Merge "[dev.ssa] Merge remote-tracking branch 'origin/master' into ssamerge" into...

[gostls13.git] / src / runtime / mgc.go

// Copyright 2009 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.

// TODO(rsc): The code having to do with the heap bitmap needs very serious cleanup.
// It has gotten completely out of control.

// Garbage collector (GC).
//
// The GC runs concurrently with mutator threads, is type accurate (aka precise), allows multiple
// GC thread to run in parallel. It is a concurrent mark and sweep that uses a write barrier. It is
// non-generational and non-compacting. Allocation is done using size segregated per P allocation
// areas to minimize fragmentation while eliminating locks in the common case.
//
// The algorithm decomposes into several steps.
// This is a high level description of the algorithm being used. For an overview of GC a good
// place to start is Richard Jones' gchandbook.org.
//
// The algorithm's intellectual heritage includes Dijkstra's on-the-fly algorithm, see
// Edsger W. Dijkstra, Leslie Lamport, A. J. Martin, C. S. Scholten, and E. F. M. Steffens. 1978.
// On-the-fly garbage collection: an exercise in cooperation. Commun. ACM 21, 11 (November 1978),
// 966-975.
// For journal quality proofs that these steps are complete, correct, and terminate see
// Hudson, R., and Moss, J.E.B. Copying Garbage Collection without stopping the world.
// Concurrency and Computation: Practice and Experience 15(3-5), 2003.
//
//  0. Set phase = GCscan from GCoff.
//  1. Wait for all P's to acknowledge phase change.
//         At this point all goroutines have passed through a GC safepoint and
//         know we are in the GCscan phase.
//  2. GC scans all goroutine stacks, mark and enqueues all encountered pointers
//       (marking avoids most duplicate enqueuing but races may produce benign duplication).
//       Preempted goroutines are scanned before P schedules next goroutine.
//  3. Set phase = GCmark.
//  4. Wait for all P's to acknowledge phase change.
//  5. Now write barrier marks and enqueues black, grey, or white to white pointers.
//       Malloc still allocates white (non-marked) objects.
//  6. Meanwhile GC transitively walks the heap marking reachable objects.
//  7. When GC finishes marking heap, it preempts P's one-by-one and
//       retakes partial wbufs (filled by write barrier or during a stack scan of the goroutine
//       currently scheduled on the P).
//  8. Once the GC has exhausted all available marking work it sets phase = marktermination.
//  9. Wait for all P's to acknowledge phase change.
// 10. Malloc now allocates black objects, so number of unmarked reachable objects
//        monotonically decreases.
// 11. GC preempts P's one-by-one taking partial wbufs and marks all unmarked yet
//        reachable objects.
// 12. When GC completes a full cycle over P's and discovers no new grey
//         objects, (which means all reachable objects are marked) set phase = GCoff.
// 13. Wait for all P's to acknowledge phase change.
// 14. Now malloc allocates white (but sweeps spans before use).
//         Write barrier becomes nop.
// 15. GC does background sweeping, see description below.
// 16. When sufficient allocation has taken place replay the sequence starting at 0 above,
//         see discussion of GC rate below.

// Changing phases.
// Phases are changed by setting the gcphase to the next phase and possibly calling ackgcphase.
// All phase action must be benign in the presence of a change.
// Starting with GCoff
// GCoff to GCscan
//     GSscan scans stacks and globals greying them and never marks an object black.
//     Once all the P's are aware of the new phase they will scan gs on preemption.
//     This means that the scanning of preempted gs can't start until all the Ps
//     have acknowledged.
//     When a stack is scanned, this phase also installs stack barriers to
//     track how much of the stack has been active.
//     This transition enables write barriers because stack barriers
//     assume that writes to higher frames will be tracked by write
//     barriers. Technically this only needs write barriers for writes
//     to stack slots, but we enable write barriers in general.
// GCscan to GCmark
//     In GCmark, work buffers are drained until there are no more
//     pointers to scan.
//     No scanning of objects (making them black) can happen until all
//     Ps have enabled the write barrier, but that already happened in
//     the transition to GCscan.
// GCmark to GCmarktermination
//     The only change here is that we start allocating black so the Ps must acknowledge
//     the change before we begin the termination algorithm
// GCmarktermination to GSsweep
//     Object currently on the freelist must be marked black for this to work.
//     Are things on the free lists black or white? How does the sweep phase work?

// Concurrent sweep.
//
// The sweep phase proceeds concurrently with normal program execution.
// The heap is swept span-by-span both lazily (when a goroutine needs another span)
// and concurrently in a background goroutine (this helps programs that are not CPU bound).
// At the end of STW mark termination all spans are marked as "needs sweeping".
//
// The background sweeper goroutine simply sweeps spans one-by-one.
//
// To avoid requesting more OS memory while there are unswept spans, when a
// goroutine needs another span, it first attempts to reclaim that much memory
// by sweeping. When a goroutine needs to allocate a new small-object span, it
// sweeps small-object spans for the same object size until it frees at least
// one object. When a goroutine needs to allocate large-object span from heap,
// it sweeps spans until it frees at least that many pages into heap. There is
// one case where this may not suffice: if a goroutine sweeps and frees two
// nonadjacent one-page spans to the heap, it will allocate a new two-page
// span, but there can still be other one-page unswept spans which could be
// combined into a two-page span.
//
// It's critical to ensure that no operations proceed on unswept spans (that would corrupt
// mark bits in GC bitmap). During GC all mcaches are flushed into the central cache,
// so they are empty. When a goroutine grabs a new span into mcache, it sweeps it.
// When a goroutine explicitly frees an object or sets a finalizer, it ensures that
// the span is swept (either by sweeping it, or by waiting for the concurrent sweep to finish).
// The finalizer goroutine is kicked off only when all spans are swept.
// When the next GC starts, it sweeps all not-yet-swept spans (if any).

