWork atomic.Float64
/ fractionalUtilizationGoal is the fraction of wall clock
/ time that should be spent in the fractional mark worker on
/ each P that isn't running a dedicated worker.
/
/ For example, if the utilization goal is 25% and there are
/ no dedicated workers, this will be 0.25. If the goal is
/ 25%, there is one dedicated worker, and GOMAXPROCS is 5,
/ this will be 0.05 to make up the missing 5%.
/
/ If this is zero, no fractional workers are needed.
fractionalUtilizationGoal float64
/ These memory stats are effectively duplicates of fields from
/ memstats.heapStats but are updated atomically or with the world
/ stopped and don't provide the same consistency guarantees.
/
/ Because the runtime is responsible for managing a memory limit, it's
/ useful to couple these stats more tightly to the gcController, which
/ is intimately connected to how that memory limit is maintained.
heapInUse sysMemStat / bytes in mSpanInUse spans
heapReleased sysMemStat / bytes released to the OS
heapFree sysMemStat / bytes not in any span, but not released to the OS
totalAlloc atomic.Uint64 / total bytes allocated
totalFree atomic.Uint64 / total bytes freed
mappedReady atomic.Uint64 / total virtual memory in the Ready state (see mem.go).
/ test indicates that this is a test-only copy of gcControllerState.
test bool
_ cpu.CacheLinePad
}
func (c *gcControllerState) init(gcPercent int32, memoryLimit int64) {
c.heapMinimum = defaultHeapMinimum
c.triggered = ^uint64(0)
c.consMarkController = piController{
/ Tuned first via the Ziegler-Nichols process in simulation,
/ then the integral time was manually tuned against real-world
/ applications to deal with noisiness in the measured cons/mark
/ ratio.
kp: 0.9,
ti: 4.0,
/ Set a high reset time in GC cycles.
/ This is inversely proportional to the rate at which we
/ accumulate error from clipping. By making this very high
/ we make the accumulation slow. In general, clipping is
/ OK in our situation, hence the choice.
/
/ Tune this if we get unintended effects from clipping for
/ a long time.
tt: 1000,
min: -1000,
max: 1000,
}
c.setGCPercent(gcPercent)
c.setMemoryLimit(memoryLimit)
c.commit(true) / No sweep phase in the first GC cycle.
/ N.B. Don't bother calling traceHeapGoal. Tracing is never enabled at
/ initialization time.
/ N.B. No need to call revise; there's no GC enabled during
/ initialization.
}
/ startCycle resets the GC controller's state and computes estimates
/ for a new GC cycle. The caller must hold worldsema and the world
/ must be stopped.
func (c *gcControllerState) startCycle(markStartTime int64, procs int, trigger gcTrigger) {
c.heapScanWork.Store(0)
c.stackScanWork.Store(0)
c.globalsScanWork.Store(0)
c.bgScanCredit = 0
c.assistTime.Store(0)
c.dedicatedMarkTime = 0
c.fractionalMarkTime = 0
c.idleMarkTime = 0
c.markStartTime = markStartTime
/ TODO(mknyszek): This is supposed to be the actual trigger point for the heap, but
/ causes regressions in memory use. The cause is that the PI controller used to smooth
/ the cons/mark ratio measurements tends to flail when using the less accurate precomputed
/ trigger for the cons/mark calculation, and this results in the controller being more
/ conservative about steady-states it tries to find in the future.
/
/ This conservatism is transient, but these transient states tend to matter for short-lived
/ programs, especially because the PI controller is overdamped, partially because it is
/ configured with a relatively large time constant.
/
/ Ultimately, I think this is just two mistakes piled on one another: the choice of a swingy
/ smoothing function that recalls a fairly long history (due to its overdamped time constant)
/ coupled with an inaccurate cons/mark calculation. It just so happens this works better
/ today, and it makes it harder to change things in the future.
/
/ This is described in #53738. Fix this for #53892 by changing back to the actual trigger
/ point and simplifying the smoothing function.
heapTrigger, heapGoal := c.trigger()
c.triggered = heapTrigger
/ Compute the background mark utilization goal. In general,
/ this may not come out exactly. We round the number of
/ dedicated workers so that the utilization is closest to
/ 25%. For small GOMAXPROCS, this would introduce too much
/ error, so we add fractional workers in that case.
totalUtilizationGoal := float64(procs) * gcBackgroundUtilization
c.dedicatedMarkWorkersNeeded = int64(totalUtilizationGoal + 0.5)
utilError := float64(c.dedicatedMarkWorkersNeeded)/totalUtilizationGoal - 1
const maxUtilError = 0.3
if utilError < -maxUtilError || utilError > maxUtilError {
/ Rounding put us more than 30% off our goal. With
/ gcBackgroundUtilization of 25%, this happens for
/ GOMAXPROCS<=3 or GOMAXPROCS=6. Enable fractional
/ workers to compensate.
if float64(c.dedicatedMarkWorkersNeeded) > totalUtilizationGoal {
/ Too many dedicated workers.
c.dedicatedMarkWorkersNeeded--
}
c.fractionalUtilizationGoal = (totalUtilizationGoal - float64(c.dedicatedMarkWorkersNeeded)) / float64(procs)
} else {
c.fractionalUtilizationGoal = 0
}
/ In STW mode, we just want dedicated workers.
if debug.gcstoptheworld > 0 {
c.dedicatedMarkWorkersNeeded = int64(procs)
c.fractionalUtilizationGoal = 0
}
/ Clear per-P state
for _, p := range allp {
p.gcAssistTime = 0
p.gcFractionalMarkTime = 0
}
if trigger.kind == gcTriggerTime {
/ During a periodic GC cycle, reduce the number of idle mark workers
/ required. However, we need at least one dedicated mark worker or
/ idle GC worker to ensure GC progress in some scenarios (see comment
/ on maxIdleMarkWorkers).
if c.dedicatedMarkWorkersNeeded > 0 {
c.setMaxIdleMarkWorkers(0)
} else {
/ TODO(mknyszek): The fundamental reason why we need this is because
/ we can't count on the fractional mark worker to get scheduled.
