Adding upstream version 1.20.14.upstream/1.20.14upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 1426 insertions, 0 deletions

diff --git a/src/runtime/mgcpacer.go b/src/runtime/mgcpacer.go
new file mode 100644
index 0000000..9d9840e
--- /dev/null
+++ b/src/runtime/mgcpacer.go
@@ -0,0 +1,1426 @@
+// 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
+)
+
+// 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
+)
+
+// 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 GC
+	// cycle. Note that this is *not* the last value of cons/mark, but the
+	// actual computed value. See endCycle for details.
+	lastConsMark 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 raw value above, but averaged over 2 GC cycles
+	// because it tends to be jittery, even in the steady-state. The smoothing helps the GC to
+	// maintain much more stable cycle-by-cycle behavior.
+	oldConsMark := c.consMark
+	c.consMark = (currentConsMark + c.lastConsMark) / 2
+	c.lastConsMark = 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(); 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(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)
+}