// GC rate.
// Next GC is after we've allocated an extra amount of memory proportional to
// the amount already in use. The proportion is controlled by GOGC environment variable
// (100 by default). If GOGC=100 and we're using 4M, we'll GC again when we get to 8M
// (this mark is tracked in next_gc variable). This keeps the GC cost in linear
// proportion to the allocation cost. Adjusting GOGC just changes the linear constant
// (and also the amount of extra memory used).

package runtime

import (
        "runtime/internal/atomic"
        "runtime/internal/sys"
        "unsafe"
)

const (
        _DebugGC         = 0
        _ConcurrentSweep = true
        _FinBlockSize    = 4 * 1024

        // sweepMinHeapDistance is a lower bound on the heap distance
        // (in bytes) reserved for concurrent sweeping between GC
        // cycles. This will be scaled by gcpercent/100.
        sweepMinHeapDistance = 1024 * 1024
)

// 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.
var heapminimum uint64 = defaultHeapMinimum

// defaultHeapMinimum is the value of heapminimum for GOGC==100.
const defaultHeapMinimum = 4 << 20

// Initialized from $GOGC.  GOGC=off means no GC.
var gcpercent int32

func gcinit() {
        if unsafe.Sizeof(workbuf{}) != _WorkbufSize {
                throw("size of Workbuf is suboptimal")
        }

        _ = setGCPercent(readgogc())
        for datap := &firstmoduledata; datap != nil; datap = datap.next {
                datap.gcdatamask = progToPointerMask((*byte)(unsafe.Pointer(datap.gcdata)), datap.edata-datap.data)
                datap.gcbssmask = progToPointerMask((*byte)(unsafe.Pointer(datap.gcbss)), datap.ebss-datap.bss)
        }
        memstats.next_gc = heapminimum
        work.startSema = 1
        work.markDoneSema = 1
}

func readgogc() int32 {
        p := gogetenv("GOGC")
        if p == "" {
                return 100
        }
        if p == "off" {
                return -1
        }
        return int32(atoi(p))
}

// gcenable is called after the bulk of the runtime initialization,
// just before we're about to start letting user code run.
// It kicks off the background sweeper goroutine and enables GC.
func gcenable() {
        c := make(chan int, 1)
        go bgsweep(c)
        <-c
        memstats.enablegc = true // now that runtime is initialized, GC is okay
}

//go:linkname setGCPercent runtime/debug.setGCPercent
func setGCPercent(in int32) (out int32) {
        lock(&mheap_.lock)
        out = gcpercent
        if in < 0 {
                in = -1
        }
        gcpercent = in
        heapminimum = defaultHeapMinimum * uint64(gcpercent) / 100
        if gcController.triggerRatio > float64(gcpercent)/100 {
                gcController.triggerRatio = float64(gcpercent) / 100
        }
        unlock(&mheap_.lock)
        return out
}

// Garbage collector phase.
// Indicates to write barrier and sychronization task to preform.
var gcphase uint32

// The compiler knows about this variable.
// If you change it, you must change the compiler too.
var writeBarrier struct {
        enabled bool   // compiler emits a check of this before calling write barrier
        needed  bool   // whether we need a write barrier for current GC phase
        cgo     bool   // whether we need a write barrier for a cgo check
        alignme uint64 // guarantee alignment so that compiler can use a 32 or 64-bit load
}

// gcBlackenEnabled is 1 if mutator assists and background mark
// workers are allowed to blacken objects. This must only be set when
// gcphase == _GCmark.
var gcBlackenEnabled uint32

// gcBlackenPromptly indicates that optimizations that may
// hide work from the global work queue should be disabled.
//
// If gcBlackenPromptly is true, per-P gcWork caches should
// be flushed immediately and new objects should be allocated black.
//
// There is a tension between allocating objects white and
// allocating them black. If white and the objects die before being
// marked they can be collected during this GC cycle. On the other
// hand allocating them black will reduce _GCmarktermination latency
// since more work is done in the mark phase. This tension is resolved
// by allocating white until the mark phase is approaching its end and
// then allocating black for the remainder of the mark phase.
var gcBlackenPromptly bool

const (
        _GCoff             = iota // GC not running; sweeping in background, write barrier disabled
        _GCmark                   // GC marking roots and workbufs, write barrier ENABLED
        _GCmarktermination        // GC mark termination: allocate black, P's help GC, write barrier ENABLED
)

//go:nosplit
func setGCPhase(x uint32) {
        atomic.Store(&gcphase, x)
        writeBarrier.needed = gcphase == _GCmark || gcphase == _GCmarktermination
        writeBarrier.enabled = writeBarrier.needed || writeBarrier.cgo
}

// gcMarkWorkerMode represents the mode that a concurrent mark worker
// should operate in.
//
// Concurrent marking happens through four different mechanisms. One
// is mutator assists, which happen in response to allocations and are
// not scheduled. The other three are variations in the per-P mark
// workers and are distinguished by gcMarkWorkerMode.
type gcMarkWorkerMode int

const (
        // gcMarkWorkerDedicatedMode indicates that the P of a mark
        // worker is dedicated to running that mark worker. The mark
        // worker should run without preemption.
        gcMarkWorkerDedicatedMode gcMarkWorkerMode = iota