/ Fix that by ensuring it gets scheduled according to its quota even
/ if the rest of the application is idle.
c.setMaxIdleMarkWorkers(1)
}
} else {
/ N.B. gomaxprocs and dedicatedMarkWorkersNeeded is guaranteed not to
/ change during a GC cycle.
c.setMaxIdleMarkWorkers(int32(procs) - int32(c.dedicatedMarkWorkersNeeded))
}
/ Compute initial values for controls that are updated
/ throughout the cycle.
c.revise()
if debug.gcpacertrace > 0 {
assistRatio := c.assistWorkPerByte.Load()
print("pacer: assist ratio=", assistRatio,
" (scan ", gcController.heapScan>>20, " MB in ",
work.initialHeapLive>>20, "->",
heapGoal>>20, " MB)",
" workers=", c.dedicatedMarkWorkersNeeded,
"+", c.fractionalUtilizationGoal, "\n")
}
}
/ revise updates the assist ratio during the GC cycle to account for
/ improved estimates. This should be called whenever gcController.heapScan,
/ gcController.heapLive, or if any inputs to gcController.heapGoal are
/ updated. It is safe to call concurrently, but it may race with other
/ calls to revise.
/
/ The result of this race is that the two assist ratio values may not line
/ up or may be stale. In practice this is OK because the assist ratio
/ moves slowly throughout a GC cycle, and the assist ratio is a best-effort
/ heuristic anyway. Furthermore, no part of the heuristic depends on
/ the two assist ratio values being exact reciprocals of one another, since
/ the two values are used to convert values from different sources.
/
/ The worst case result of this raciness is that we may miss a larger shift
/ in the ratio (say, if we decide to pace more aggressively against the
/ hard heap goal) but even this "hard goal" is best-effort (see #40460).
/ The dedicated GC should ensure we don't exceed the hard goal by too much
/ in the rare case we do exceed it.
/
/ It should only be called when gcBlackenEnabled != 0 (because this
/ is when assists are enabled and the necessary statistics are
/ available).
func (c *gcControllerState) revise() {
gcPercent := c.gcPercent.Load()
if gcPercent < 0 {
/ If GC is disabled but we're running a forced GC,
/ act like GOGC is huge for the below calculations.
gcPercent = 100000
}
live := atomic.Load64(&c.heapLive)
scan := atomic.Load64(&c.heapScan)
work := c.heapScanWork.Load() + c.stackScanWork.Load() + c.globalsScanWork.Load()
/ Assume we're under the soft goal. Pace GC to complete at
/ heapGoal assuming the heap is in steady-state.
heapGoal := int64(c.heapGoal())
/ The expected scan work is computed as the amount of bytes scanned last
/ GC cycle (both heap and stack), plus our estimate of globals work for this cycle.
scanWorkExpected := int64(c.lastHeapScan + c.lastStackScan + c.globalsScan)
/ maxScanWork is a worst-case estimate of the amount of scan work that
/ needs to be performed in this GC cycle. Specifically, it represents
/ the case where *all* scannable memory turns out to be live, and
/ *all* allocated stack space is scannable.
maxStackScan := atomic.Load64(&c.maxStackScan)
maxScanWork := int64(scan + maxStackScan + c.globalsScan)
if work > scanWorkExpected {
/ We've already done more scan work than expected. Because our expectation
/ is based on a steady-state scannable heap size, we assume this means our
/ heap is growing. Compute a new heap goal that takes our existing runway
/ computed for scanWorkExpected and extrapolates it to maxScanWork, the worst-case
/ scan work. This keeps our assist ratio stable if the heap continues to grow.
/
/ The effect of this mechanism is that assists stay flat in the face of heap
/ growths. It's OK to use more memory this cycle to scan all the live heap,
/ because the next GC cycle is inevitably going to use *at least* that much
/ memory anyway.
extHeapGoal := int64(float64(heapGoal-int64(c.triggered))/float64(scanWorkExpected)*float64(maxScanWork)) + int64(c.triggered)
scanWorkExpected = maxScanWork
/ hardGoal is a hard limit on the amount that we're willing to push back the
/ heap goal, and that's twice the heap goal (i.e. if GOGC=100 and the heap and/or
/ stacks and/or globals grow to twice their size, this limits the current GC cycle's
/ growth to 4x the original live heap's size).
/
/ This maintains the invariant that we use no more memory than the next GC cycle
/ will anyway.
hardGoal := int64((1.0 + float64(gcPercent)/100.0) * float64(heapGoal))
if extHeapGoal > hardGoal {
extHeapGoal = hardGoal
}
heapGoal = extHeapGoal
}
if int64(live) > heapGoal {
/ We're already past our heap goal, even the extrapolated one.
/ Leave ourselves some extra runway, so in the worst case we
/ finish by that point.
const maxOvershoot = 1.1
heapGoal = int64(float64(heapGoal) * maxOvershoot)
/ Compute the upper bound on the scan work remaining.
scanWorkExpected = maxScanWork
}
/ Compute the remaining scan work estimate.
/
/ Note that we currently count allocations during GC as both
/ scannable heap (heapScan) and scan work completed
/ (scanWork), so allocation will change this difference
/ slowly in the soft regime and not at all in the hard
/ regime.
scanWorkRemaining := scanWorkExpected - work
if scanWorkRemaining < 1000 {
/ We set a somewhat arbitrary lower bound on
/ remaining scan work since if we aim a little high,
/ we can miss by a little.
