runtime: bias the pacer's cons/mark smoothing against noise

[gostls13.git] / 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" // for go:linkname
)

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
)

// 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

        // 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

        // lastConsMark is the computed cons/mark value for the previous 4 GC
        // cycles. Note that this is *not* the last value of consMark, but the
        // measured cons/mark value in endCycle.
        lastConsMark [4]float64

        // 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).
        //
        // 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.
        //
        // Whenever this is updated, call traceHeapAlloc() and
        // this gcControllerState's revise() method.
        heapLive atomic.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. It represents the
        // maximum scannable heap.
        heapScan atomic.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 atomic.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.
        maxStackScan atomic.Uint64

        // globalsScan is the total amount of global variable space
        // that is scannable.
        globalsScan atomic.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.  Updates occur in bounded batches,
        // since it is both written and read throughout the cycle.
        bgScanCredit atomic.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 at the end of the concurrent mark
        // phase.
        dedicatedMarkTime atomic.Int64

        // fractionalMarkTime is the nanoseconds spent in the fractional mark
        // worker during this cycle. This is updated throughout the cycle and
        // will be up-to-date if the fractional mark worker is not currently
        // running.
        fractionalMarkTime atomic.Int64

        // idleMarkTime is the nanoseconds spent in idle marking during this
        // cycle. This is updated throughout the cycle.
        idleMarkTime atomic.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 as dedicated mark workers get started.
        dedicatedMarkWorkersNeeded atomic.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.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.Store(0)
        c.assistTime.Store(0)
        c.dedicatedMarkTime.Store(0)
        c.fractionalMarkTime.Store(0)
        c.idleMarkTime.Store(0)
        c.markStartTime = markStartTime
        c.triggered = c.heapLive.Load()

        // 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
        dedicatedMarkWorkersNeeded := int64(totalUtilizationGoal + 0.5)
        utilError := float64(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(dedicatedMarkWorkersNeeded) > totalUtilizationGoal {
                        // Too many dedicated workers.
                        dedicatedMarkWorkersNeeded--
                }
                c.fractionalUtilizationGoal = (totalUtilizationGoal - float64(dedicatedMarkWorkersNeeded)) / float64(procs)
        } else {
                c.fractionalUtilizationGoal = 0
        }

        // In STW mode, we just want dedicated workers.
        if debug.gcstoptheworld > 0 {
                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 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 are guaranteed not to
                // change during a GC cycle.
                c.setMaxIdleMarkWorkers(int32(procs) - int32(dedicatedMarkWorkersNeeded))
        }

        // Compute initial values for controls that are updated
        // throughout the cycle.
        c.dedicatedMarkWorkersNeeded.Store(dedicatedMarkWorkersNeeded)
        c.revise()

        if debug.gcpacertrace > 0 {
                heapGoal := c.heapGoal()
                assistRatio := c.assistWorkPerByte.Load()
                print("pacer: assist ratio=", assistRatio,
                        " (scan ", gcController.heapScan.Load()>>20, " MB in ",
                        work.initialHeapLive>>20, "->",
                        heapGoal>>20, " MB)",
                        " workers=", 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 := c.heapLive.Load()
        scan := c.heapScan.Load()
        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.Load() + c.globalsScan.Load())

        // 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 := c.maxStackScan.Load()
        maxScanWork := int64(scan + maxStackScan + c.globalsScan.Load())
        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.Load() <= 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.Load()) / 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 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.Load()-c.triggered) * (utilization + idleUtilization)) /
                (float64(scanWork) * (1 - utilization))

        // Update our cons/mark estimate. This is the maximum of the value we just computed and the last
        // 4 cons/mark values we measured. The reason we take the maximum here is to bias a noisy
        // cons/mark measurement toward fewer assists at the expense of additional GC cycles (starting
        // earlier).
        oldConsMark := c.consMark
        c.consMark = currentConsMark
        for i := range c.lastConsMark {
                if c.lastConsMark[i] > c.consMark {
                        c.consMark = c.lastConsMark[i]
                }
        }
        copy(c.lastConsMark[:], c.lastConsMark[1:])
        c.lastConsMark[len(c.lastConsMark)-1] = currentConsMark

        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.Load()+c.globalsScan.Load(), " B exp.) ")
                live := c.heapLive.Load()
                print("in ", c.triggered, " B -> ", live, " B (∆goal ", int64(live)-int64(c.lastHeapGoal), ", cons/mark ", oldConsMark, ")")
                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 sched.npidle.Load() != 0 && sched.nmspinning.Load() == 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.Load() <= 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 pp if it
// should be run. This must only be called when gcBlackenEnabled != 0.
func (c *gcControllerState) findRunnableGCWorker(pp *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(pp) {
                // 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(val *atomic.Int64) bool {
                for {
                        v := val.Load()
                        if v <= 0 {
                                return false
                        }

                        if val.CompareAndSwap(v, v-1) {
                                return true
                        }
                }
        }

        if decIfPositive(&c.dedicatedMarkWorkersNeeded) {
                // This P is now dedicated to marking until the end of
                // the concurrent mark phase.
                pp.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(pp.gcFractionalMarkTime)/float64(delta) > c.fractionalUtilizationGoal {
                        // Nope. No need to run a fractional worker.
                        gcBgMarkWorkerPool.push(&node.node)
                        return nil, now
                }
                // Run a fractional worker.
                pp.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.Store(bytesMarked)
        c.heapScan.Store(uint64(c.heapScanWork.Load()))
        c.lastHeapScan = uint64(c.heapScanWork.Load())
        c.lastStackScan.Store(uint64(c.stackScanWork.Load()))
        c.triggered = ^uint64(0) // Reset triggered.

        // heapLive was updated, so emit a trace event.
        if trace.enabled {
                traceHeapAlloc(bytesMarked)
        }
}

// 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:
                c.dedicatedMarkTime.Add(duration)
                c.dedicatedMarkWorkersNeeded.Add(1)
        case gcMarkWorkerFractionalMode:
                c.fractionalMarkTime.Add(duration)
        case gcMarkWorkerIdleMode:
                c.idleMarkTime.Add(duration)
                c.removeIdleMarkWorker()
        default:
                throw("markWorkerStop: unknown mark worker mode")
        }
}

func (c *gcControllerState) update(dHeapLive, dHeapScan int64) {
        if dHeapLive != 0 {
                live := gcController.heapLive.Add(dHeapLive)
                if trace.enabled {
                        // gcController.heapLive changed.
                        traceHeapAlloc(live)
                }
        }
        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 {
                        gcController.heapScan.Add(dHeapScan)
                }
        } else {
                // gcController.heapLive changed.
                c.revise()
        }
}

func (c *gcControllerState) addScannableStack(pp *p, amount int64) {
        if pp == nil {
                c.maxStackScan.Add(amount)
                return
        }
        pp.maxStackScanDelta += amount
        if pp.maxStackScanDelta >= maxStackScanSlack || pp.maxStackScanDelta <= -maxStackScanSlack {
                c.maxStackScan.Add(pp.maxStackScanDelta)
                pp.maxStackScanDelta = 0
        }
}

func (c *gcControllerState) addGlobals(amount int64) {
        c.globalsScan.Add(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(); 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(c.heapLive.Load() + 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+c.lastStackScan.Load()+c.globalsScan.Load())*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.Load()+c.globalsScan.Load())))
}

// 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(work.cycles.Load())
        }

        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
}

// 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)
}