        // gcMarkWorkerFractionalMode indicates that a P is currently
        // running the "fractional" mark worker. The fractional worker
        // is necessary when GOMAXPROCS*gcGoalUtilization is not an
        // integer. The fractional worker should run until it is
        // preempted and will be scheduled to pick up the fractional
        // part of GOMAXPROCS*gcGoalUtilization.
        gcMarkWorkerFractionalMode

        // gcMarkWorkerIdleMode indicates that a P is running the mark
        // worker because it has nothing else to do. The idle worker
        // should run until it is preempted and account its time
        // against gcController.idleMarkTime.
        gcMarkWorkerIdleMode
)

// 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 uses a feedback control algorithm to adjust the memstats.next_gc
// trigger based on the heap growth and GC CPU utilization each cycle.
// This algorithm optimizes for heap growth to match GOGC and for CPU
// utilization between assist and background marking to be 25% of
// GOMAXPROCS. The high-level design of this algorithm is documented
// at https://golang.org/s/go15gcpacing.
var gcController = gcControllerState{
        // Initial trigger ratio guess.
        triggerRatio: 7 / 8.0,
}

type gcControllerState struct {
        // scanWork is the total scan work performed this cycle. This
        // is updated atomically during the cycle. Updates occur in
        // bounded batches, since it is both written and read
        // throughout the cycle.
        //
        // Currently this is the bytes of heap scanned. For most uses,
        // this is an opaque unit of work, but for estimation the
        // definition is important.
        scanWork int64

        // bgScanCredit is the scan work credit accumulated by the
        // concurrent background scan. This credit is accumulated by
        // the background scan and stolen by mutator assists. This is
        // updated atomically. Updates occur in bounded batches, since
        // it is both written and read throughout the cycle.
        bgScanCredit int64

        // assistTime is the nanoseconds spent in mutator assists
        // during this cycle. This is updated atomically. Updates
        // occur in bounded batches, since it is both written and read
        // throughout the cycle.
        assistTime int64

        // dedicatedMarkTime is the nanoseconds spent in dedicated
        // mark workers during this cycle. This is updated atomically
        // at the end of the concurrent mark phase.
        dedicatedMarkTime int64

        // fractionalMarkTime is the nanoseconds spent in the
        // fractional mark worker during this cycle. This is updated
        // atomically throughout the cycle and will be up-to-date if
        // the fractional mark worker is not currently running.
        fractionalMarkTime int64

        // idleMarkTime is the nanoseconds spent in idle marking
        // during this cycle. This is updated atomically throughout
        // the cycle.
        idleMarkTime int64

        // bgMarkStartTime is the absolute start time in nanoseconds
        // that the background mark phase started.
        bgMarkStartTime int64

        // assistTime is the absolute start time in nanoseconds that
        // mutator assists were enabled.
        assistStartTime int64

        // heapGoal is the goal memstats.heap_live for when this cycle
        // ends. This is computed at the beginning of each cycle.
        heapGoal uint64

        // dedicatedMarkWorkersNeeded is the number of dedicated mark
        // workers that need to be started. This is computed at the
        // beginning of each cycle and decremented atomically as
        // dedicated mark workers get started.
        dedicatedMarkWorkersNeeded int64

        // 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 heap_scan is updated.
        assistWorkPerByte float64

        // assistBytesPerWork is 1/assistWorkPerByte.
        assistBytesPerWork float64

        // fractionalUtilizationGoal is the fraction of wall clock
        // time that should be spent in the fractional mark worker.
        // For example, if the overall mark utilization goal is 25%
        // and GOMAXPROCS is 6, one P will be a dedicated mark worker
        // and this will be set to 0.5 so that 50% of the time some P
        // is in a fractional mark worker. This is computed at the
        // beginning of each cycle.
        fractionalUtilizationGoal float64

        // triggerRatio is the heap growth ratio at which the garbage
        // collection cycle should start. E.g., if this is 0.6, then
        // GC should start when the live heap has reached 1.6 times
        // the heap size marked by the previous cycle. This is updated
        // at the end of of each cycle.
        triggerRatio float64

        _ [sys.CacheLineSize]byte

        // fractionalMarkWorkersNeeded is the number of fractional
        // mark workers that need to be started. This is either 0 or
        // 1. This is potentially updated atomically at every
        // scheduling point (hence it gets its own cache line).
        fractionalMarkWorkersNeeded int64

        _ [sys.CacheLineSize]byte
}

// startCycle resets the GC controller's state and computes estimates
// for a new GC cycle. The caller must hold worldsema.
func (c *gcControllerState) startCycle() {
        c.scanWork = 0
        c.bgScanCredit = 0
        c.assistTime = 0
        c.dedicatedMarkTime = 0
        c.fractionalMarkTime = 0
        c.idleMarkTime = 0

        // If this is the first GC cycle or we're operating on a very
        // small heap, fake heap_marked so it looks like next_gc is
        // the appropriate growth from heap_marked, even though the
        // real heap_marked may not have a meaningful value (on the
        // first cycle) or may be much smaller (resulting in a large
        // error response).
        if memstats.next_gc <= heapminimum {
                memstats.heap_marked = uint64(float64(memstats.next_gc) / (1 + c.triggerRatio))
                memstats.heap_reachable = memstats.heap_marked
        }

        // Compute the heap goal for this cycle
        c.heapGoal = memstats.heap_reachable + memstats.heap_reachable*uint64(gcpercent)/100