/
/ We *do* need to enforce that this is at least 1,
/ since marking is racy and double-scanning objects
/ may legitimately make the remaining scan work
/ negative, even in the hard goal regime.
scanWorkRemaining = 1000
}
/ Compute the heap distance remaining.
heapRemaining := heapGoal - int64(live)
if heapRemaining <= 0 {
/ This shouldn't happen, but if it does, avoid
/ dividing by zero or setting the assist negative.
heapRemaining = 1
}
/ Compute the mutator assist ratio so by the time the mutator
/ allocates the remaining heap bytes up to heapGoal, it will
/ have done (or stolen) the remaining amount of scan work.
/ Note that the assist ratio values are updated atomically
/ but not together. This means there may be some degree of
/ skew between the two values. This is generally OK as the
/ values shift relatively slowly over the course of a GC
/ cycle.
assistWorkPerByte := float64(scanWorkRemaining) / float64(heapRemaining)
assistBytesPerWork := float64(heapRemaining) / float64(scanWorkRemaining)
c.assistWorkPerByte.Store(assistWorkPerByte)
c.assistBytesPerWork.Store(assistBytesPerWork)
}
/ endCycle computes the consMark estimate for the next cycle.
/ userForced indicates whether the current GC cycle was forced
/ by the application.
func (c *gcControllerState) endCycle(now int64, procs int, userForced bool) {
/ Record last heap goal for the scavenger.
/ We'll be updating the heap goal soon.
gcController.lastHeapGoal = c.heapGoal()
/ Compute the duration of time for which assists were turned on.
assistDuration := now - c.markStartTime
/ Assume background mark hit its utilization goal.
utilization := gcBackgroundUtilization
/ Add assist utilization; avoid divide by zero.
if assistDuration > 0 {
utilization += float64(c.assistTime.Load()) / float64(assistDuration*int64(procs))
}
if c.heapLive <= c.triggered {
/ Shouldn't happen, but let's be very safe about this in case the
/ GC is somehow extremely short.
/
/ In this case though, the only reasonable value for c.heapLive-c.triggered
/ would be 0, which isn't really all that useful, i.e. the GC was so short
/ that it didn't matter.
/
/ Ignore this case and don't update anything.
return
}
idleUtilization := 0.0
if assistDuration > 0 {
idleUtilization = float64(c.idleMarkTime) / float64(assistDuration*int64(procs))
}
/ Determine the cons/mark ratio.
/
/ The units we want for the numerator and denominator are both B / cpu-ns.
/ We get this by taking the bytes allocated or scanned, and divide by the amount of
/ CPU time it took for those operations. For allocations, that CPU time is
/
/ assistDuration * procs * (1 - utilization)
/
/ Where utilization includes just background GC workers and assists. It does *not*
/ include idle GC work time, because in theory the mutator is free to take that at
/ any point.
/
/ For scanning, that CPU time is
/
/ assistDuration * procs * (utilization + idleUtilization)
/
/ In this case, we *include* idle utilization, because that is additional CPU time that the
/ the GC had available to it.
/
/ In effect, idle GC time is sort of double-counted here, but it's very weird compared
/ to other kinds of GC work, because of how fluid it is. Namely, because the mutator is
/ *always* free to take it.
/
/ So this calculation is really:
/ (heapLive-trigger) / (assistDuration * procs * (1-utilization)) /
/ (scanWork) / (assistDuration * procs * (utilization+idleUtilization)
/
/ Note that because we only care about the ratio, assistDuration and procs cancel out.
scanWork := c.heapScanWork.Load() + c.stackScanWork.Load() + c.globalsScanWork.Load()
currentConsMark := (float64(c.heapLive-c.triggered) * (utilization + idleUtilization)) /
(float64(scanWork) * (1 - utilization))
/ Update cons/mark controller. The time period for this is 1 GC cycle.
/
/ This use of a PI controller might seem strange. So, here's an explanation:
/
/ currentConsMark represents the consMark we *should've* had to be perfectly
/ on-target for this cycle. Given that we assume the next GC will be like this
/ one in the steady-state, it stands to reason that we should just pick that
/ as our next consMark. In practice, however, currentConsMark is too noisy:
/ we're going to be wildly off-target in each GC cycle if we do that.
/
/ What we do instead is make a long-term assumption: there is some steady-state
/ consMark value, but it's obscured by noise. By constantly shooting for this
/ noisy-but-perfect consMark value, the controller will bounce around a bit,
/ but its average behavior, in aggregate, should be less noisy and closer to
/ the true long-term consMark value, provided its tuned to be slightly overdamped.
var ok bool
oldConsMark := c.consMark
c.consMark, ok = c.consMarkController.next(c.consMark, currentConsMark, 1.0)
if !ok {
/ The error spiraled out of control. This is incredibly unlikely seeing
/ as this controller is essentially just a smoothing function, but it might
/ mean that something went very wrong with how currentConsMark was calculated.
/ Just reset consMark and keep going.
c.consMark = 0
}
if debug.gcpacertrace > 0 {
printlock()
goal := gcGoalUtilization * 100
print("pacer: ", int(utilization*100), "% CPU (", int(goal), " exp.) for ")
print(c.heapScanWork.Load(), "+", c.stackScanWork.Load(), "+", c.globalsScanWork.Load(), " B work (", c.lastHeapScan+c.lastStackScan+c.globalsScan, " B exp.) ")
print("in ", c.triggered, " B -> ", c.heapLive, " B (∆goal ", int64(c.heapLive)-int64(c.lastHeapGoal), ", cons/mark ", oldConsMark, ")")
if !ok {
print("[controller reset]")
}
println()
printunlock()
}
}
/ enlistWorker encourages another dedicated mark worker to start on
/ another P if there are spare worker slots. It is used by putfull
/ when more work is made available.
/
/go:nowritebarrier
func (c *gcControllerState) enlistWorker() {
/ If there are idle Ps, wake one so it will run an idle worker.