        // Ensure that the heap goal is at least a little larger than
        // the current live heap size. This may not be the case if GC
        // start is delayed or if the allocation that pushed heap_live
        // over next_gc 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.
        if c.heapGoal < memstats.heap_live+1024*1024 {
                c.heapGoal = memstats.heap_live + 1024*1024
        }

        // Compute the total mark utilization goal and divide it among
        // dedicated and fractional workers.
        totalUtilizationGoal := float64(gomaxprocs) * gcGoalUtilization
        c.dedicatedMarkWorkersNeeded = int64(totalUtilizationGoal)
        c.fractionalUtilizationGoal = totalUtilizationGoal - float64(c.dedicatedMarkWorkersNeeded)
        if c.fractionalUtilizationGoal > 0 {
                c.fractionalMarkWorkersNeeded = 1
        } else {
                c.fractionalMarkWorkersNeeded = 0
        }

        // Clear per-P state
        for _, p := range &allp {
                if p == nil {
                        break
                }
                p.gcAssistTime = 0
        }

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

        if debug.gcpacertrace > 0 {
                print("pacer: assist ratio=", c.assistWorkPerByte,
                        " (scan ", memstats.heap_scan>>20, " MB in ",
                        work.initialHeapLive>>20, "->",
                        c.heapGoal>>20, " MB)",
                        " workers=", c.dedicatedMarkWorkersNeeded,
                        "+", c.fractionalMarkWorkersNeeded, "\n")
        }
}

// revise updates the assist ratio during the GC cycle to account for
// improved estimates. This should be called either under STW or
// whenever memstats.heap_scan or memstats.heap_live is updated (with
// mheap_.lock held).
//
// 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() {
        // Compute the expected scan work remaining.
        //
        // Note that the scannable heap size is likely to increase
        // during the GC cycle. This is why it's important to revise
        // the assist ratio throughout the cycle: if the scannable
        // heap size increases, the assist ratio based on the initial
        // scannable heap size may target too little scan work.
        //
        // This particular estimate is a strict upper bound on the
        // possible remaining scan work for the current heap.
        // You might consider dividing this by 2 (or by
        // (100+GOGC)/100) to counter this over-estimation, but
        // benchmarks show that this has almost no effect on mean
        // mutator utilization, heap size, or assist time and it
        // introduces the danger of under-estimating and letting the
        // mutator outpace the garbage collector.
        scanWorkExpected := int64(memstats.heap_scan) - c.scanWork
        if scanWorkExpected < 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 expected scan work
                // negative.
                scanWorkExpected = 1000
        }

        // Compute the heap distance remaining.
        heapDistance := int64(c.heapGoal) - int64(memstats.heap_live)
        if heapDistance <= 0 {
                // This shouldn't happen, but if it does, avoid
                // dividing by zero or setting the assist negative.
                heapDistance = 1
        }

        // Compute the mutator assist ratio so by the time the mutator
        // allocates the remaining heap bytes up to next_gc, it will
        // have done (or stolen) the remaining amount of scan work.
        c.assistWorkPerByte = float64(scanWorkExpected) / float64(heapDistance)
        c.assistBytesPerWork = float64(heapDistance) / float64(scanWorkExpected)
}

// endCycle updates the GC controller state at the end of the
// concurrent part of the GC cycle.
func (c *gcControllerState) endCycle() {
        h_t := c.triggerRatio // For debugging

        // Proportional response gain for the trigger controller. Must
        // be in [0, 1]. Lower values smooth out transient effects but
        // take longer to respond to phase changes. Higher values
        // react to phase changes quickly, but are more affected by
        // transient changes. Values near 1 may be unstable.
        const triggerGain = 0.5

        // Compute next cycle trigger ratio. First, this computes the
        // "error" for this cycle; that is, how far off the trigger
        // was from what it should have been, accounting for both heap
        // growth and GC CPU utilization. We compute the actual heap
        // growth during this cycle and scale that by how far off from
        // the goal CPU utilization we were (to estimate the heap
        // growth if we had the desired CPU utilization). The
        // difference between this estimate and the GOGC-based goal
        // heap growth is the error.
        //
        // TODO(austin): next_gc is based on heap_reachable, not
        // heap_marked, which means the actual growth ratio
        // technically isn't comparable to the trigger ratio.
        goalGrowthRatio := float64(gcpercent) / 100
        actualGrowthRatio := float64(memstats.heap_live)/float64(memstats.heap_marked) - 1
        assistDuration := nanotime() - c.assistStartTime

        // Assume background mark hit its utilization goal.
        utilization := gcGoalUtilization
        // Add assist utilization; avoid divide by zero.
        if assistDuration > 0 {
                utilization += float64(c.assistTime) / float64(assistDuration*int64(gomaxprocs))
        }

        triggerError := goalGrowthRatio - c.triggerRatio - utilization/gcGoalUtilization*(actualGrowthRatio-c.triggerRatio)

        // Finally, we adjust the trigger for next time by this error,
        // damped by the proportional gain.
        c.triggerRatio += triggerGain * triggerError
        if c.triggerRatio < 0 {
                // This can happen if the mutator is allocating very
                // quickly or the GC is scanning very slowly.
                c.triggerRatio = 0
        } else if c.triggerRatio > goalGrowthRatio*0.95 {
                // Ensure there's always a little margin so that the
                // mutator assist ratio isn't infinity.
                c.triggerRatio = goalGrowthRatio * 0.95
        }

        if debug.gcpacertrace > 0 {
                // Print controller state in terms of the design
                // document.
                H_m_prev := memstats.heap_marked
                H_T := memstats.next_gc
                h_a := actualGrowthRatio
                H_a := memstats.heap_live
                h_g := goalGrowthRatio
                H_g := int64(float64(H_m_prev) * (1 + h_g))
                u_a := utilization
                u_g := gcGoalUtilization
                W_a := c.scanWork
                print("pacer: H_m_prev=", H_m_prev,
                        " h_t=", h_t, " H_T=", H_T,
                        " h_a=", h_a, " H_a=", H_a,
                        " h_g=", h_g, " H_g=", H_g,
                        " u_a=", u_a, " u_g=", u_g,
                        " W_a=", W_a,
                        " goalΔ=", goalGrowthRatio-h_t,
                        " actualΔ=", h_a-h_t,
                        " u_a/u_g=", u_a/u_g,
                        "\n")
        }
}

// 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 c.dedicatedMarkWorkersNeeded <= 0 {
                return
        }
        // Pick a random other P to preempt.
        if gomaxprocs <= 1 {
                return
        }
        gp := getg()
        if gp == nil || gp.m == nil || gp.m.p == 0 {
                return
        }
        myID := gp.m.p.ptr().id
        for tries := 0; tries < 5; tries++ {
                id := int32(fastrand1() % uint32(gomaxprocs-1))
                if id >= myID {
                        id++
                }
                p := allp[id]
                if p.status != _Prunning {
                        continue
                }
                if preemptone(p) {
                        return
                }
        }
}

// findRunnableGCWorker returns the background mark worker for _p_ if it
// should be run. This must only be called when gcBlackenEnabled != 0.
func (c *gcControllerState) findRunnableGCWorker(_p_ *p) *g {
        if gcBlackenEnabled == 0 {
                throw("gcControllerState.findRunnable: blackening not enabled")
        }
        if _p_.gcBgMarkWorker == 0 {
                // The mark worker associated with this P is blocked
                // performing a mark transition. We can't run it
                // because it may be on some other run or wait queue.
                return nil
        }

        if !gcMarkWorkAvailable(_p_) {
                // No work to be done right now. This can happen at
                // the end of the mark phase when there are still
                // assists tapering off. Don't bother running a worker
                // now because it'll just return immediately.
                return nil
        }

        decIfPositive := func(ptr *int64) bool {
                if *ptr > 0 {
                        if atomic.Xaddint64(ptr, -1) >= 0 {
                                return true
                        }
                        // We lost a race
                        atomic.Xaddint64(ptr, +1)
                }
                return false
        }

        if decIfPositive(&c.dedicatedMarkWorkersNeeded) {
                // This P is now dedicated to marking until the end of
                // the concurrent mark phase.
                _p_.gcMarkWorkerMode = gcMarkWorkerDedicatedMode
                // TODO(austin): This P isn't going to run anything
                // else for a while, so kick everything out of its run
                // queue.
        } else {
                if !decIfPositive(&c.fractionalMarkWorkersNeeded) {
                        // No more workers are need right now.
                        return nil
                }

                // This P has picked the token for the fractional worker.
                // Is the GC currently under or at the utilization goal?
                // If so, do more work.
                //
                // We used to check whether doing one time slice of work
                // would remain under the utilization goal, but that has the
                // effect of delaying work until the mutator has run for
                // enough time slices to pay for the work. During those time
                // slices, write barriers are enabled, so the mutator is running slower.
                // Now instead we do the work whenever we're under or at the
                // utilization work and pay for it by letting the mutator run later.
                // This doesn't change the overall utilization averages, but it
                // front loads the GC work so that the GC finishes earlier and
                // write barriers can be turned off sooner, effectively giving
                // the mutator a faster machine.
                //
                // The old, slower behavior can be restored by setting
                //      gcForcePreemptNS = forcePreemptNS.
                const gcForcePreemptNS = 0

                // TODO(austin): We could fast path this and basically
                // eliminate contention on c.fractionalMarkWorkersNeeded by
                // precomputing the minimum time at which it's worth
                // next scheduling the fractional worker. Then Ps
                // don't have to fight in the window where we've
                // passed that deadline and no one has started the
                // worker yet.
                //
                // TODO(austin): Shorter preemption interval for mark
                // worker to improve fairness and give this
                // finer-grained control over schedule?
                now := nanotime() - gcController.bgMarkStartTime
                then := now + gcForcePreemptNS
                timeUsed := c.fractionalMarkTime + gcForcePreemptNS
                if then > 0 && float64(timeUsed)/float64(then) > c.fractionalUtilizationGoal {
                        // Nope, we'd overshoot the utilization goal
                        atomic.Xaddint64(&c.fractionalMarkWorkersNeeded, +1)
                        return nil
                }
                _p_.gcMarkWorkerMode = gcMarkWorkerFractionalMode
        }

        // Run the background mark worker
        gp := _p_.gcBgMarkWorker.ptr()
        casgstatus(gp, _Gwaiting, _Grunnable)
        if trace.enabled {
                traceGoUnpark(gp, 0)
        }
        return gp
}

// gcGoalUtilization is the goal CPU utilization for background
// marking as a fraction of GOMAXPROCS.
const gcGoalUtilization = 0.25

// gcCreditSlack is the amount of scan work credit that can can
// accumulate locally before updating gcController.scanWork 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.
const gcCreditSlack = 2000

// gcAssistTimeSlack is the nanoseconds of mutator assist time that
// can accumulate on a P before updating gcController.assistTime.
const gcAssistTimeSlack = 5000