/ NOTE: This is suspected of causing deadlocks. See golang.org/issue/19112.
/
/ if atomic.Load(&sched.npidle) != 0 && atomic.Load(&sched.nmspinning) == 0 {
/ wakep()
/ return
/ }
/ There are no idle Ps. If we need more dedicated workers,
/ try to preempt a running P so it will switch to a worker.
if c.dedicatedMarkWorkersNeeded <= 0 {
return
}
/ Pick a random other P to preempt.
if gomaxprocs <= 1 {
return
}
gp := getg()
if gp == nil || gp.m == nil || gp.m.p == 0 {
return
}
myID := gp.m.p.ptr().id
for tries := 0; tries < 5; tries++ {
id := int32(fastrandn(uint32(gomaxprocs - 1)))
if id >= myID {
id++
}
p := allp[id]
if p.status != _Prunning {
continue
}
if preemptone(p) {
return
}
}
}
/ findRunnableGCWorker returns a background mark worker for _p_ if it
/ should be run. This must only be called when gcBlackenEnabled != 0.
func (c *gcControllerState) findRunnableGCWorker(_p_ *p, now int64) (*g, int64) {
if gcBlackenEnabled == 0 {
throw("gcControllerState.findRunnable: blackening not enabled")
}
/ Since we have the current time, check if the GC CPU limiter
/ hasn't had an update in a while. This check is necessary in
/ case the limiter is on but hasn't been checked in a while and
/ so may have left sufficient headroom to turn off again.
if now == 0 {
now = nanotime()
}
if gcCPULimiter.needUpdate(now) {
gcCPULimiter.update(now)
}
if !gcMarkWorkAvailable(_p_) {
/ No work to be done right now. This can happen at
/ the end of the mark phase when there are still
/ assists tapering off. Don't bother running a worker
/ now because it'll just return immediately.
return nil, now
}
/ Grab a worker before we commit to running below.
node := (*gcBgMarkWorkerNode)(gcBgMarkWorkerPool.pop())
if node == nil {
/ There is at least one worker per P, so normally there are
/ enough workers to run on all Ps, if necessary. However, once
/ a worker enters gcMarkDone it may park without rejoining the
/ pool, thus freeing a P with no corresponding worker.
/ gcMarkDone never depends on another worker doing work, so it
/ is safe to simply do nothing here.
/
/ If gcMarkDone bails out without completing the mark phase,
/ it will always do so with queued global work. Thus, that P
/ will be immediately eligible to re-run the worker G it was
/ just using, ensuring work can complete.
return nil, now
}
decIfPositive := func(ptr *int64) bool {
for {
v := atomic.Loadint64(ptr)
if v <= 0 {
return false
}
if atomic.Casint64(ptr, v, v-1) {
return true
}
}
}
if decIfPositive(&c.dedicatedMarkWorkersNeeded) {
/ This P is now dedicated to marking until the end of
/ the concurrent mark phase.
_p_.gcMarkWorkerMode = gcMarkWorkerDedicatedMode
} else if c.fractionalUtilizationGoal == 0 {
/ No need for fractional workers.
gcBgMarkWorkerPool.push(&node.node)
return nil, now
} else {
/ Is this P behind on the fractional utilization
/ goal?
/
/ This should be kept in sync with pollFractionalWorkerExit.
delta := now - c.markStartTime
if delta > 0 && float64(_p_.gcFractionalMarkTime)/float64(delta) > c.fractionalUtilizationGoal {
/ Nope. No need to run a fractional worker.
gcBgMarkWorkerPool.push(&node.node)
return nil, now
}
/ Run a fractional worker.
_p_.gcMarkWorkerMode = gcMarkWorkerFractionalMode
}
/ Run the background mark worker.
gp := node.gp.ptr()
casgstatus(gp, _Gwaiting, _Grunnable)
if trace.enabled {
traceGoUnpark(gp, 0)
}
return gp, now
}
/ resetLive sets up the controller state for the next mark phase after the end
/ of the previous one. Must be called after endCycle and before commit, before
/ the world is started.
/
/ The world must be stopped.
func (c *gcControllerState) resetLive(bytesMarked uint64) {
c.heapMarked = bytesMarked
c.heapLive = bytesMarked
c.heapScan = uint64(c.heapScanWork.Load())
c.lastHeapScan = uint64(c.heapScanWork.Load())
c.lastStackScan = uint64(c.stackScanWork.Load())
c.triggered = ^uint64(0) / Reset triggered.
/ heapLive was updated, so emit a trace event.
if trace.enabled {
traceHeapAlloc()
}
}
/ markWorkerStop must be called whenever a mark worker stops executing.
/
/ It updates mark work accounting in the controller by a duration of
/ work in nanoseconds and other bookkeeping.
/
/ Safe to execute at any time.
func (c *gcControllerState) markWorkerStop(mode gcMarkWorkerMode, duration int64) {
switch mode {
case gcMarkWorkerDedicatedMode:
atomic.Xaddint64(&c.dedicatedMarkTime, duration)
atomic.Xaddint64(&c.dedicatedMarkWorkersNeeded, 1)
case gcMarkWorkerFractionalMode:
atomic.Xaddint64(&c.fractionalMarkTime, duration)
case gcMarkWorkerIdleMode:
atomic.Xaddint64(&c.idleMarkTime, duration)
c.removeIdleMarkWorker()
default:
throw("markWorkerStop: unknown mark worker mode")
}
}
func (c *gcControllerState) update(dHeapLive, dHeapScan int64) {
if dHeapLive != 0 {
atomic.Xadd64(&gcController.heapLive, dHeapLive)
if trace.enabled {
/ gcController.heapLive changed.
traceHeapAlloc()
}
}
if gcBlackenEnabled == 0 {
/ Update heapScan when we're not in a current GC. It is fixed
/ at the beginning of a cycle.