// gcOverAssistBytes determines how many extra allocation bytes of
// assist credit a GC assist builds up when an assist happens. This
// amortizes the cost of an assist by pre-paying for this many bytes
// of future allocations.
const gcOverAssistBytes = 1 << 20

var work struct {
        full  uint64                   // lock-free list of full blocks workbuf
        empty uint64                   // lock-free list of empty blocks workbuf
        pad0  [sys.CacheLineSize]uint8 // prevents false-sharing between full/empty and nproc/nwait

        markrootNext uint32 // next markroot job
        markrootJobs uint32 // number of markroot jobs

        nproc   uint32
        tstart  int64
        nwait   uint32
        ndone   uint32
        alldone note

        // Number of roots of various root types. Set by gcMarkRootPrepare.
        nDataRoots, nBSSRoots, nSpanRoots, nStackRoots int

        // finalizersDone indicates that finalizers and objects with
        // finalizers have been scanned by markroot. During concurrent
        // GC, this happens during the concurrent scan phase. During
        // STW GC, this happens during mark termination.
        finalizersDone bool

        // Each type of GC state transition is protected by a lock.
        // Since multiple threads can simultaneously detect the state
        // transition condition, any thread that detects a transition
        // condition must acquire the appropriate transition lock,
        // re-check the transition condition and return if it no
        // longer holds or perform the transition if it does.
        // Likewise, any transition must invalidate the transition
        // condition before releasing the lock. This ensures that each
        // transition is performed by exactly one thread and threads
        // that need the transition to happen block until it has
        // happened.
        //
        // startSema protects the transition from "off" to mark or
        // mark termination.
        startSema uint32
        // markDoneSema protects transitions from mark 1 to mark 2 and
        // from mark 2 to mark termination.
        markDoneSema uint32

        bgMarkReady note   // signal background mark worker has started
        bgMarkDone  uint32 // cas to 1 when at a background mark completion point
        // Background mark completion signaling

        // mode is the concurrency mode of the current GC cycle.
        mode gcMode

        // Copy of mheap.allspans for marker or sweeper.
        spans []*mspan

        // totaltime is the CPU nanoseconds spent in GC since the
        // program started if debug.gctrace > 0.
        totaltime int64

        // bytesMarked is the number of bytes marked this cycle. This
        // includes bytes blackened in scanned objects, noscan objects
        // that go straight to black, and permagrey objects scanned by
        // markroot during the concurrent scan phase. This is updated
        // atomically during the cycle. Updates may be batched
        // arbitrarily, since the value is only read at the end of the
        // cycle.
        //
        // Because of benign races during marking, this number may not
        // be the exact number of marked bytes, but it should be very
        // close.
        bytesMarked uint64

        // initialHeapLive is the value of memstats.heap_live at the
        // beginning of this GC cycle.
        initialHeapLive uint64

        // assistQueue is a queue of assists that are blocked because
        // there was neither enough credit to steal or enough work to
        // do.
        assistQueue struct {
                lock       mutex
                head, tail guintptr
        }

        // Timing/utilization stats for this cycle.
        stwprocs, maxprocs                 int32
        tSweepTerm, tMark, tMarkTerm, tEnd int64 // nanotime() of phase start

        pauseNS    int64 // total STW time this cycle
        pauseStart int64 // nanotime() of last STW

        // debug.gctrace heap sizes for this cycle.
        heap0, heap1, heap2, heapGoal uint64
}

// GC runs a garbage collection and blocks the caller until the
// garbage collection is complete. It may also block the entire
// program.
func GC() {
        gcStart(gcForceBlockMode, false)
}

// gcMode indicates how concurrent a GC cycle should be.
type gcMode int

const (
        gcBackgroundMode gcMode = iota // concurrent GC and sweep
        gcForceMode                    // stop-the-world GC now, concurrent sweep
        gcForceBlockMode               // stop-the-world GC now and STW sweep
)

// gcShouldStart returns true if the exit condition for the _GCoff
// phase has been met. The exit condition should be tested when
// allocating.
//
// If forceTrigger is true, it ignores the current heap size, but
// checks all other conditions. In general this should be false.
func gcShouldStart(forceTrigger bool) bool {
        return gcphase == _GCoff && (forceTrigger || memstats.heap_live >= memstats.next_gc) && memstats.enablegc && panicking == 0 && gcpercent >= 0
}

// gcStart transitions the GC from _GCoff to _GCmark (if mode ==
// gcBackgroundMode) or _GCmarktermination (if mode !=
// gcBackgroundMode) by performing sweep termination and GC
// initialization.
//
// This may return without performing this transition in some cases,
// such as when called on a system stack or with locks held.
func gcStart(mode gcMode, forceTrigger bool) {
        // Since this is called from malloc and malloc is called in
        // the guts of a number of libraries that might be holding
        // locks, don't attempt to start GC in non-preemptible or
        // potentially unstable situations.
        mp := acquirem()
        if gp := getg(); gp == mp.g0 || mp.locks > 1 || mp.preemptoff != "" {
                releasem(mp)
                return
        }
        releasem(mp)
        mp = nil

        // Pick up the remaining unswept/not being swept spans concurrently
        //
        // This shouldn't happen if we're being invoked in background
        // mode since proportional sweep should have just finished
        // sweeping everything, but rounding errors, etc, may leave a
        // few spans unswept. In forced mode, this is necessary since
        // GC can be forced at any point in the sweeping cycle.
        //
        // We check the transition condition continuously here in case
        // this G gets delayed in to the next GC cycle.
        for (mode != gcBackgroundMode || gcShouldStart(forceTrigger)) && gosweepone() != ^uintptr(0) {
                sweep.nbgsweep++
        }

        // Perform GC initialization and the sweep termination
        // transition.
        //
        // If this is a forced GC, don't acquire the transition lock
        // or re-check the transition condition because we
        // specifically *don't* want to share the transition with
        // another thread.
        useStartSema := mode == gcBackgroundMode
        if useStartSema {
                semacquire(&work.startSema, false)
                // Re-check transition condition under transition lock.
                if !gcShouldStart(forceTrigger) {
                        semrelease(&work.startSema)
                        return
                }
        }

        // In gcstoptheworld debug mode, upgrade the mode accordingly.
        // We do this after re-checking the transition condition so
        // that multiple goroutines that detect the heap trigger don't
        // start multiple STW GCs.
        if mode == gcBackgroundMode {
                if debug.gcstoptheworld == 1 {
                        mode = gcForceMode
                } else if debug.gcstoptheworld == 2 {
                        mode = gcForceBlockMode
                }
        }

        // Ok, we're doing it!  Stop everybody else
        semacquire(&worldsema, false)

        if trace.enabled {
                traceGCStart()
        }

        if mode == gcBackgroundMode {
                gcBgMarkStartWorkers()
        }
        now := nanotime()
        work.stwprocs, work.maxprocs = gcprocs(), gomaxprocs
        work.tSweepTerm = now
        work.heap0 = memstats.heap_live
        work.pauseNS = 0
        work.mode = mode

        work.pauseStart = now
        systemstack(stopTheWorldWithSema)
        // Finish sweep before we start concurrent scan.
        systemstack(func() {
                finishsweep_m(true)
        })
        // clearpools before we start the GC. If we wait they memory will not be
        // reclaimed until the next GC cycle.
        clearpools()

        gcResetMarkState()

        work.finalizersDone = false

        if mode == gcBackgroundMode { // Do as much work concurrently as possible
                gcController.startCycle()
                work.heapGoal = gcController.heapGoal

                // Enter concurrent mark phase and enable
                // write barriers.
                //
                // Because the world is stopped, all Ps will
                // observe that write barriers are enabled by
                // the time we start the world and begin
                // scanning.
                //
                // It's necessary to enable write barriers
                // during the scan phase for several reasons:
                //
                // They must be enabled for writes to higher
                // stack frames before we scan stacks and
                // install stack barriers because this is how
                // we track writes to inactive stack frames.
                // (Alternatively, we could not install stack
                // barriers over frame boundaries with
                // up-pointers).
                //
                // They must be enabled before assists are
                // enabled because they must be enabled before
                // any non-leaf heap objects are marked. Since
                // allocations are blocked until assists can
                // happen, we want enable assists as early as
                // possible.
                setGCPhase(_GCmark)

                // markrootSpans uses work.spans, so make sure
                // it is up to date.
                gcCopySpans()

                gcBgMarkPrepare() // Must happen before assist enable.
                gcMarkRootPrepare()

                // At this point all Ps have enabled the write
                // barrier, thus maintaining the no white to
                // black invariant. Enable mutator assists to
                // put back-pressure on fast allocating
                // mutators.
                atomic.Store(&gcBlackenEnabled, 1)

                // Assists and workers can start the moment we start
                // the world.
                gcController.assistStartTime = now
                gcController.bgMarkStartTime = now

                // Concurrent mark.
                systemstack(startTheWorldWithSema)
                now = nanotime()
                work.pauseNS += now - work.pauseStart
                work.tMark = now
        } else {
                t := nanotime()
                work.tMark, work.tMarkTerm = t, t
                work.heapGoal = work.heap0

                // Perform mark termination. This will restart the world.
                gcMarkTermination()
        }

        if useStartSema {
                semrelease(&work.startSema)
        }
}

// gcMarkDone transitions the GC from mark 1 to mark 2 and from mark 2
// to mark termination.
//
// This should be called when all mark work has been drained. In mark
// 1, this includes all root marking jobs, global work buffers, and
// active work buffers in assists and background workers; however,
// work may still be cached in per-P work buffers. In mark 2, per-P
// caches are disabled.
//
// The calling context must be preemptible.
//
// Note that it is explicitly okay to have write barriers in this
// function because completion of concurrent mark is best-effort
// anyway. Any work created by write barriers here will be cleaned up
// by mark termination.
func gcMarkDone() {
top:
        semacquire(&work.markDoneSema, false)

        // Re-check transition condition under transition lock.
        if !(gcphase == _GCmark && work.nwait == work.nproc && !gcMarkWorkAvailable(nil)) {
                semrelease(&work.markDoneSema)
                return
        }

        // Disallow starting new workers so that any remaining workers
        // in the current mark phase will drain out.
        //
        // TODO(austin): Should dedicated workers keep an eye on this
        // and exit gcDrain promptly?
        atomic.Xaddint64(&gcController.dedicatedMarkWorkersNeeded, -0xffffffff)
        atomic.Xaddint64(&gcController.fractionalMarkWorkersNeeded, -0xffffffff)

        if !gcBlackenPromptly {
                // Transition from mark 1 to mark 2.
                //
                // The global work list is empty, but there can still be work
                // sitting in the per-P work caches and there can be more
                // objects reachable from global roots since they don't have write
                // barriers. Rescan some roots and flush work caches.

                gcMarkRootCheck()

                // Disallow caching workbufs and indicate that we're in mark 2.
                gcBlackenPromptly = true

                // Prevent completion of mark 2 until we've flushed
                // cached workbufs.
                atomic.Xadd(&work.nwait, -1)