if dHeapScan != 0 {
atomic.Xadd64(&gcController.heapScan, dHeapScan)
}
} else {
/ gcController.heapLive changed.
c.revise()
}
}
func (c *gcControllerState) addScannableStack(pp *p, amount int64) {
if pp == nil {
atomic.Xadd64(&c.maxStackScan, amount)
return
}
pp.maxStackScanDelta += amount
if pp.maxStackScanDelta >= maxStackScanSlack || pp.maxStackScanDelta <= -maxStackScanSlack {
atomic.Xadd64(&c.maxStackScan, pp.maxStackScanDelta)
pp.maxStackScanDelta = 0
}
}
func (c *gcControllerState) addGlobals(amount int64) {
atomic.Xadd64(&c.globalsScan, amount)
}
/ heapGoal returns the current heap goal.
func (c *gcControllerState) heapGoal() uint64 {
goal, _ := c.heapGoalInternal()
return goal
}
/ heapGoalInternal is the implementation of heapGoal which returns additional
/ information that is necessary for computing the trigger.
/
/ The returned minTrigger is always <= goal.
func (c *gcControllerState) heapGoalInternal() (goal, minTrigger uint64) {
/ Start with the goal calculated for gcPercent.
goal = c.gcPercentHeapGoal.Load()
/ Check if the memory-limit-based goal is smaller, and if so, pick that.
if newGoal := c.memoryLimitHeapGoal(); go119MemoryLimitSupport && newGoal < goal {
goal = newGoal
} else {
/ We're not limited by the memory limit goal, so perform a series of
/ adjustments that might move the goal forward in a variety of circumstances.
sweepDistTrigger := c.sweepDistMinTrigger.Load()
if sweepDistTrigger > goal {
/ Set the goal to maintain a minimum sweep distance since
/ the last call to commit. Note that we never want to do this
/ if we're in the memory limit regime, because it could push
/ the goal up.
goal = sweepDistTrigger
}
/ Since we ignore the sweep distance trigger in the memory
/ limit regime, we need to ensure we don't propagate it to
/ the trigger, because it could cause a violation of the
/ invariant that the trigger < goal.
minTrigger = sweepDistTrigger
/ Ensure that the heap goal is at least a little larger than
/ the point at which we triggered. This may not be the case if GC
/ start is delayed or if the allocation that pushed gcController.heapLive
/ over trigger is large or if the trigger is really close to
/ GOGC. Assist is proportional to this distance, so enforce a
/ minimum distance, even if it means going over the GOGC goal
/ by a tiny bit.
/
/ Ignore this if we're in the memory limit regime: we'd prefer to
/ have the GC respond hard about how close we are to the goal than to
/ push the goal back in such a manner that it could cause us to exceed
/ the memory limit.
const minRunway = 64 << 10
if c.triggered != ^uint64(0) && goal < c.triggered+minRunway {
goal = c.triggered + minRunway
}
}
return
}
/ memoryLimitHeapGoal returns a heap goal derived from memoryLimit.
func (c *gcControllerState) memoryLimitHeapGoal() uint64 {
/ Start by pulling out some values we'll need. Be careful about overflow.
var heapFree, heapAlloc, mappedReady uint64
for {
heapFree = c.heapFree.load() / Free and unscavenged memory.
heapAlloc = c.totalAlloc.Load() - c.totalFree.Load() / Heap object bytes in use.
mappedReady = c.mappedReady.Load() / Total unreleased mapped memory.
if heapFree+heapAlloc <= mappedReady {
break
}
/ It is impossible for total unreleased mapped memory to exceed heap memory, but
/ because these stats are updated independently, we may observe a partial update
/ including only some values. Thus, we appear to break the invariant. However,
/ this condition is necessarily transient, so just try again. In the case of a
/ persistent accounting error, we'll deadlock here.
}
/ Below we compute a goal from memoryLimit. There are a few things to be aware of.
/ Firstly, the memoryLimit does not easily compare to the heap goal: the former
/ is total mapped memory by the runtime that hasn't been released, while the latter is
/ only heap object memory. Intuitively, the way we convert from one to the other is to
/ subtract everything from memoryLimit that both contributes to the memory limit (so,
/ ignore scavenged memory) and doesn't contain heap objects. This isn't quite what
/ lines up with reality, but it's a good starting point.
/
/ In practice this computation looks like the following:
/
/ memoryLimit - ((mappedReady - heapFree - heapAlloc) + max(mappedReady - memoryLimit, 0)) - memoryLimitHeapGoalHeadroom
/ ^1 ^2 ^3
/
/ Let's break this down.
/
/ The first term (marker 1) is everything that contributes to the memory limit and isn't
/ or couldn't become heap objects. It represents, broadly speaking, non-heap overheads.
/ One oddity you may have noticed is that we also subtract out heapFree, i.e. unscavenged
/ memory that may contain heap objects in the future.
/
/ Let's take a step back. In an ideal world, this term would look something like just
/ the heap goal. That is, we "reserve" enough space for the heap to grow to the heap
/ goal, and subtract out everything else. This is of course impossible; the definition
/ is circular! However, this impossible definition contains a key insight: the amount
/ we're *going* to use matters just as much as whatever we're currently using.
/
/ Consider if the heap shrinks to 1/10th its size, leaving behind lots of free and
/ unscavenged memory. mappedReady - heapAlloc will be quite large, because of that free
/ and unscavenged memory, pushing the goal down significantly.
/
/ heapFree is also safe to exclude from the memory limit because in the steady-state, it's
/ just a pool of memory for future heap allocations, and making new allocations from heapFree
/ memory doesn't increase overall memory use. In transient states, the scavenger and the
/ allocator actively manage the pool of heapFree memory to maintain the memory limit.
/
/ The second term (marker 2) is the amount of memory we've exceeded the limit by, and is
/ intended to help recover from such a situation. By pushing the heap goal down, we also
/ push the trigger down, triggering and finishing a GC sooner in order to make room for
/ other memory sources. Note that since we're effectively reducing the heap goal by X bytes,
/ we're actually giving more than X bytes of headroom back, because the heap goal is in
/ terms of heap objects, but it takes more than X bytes (e.g. due to fragmentation) to store
/ X bytes worth of objects.
/
/ The third term (marker 3) subtracts an additional memoryLimitHeapGoalHeadroom bytes from the
/ heap goal. As the name implies, this is to provide additional headroom in the face of pacing
/ inaccuracies. This is a fixed number of bytes because these inaccuracies disproportionately
/ affect small heaps: as heaps get smaller, the pacer's inputs get fuzzier. Shorter GC cycles
/ and less GC work means noisy external factors like the OS scheduler have a greater impact.
memoryLimit := uint64(c.memoryLimit.Load())
/ Compute term 1.
nonHeapMemory := mappedReady - heapFree - heapAlloc
/ Compute term 2.
var overage uint64
if mappedReady > memoryLimit {
overage = mappedReady - memoryLimit
}
if nonHeapMemory+overage >= memoryLimit {
/ We're at a point where non-heap memory exceeds the memory limit on its own.
/ There's honestly not much we can do here but just trigger GCs continuously
/ and let the CPU limiter reign that in. Something has to give at this point.
/ Set it to heapMarked, the lowest possible goal.
return c.heapMarked
}
/ Compute the goal.
goal := memoryLimit - (nonHeapMemory + overage)
/ Apply some headroom to the goal to account for pacing inaccuracies.
/ Be careful about small limits.
if goal < memoryLimitHeapGoalHeadroom || goal-memoryLimitHeapGoalHeadroom < memoryLimitHeapGoalHeadroom {
goal = memoryLimitHeapGoalHeadroom
} else {
goal = goal - memoryLimitHeapGoalHeadroom
}
/ Don't let us go below the live heap. A heap goal below the live heap doesn't make sense.
if goal < c.heapMarked {
goal = c.heapMarked
}
return goal
}
const (
/ These constants determine the bounds on the GC trigger as a fraction
/ of heap bytes allocated between the start of a GC (heapLive == heapMarked)
/ and the end of a GC (heapLive == heapGoal).
/
/ The constants are obscured in this way for efficiency. The denominator
/ of the fraction is always a power-of-two for a quick division, so that
/ the numerator is a single constant integer multiplication.
triggerRatioDen = 64
/ The minimum trigger constant was chosen empirically: given a sufficiently
/ fast/scalable allocator with 48 Ps that could drive the trigger ratio
/ to <0.05, this constant causes applications to retain the same peak
/ RSS compared to not having this allocator.
minTriggerRatioNum = 45 / ~0.7
/ The maximum trigger constant is chosen somewhat arbitrarily, but the
/ current constant has served us well over the years.
maxTriggerRatioNum = 61 / ~0.95
)
/ trigger returns the current point at which a GC should trigger along with
/ the heap goal.
/
/ The returned value may be compared against heapLive to determine whether
/ the GC should trigger. Thus, the GC trigger condition should be (but may
/ not be, in the case of small movements for efficiency) checked whenever
/ the heap goal may change.
func (c *gcControllerState) trigger() (uint64, uint64) {
goal, minTrigger := c.heapGoalInternal()
/ Invariant: the trigger must always be less than the heap goal.
/
/ Note that the memory limit sets a hard maximum on our heap goal,
/ but the live heap may grow beyond it.
if c.heapMarked >= goal {
/ The goal should never be smaller than heapMarked, but let's be
/ defensive about it. The only reasonable trigger here is one that
/ causes a continuous GC cycle at heapMarked, but respect the goal
/ if it came out as smaller than that.
return goal, goal
}
/ Below this point, c.heapMarked < goal.
/ heapMarked is our absolute minimum, and it's possible the trigger
/ bound we get from heapGoalinternal is less than that.
if minTrigger < c.heapMarked {
minTrigger = c.heapMarked
}
/ If we let the trigger go too low, then if the application
/ is allocating very rapidly we might end up in a situation
/ where we're allocating black during a nearly always-on GC.
/ The result of this is a growing heap and ultimately an
/ increase in RSS. By capping us at a point >0, we're essentially
/ saying that we're OK using more CPU during the GC to prevent
/ this growth in RSS.
triggerLowerBound := uint64(((goal-c.heapMarked)/triggerRatioDen)*minTriggerRatioNum) + c.heapMarked
if minTrigger < triggerLowerBound {
minTrigger = triggerLowerBound
}
/ For small heaps, set the max trigger point at maxTriggerRatio of the way
/ from the live heap to the heap goal. This ensures we always have *some*
/ headroom when the GC actually starts. For larger heaps, set the max trigger
/ point at the goal, minus the minimum heap size.
/
/ This choice follows from the fact that the minimum heap size is chosen
/ to reflect the costs of a GC with no work to do. With a large heap but
/ very little scan work to perform, this gives us exactly as much runway
/ as we would need, in the worst case.
maxTrigger := uint64(((goal-c.heapMarked)/triggerRatioDen)*maxTriggerRatioNum) + c.heapMarked
if goal > defaultHeapMinimum && goal-defaultHeapMinimum > maxTrigger {
maxTrigger = goal - defaultHeapMinimum
}
if maxTrigger < minTrigger {
maxTrigger = minTrigger
}
/ Compute the trigger from our bounds and the runway stored by commit.
var trigger uint64
runway := c.runway.Load()
if runway > goal {
trigger = minTrigger
} else {
trigger = goal - runway
}
if trigger < minTrigger {
trigger = minTrigger
}
if trigger > maxTrigger {
trigger = maxTrigger
}
if trigger > goal {
print("trigger=", trigger, " heapGoal=", goal, "\n")
print("minTrigger=", minTrigger, " maxTrigger=", maxTrigger, "\n")
throw("produced a trigger greater than the heap goal")
}
return trigger, goal
}
/ commit recomputes all pacing parameters needed to derive the
/ trigger and the heap goal. Namely, the gcPercent-based heap goal,
/ and the amount of runway we want to give the GC this cycle.
/
/ This can be called any time. If GC is the in the middle of a
/ concurrent phase, it will adjust the pacing of that phase.
/
/ isSweepDone should be the result of calling isSweepDone(),
/ unless we're testing or we know we're executing during a GC cycle.
/
/ This depends on gcPercent, gcController.heapMarked, and
/ gcController.heapLive. These must be up to date.
/
/ Callers must call gcControllerState.revise after calling this
/ function if the GC is enabled.
/
/ mheap_.lock must be held or the world must be stopped.
func (c *gcControllerState) commit(isSweepDone bool) {
if !c.test {
assertWorldStoppedOrLockHeld(&mheap_.lock)
}
if isSweepDone {
/ The sweep is done, so there aren't any restrictions on the trigger
/ we need to think about.
c.sweepDistMinTrigger.Store(0)
} else {
/ Concurrent sweep happens in the heap growth
/ from gcController.heapLive to trigger. Make sure we
/ give the sweeper some runway if it doesn't have enough.
c.sweepDistMinTrigger.Store(atomic.Load64(&c.heapLive) + sweepMinHeapDistance)
}
/ Compute the next GC goal, which is when the allocated heap
/ has grown by GOGC/100 over where it started the last cycle,
/ plus additional runway for non-heap sources of GC work.
gcPercentHeapGoal := ^uint64(0)
if gcPercent := c.gcPercent.Load(); gcPercent >= 0 {
gcPercentHeapGoal = c.heapMarked + (c.heapMarked+atomic.Load64(&c.lastStackScan)+atomic.Load64(&c.globalsScan))*uint64(gcPercent)/100
}
/ Apply the minimum heap size here. It's defined in terms of gcPercent
/ and is only updated by functions that call commit.
if gcPercentHeapGoal < c.heapMinimum {
gcPercentHeapGoal = c.heapMinimum
}
c.gcPercentHeapGoal.Store(gcPercentHeapGoal)
/ Compute the amount of runway we want the GC to have by using our
/ estimate of the cons/mark ratio.
/
/ The idea is to take our expected scan work, and multiply it by
/ the cons/mark ratio to determine how long it'll take to complete
/ that scan work in terms of bytes allocated. This gives us our GC's
/ runway.
/
/ However, the cons/mark ratio is a ratio of rates per CPU-second, but
/ here we care about the relative rates for some division of CPU
/ resources among the mutator and the GC.
/
/ To summarize, we have B / cpu-ns, and we want B / ns. We get that
/ by multiplying by our desired division of CPU resources. We choose
/ to express CPU resources as GOMAPROCS*fraction. Note that because
/ we're working with a ratio here, we can omit the number of CPU cores,
/ because they'll appear in the numerator and denominator and cancel out.
/ As a result, this is basically just "weighing" the cons/mark ratio by
/ our desired division of resources.
/
/ Furthermore, by setting the runway so that CPU resources are divided
/ this way, assuming that the cons/mark ratio is correct, we make that
/ division a reality.
c.runway.Store(uint64((c.consMark * (1 - gcGoalUtilization) / (gcGoalUtilization)) * float64(c.lastHeapScan+c.lastStackScan+c.globalsScan)))
}
/ setGCPercent updates gcPercent. commit must be called after.
/ Returns the old value of gcPercent.
/
/ The world must be stopped, or mheap_.lock must be held.
func (c *gcControllerState) setGCPercent(in int32) int32 {
if !c.test {
assertWorldStoppedOrLockHeld(&mheap_.lock)
}
out := c.gcPercent.Load()
if in < 0 {
in = -1
}
c.heapMinimum = defaultHeapMinimum * uint64(in) / 100
c.gcPercent.Store(in)
return out
}
/go:linkname setGCPercent runtime/debug.setGCPercent
func setGCPercent(in int32) (out int32) {
/ Run on the system stack since we grab the heap lock.
systemstack(func() {
lock(&mheap_.lock)
out = gcController.setGCPercent(in)
gcControllerCommit()
unlock(&mheap_.lock)
})
/ If we just disabled GC, wait for any concurrent GC mark to
/ finish so we always return with no GC running.
if in < 0 {
gcWaitOnMark(atomic.Load(&work.cycles))
}
return out
}
func readGOGC() int32 {
p := gogetenv("GOGC")
if p == "off" {
return -1
}
if n, ok := atoi32(p); ok {
return n
}
return 100
}
/ setMemoryLimit updates memoryLimit. commit must be called after
/ Returns the old value of memoryLimit.
/
/ The world must be stopped, or mheap_.lock must be held.
func (c *gcControllerState) setMemoryLimit(in int64) int64 {
if !c.test {
assertWorldStoppedOrLockHeld(&mheap_.lock)
}
out := c.memoryLimit.Load()
if in >= 0 {
c.memoryLimit.Store(in)
}
return out
}
/go:linkname setMemoryLimit runtime/debug.setMemoryLimit
func setMemoryLimit(in int64) (out int64) {
/ Run on the system stack since we grab the heap lock.
systemstack(func() {
lock(&mheap_.lock)
out = gcController.setMemoryLimit(in)
if in < 0 || out == in {
/ If we're just checking the value or not changing
/ it, there's no point in doing the rest.
unlock(&mheap_.lock)
return
}
gcControllerCommit()
unlock(&mheap_.lock)
})
return out
}
func readGOMEMLIMIT() int64 {
p := gogetenv("GOMEMLIMIT")
if p == "" || p == "off" {
return maxInt64
}
n, ok := parseByteCount(p)
if !ok {
print("GOMEMLIMIT=", p, "\n")
throw("malformed GOMEMLIMIT; see `go doc runtime/debug.SetMemoryLimit`")
}
return n
}
type piController struct {
kp float64 / Proportional constant.
ti float64 / Integral time constant.
tt float64 / Reset time.
min, max float64 / Output boundaries.
/ PI controller state.
errIntegral float64 / Integral of the error from t=0 to now.
/ Error flags.
errOverflow bool / Set if errIntegral ever overflowed.
inputOverflow bool / Set if an operation with the input overflowed.
}
/ next provides a new sample to the controller.
/
/ input is the sample, setpoint is the desired point, and period is how much
/ time (in whatever unit makes the most sense) has passed since the last sample.
/
/ Returns a new value for the variable it's controlling, and whether the operation
/ completed successfully. One reason this might fail is if error has been growing
/ in an unbounded manner, to the point of overflow.
/
/ In the specific case of an error overflow occurs, the errOverflow field will be
/ set and the rest of the controller's internal state will be fully reset.
func (c *piController) next(input, setpoint, period float64) (float64, bool) {
/ Compute the raw output value.
prop := c.kp * (setpoint - input)
rawOutput := prop + c.errIntegral
/ Clamp rawOutput into output.
output := rawOutput
if isInf(output) || isNaN(output) {
/ The input had a large enough magnitude that either it was already
/ overflowed, or some operation with it overflowed.
/ Set a flag and reset. That's the safest thing to do.
c.reset()
c.inputOverflow = true
return c.min, false
}
if output < c.min {
output = c.min
} else if output > c.max {
output = c.max
}
/ Update the controller's state.
if c.ti != 0 && c.tt != 0 {
c.errIntegral += (c.kp*period/c.ti)*(setpoint-input) + (period/c.tt)*(output-rawOutput)
if isInf(c.errIntegral) || isNaN(c.errIntegral) {
/ So much error has accumulated that we managed to overflow.
/ The assumptions around the controller have likely broken down.
/ Set a flag and reset. That's the safest thing to do.
c.reset()
c.errOverflow = true
return c.min, false
}
}
return output, true
}
/ reset resets the controller state, except for controller error flags.
func (c *piController) reset() {
c.errIntegral = 0
}
/ addIdleMarkWorker attempts to add a new idle mark worker.
/
/ If this returns true, the caller must become an idle mark worker unless
/ there's no background mark worker goroutines in the pool. This case is
/ harmless because there are already background mark workers running.
/ If this returns false, the caller must NOT become an idle mark worker.
/
/ nosplit because it may be called without a P.
/
/go:nosplit
func (c *gcControllerState) addIdleMarkWorker() bool {
for {
old := c.idleMarkWorkers.Load()
n, max := int32(old&uint64(^uint32(0))), int32(old>>32)
if n >= max {
/ See the comment on idleMarkWorkers for why
/ n > max is tolerated.
return false
}
if n < 0 {
print("n=", n, " max=", max, "\n")
throw("negative idle mark workers")
}
new := uint64(uint32(n+1)) | (uint64(max) << 32)
if c.idleMarkWorkers.CompareAndSwap(old, new) {
return true
}
}
}
/ needIdleMarkWorker is a hint as to whether another idle mark worker is needed.
/
/ The caller must still call addIdleMarkWorker to become one. This is mainly
/ useful for a quick check before an expensive operation.
/
/ nosplit because it may be called without a P.
/
/go:nosplit
func (c *gcControllerState) needIdleMarkWorker() bool {
p := c.idleMarkWorkers.Load()
n, max := int32(p&uint64(^uint32(0))), int32(p>>32)
return n < max
}
/ removeIdleMarkWorker must be called when an new idle mark worker stops executing.
func (c *gcControllerState) removeIdleMarkWorker() {
for {
old := c.idleMarkWorkers.Load()
n, max := int32(old&uint64(^uint32(0))), int32(old>>32)
if n-1 < 0 {
print("n=", n, " max=", max, "\n")
throw("negative idle mark workers")
}
new := uint64(uint32(n-1)) | (uint64(max) << 32)
if c.idleMarkWorkers.CompareAndSwap(old, new) {
return
}
}
}
/ setMaxIdleMarkWorkers sets the maximum number of idle mark workers allowed.
/
/ This method is optimistic in that it does not wait for the number of
/ idle mark workers to reduce to max before returning; it assumes the workers
/ will deschedule themselves.
func (c *gcControllerState) setMaxIdleMarkWorkers(max int32) {
for {
old := c.idleMarkWorkers.Load()
n := int32(old & uint64(^uint32(0)))
if n < 0 {
print("n=", n, " max=", max, "\n")
throw("negative idle mark workers")
}
new := uint64(uint32(n)) | (uint64(max) << 32)
if c.idleMarkWorkers.CompareAndSwap(old, new) {
return
}
}
}
/ gcControllerCommit is gcController.commit, but passes arguments from live
/ (non-test) data. It also updates any consumers of the GC pacing, such as
/ sweep pacing and the background scavenger.
/
/ Calls gcController.commit.
/
/ The heap lock must be held, so this must be executed on the system stack.
/
/go:systemstack
func gcControllerCommit() {
assertWorldStoppedOrLockHeld(&mheap_.lock)
gcController.commit(isSweepDone())
/ Update mark pacing.
if gcphase != _GCoff {
gcController.revise()
}
/ TODO(mknyszek): This isn't really accurate any longer because the heap
/ goal is computed dynamically. Still useful to snapshot, but not as useful.
if trace.enabled {
traceHeapGoal()
}
trigger, heapGoal := gcController.trigger()
gcPaceSweeper(trigger)
gcPaceScavenger(gcController.memoryLimit.Load(), heapGoal, gcController.lastHeapGoal)
}