                // Rescan global data and BSS. There may still work
                // workers running at this point, so bump "jobs" down
                // before "next" so they won't try running root jobs
                // until we set next.
                atomic.Store(&work.markrootJobs, uint32(fixedRootCount+work.nDataRoots+work.nBSSRoots))
                atomic.Store(&work.markrootNext, fixedRootCount)

                // GC is set up for mark 2. Let Gs blocked on the
                // transition lock go while we flush caches.
                semrelease(&work.markDoneSema)

                systemstack(func() {
                        // Flush all currently cached workbufs and
                        // ensure all Ps see gcBlackenPromptly. This
                        // also blocks until any remaining mark 1
                        // workers have exited their loop so we can
                        // start new mark 2 workers that will observe
                        // the new root marking jobs.
                        forEachP(func(_p_ *p) {
                                _p_.gcw.dispose()
                        })
                })

                // Now we can start up mark 2 workers.
                atomic.Xaddint64(&gcController.dedicatedMarkWorkersNeeded, 0xffffffff)
                atomic.Xaddint64(&gcController.fractionalMarkWorkersNeeded, 0xffffffff)

                incnwait := atomic.Xadd(&work.nwait, +1)
                if incnwait == work.nproc && !gcMarkWorkAvailable(nil) {
                        // This loop will make progress because
                        // gcBlackenPromptly is now true, so it won't
                        // take this same "if" branch.
                        goto top
                }
        } else {
                // Transition to mark termination.
                now := nanotime()
                work.tMarkTerm = now
                work.pauseStart = now
                getg().m.preemptoff = "gcing"
                systemstack(stopTheWorldWithSema)
                // The gcphase is _GCmark, it will transition to _GCmarktermination
                // below. The important thing is that the wb remains active until
                // all marking is complete. This includes writes made by the GC.

                // markroot is done now, so record that objects with
                // finalizers have been scanned.
                work.finalizersDone = true

                // Disable assists and background workers. We must do
                // this before waking blocked assists.
                atomic.Store(&gcBlackenEnabled, 0)

                // Flush the gcWork caches. This must be done before
                // endCycle since endCycle depends on statistics kept
                // in these caches.
                gcFlushGCWork()

                // Wake all blocked assists. These will run when we
                // start the world again.
                gcWakeAllAssists()

                // Likewise, release the transition lock. Blocked
                // workers and assists will run when we start the
                // world again.
                semrelease(&work.markDoneSema)

                gcController.endCycle()

                // Perform mark termination. This will restart the world.
                gcMarkTermination()
        }
}

func gcMarkTermination() {
        // World is stopped.
        // Start marktermination which includes enabling the write barrier.
        atomic.Store(&gcBlackenEnabled, 0)
        gcBlackenPromptly = false
        setGCPhase(_GCmarktermination)

        work.heap1 = memstats.heap_live
        startTime := nanotime()

        mp := acquirem()
        mp.preemptoff = "gcing"
        _g_ := getg()
        _g_.m.traceback = 2
        gp := _g_.m.curg
        casgstatus(gp, _Grunning, _Gwaiting)
        gp.waitreason = "garbage collection"

        // Run gc on the g0 stack.  We do this so that the g stack
        // we're currently running on will no longer change.  Cuts
        // the root set down a bit (g0 stacks are not scanned, and
        // we don't need to scan gc's internal state).  We also
        // need to switch to g0 so we can shrink the stack.
        systemstack(func() {
                gcMark(startTime)
                // Must return immediately.
                // The outer function's stack may have moved
                // during gcMark (it shrinks stacks, including the
                // outer function's stack), so we must not refer
                // to any of its variables. Return back to the
                // non-system stack to pick up the new addresses
                // before continuing.
        })

        systemstack(func() {
                work.heap2 = work.bytesMarked
                if debug.gccheckmark > 0 {
                        // Run a full stop-the-world mark using checkmark bits,
                        // to check that we didn't forget to mark anything during
                        // the concurrent mark process.
                        gcResetMarkState()
                        initCheckmarks()
                        gcMark(startTime)
                        clearCheckmarks()
                }

                // marking is complete so we can turn the write barrier off
                setGCPhase(_GCoff)
                gcSweep(work.mode)

                if debug.gctrace > 1 {
                        startTime = nanotime()
                        // The g stacks have been scanned so
                        // they have gcscanvalid==true and gcworkdone==true.
                        // Reset these so that all stacks will be rescanned.
                        gcResetMarkState()
                        finishsweep_m(true)

                        // Still in STW but gcphase is _GCoff, reset to _GCmarktermination
                        // At this point all objects will be found during the gcMark which
                        // does a complete STW mark and object scan.
                        setGCPhase(_GCmarktermination)
                        gcMark(startTime)
                        setGCPhase(_GCoff) // marking is done, turn off wb.
                        gcSweep(work.mode)
                }
        })

        _g_.m.traceback = 0
        casgstatus(gp, _Gwaiting, _Grunning)

        if trace.enabled {
                traceGCDone()
        }

        // all done
        mp.preemptoff = ""

        if gcphase != _GCoff {
                throw("gc done but gcphase != _GCoff")
        }

        // Update timing memstats
        now, unixNow := nanotime(), unixnanotime()
        work.pauseNS += now - work.pauseStart
        work.tEnd = now
        atomic.Store64(&memstats.last_gc, uint64(unixNow)) // must be Unix time to make sense to user
        memstats.pause_ns[memstats.numgc%uint32(len(memstats.pause_ns))] = uint64(work.pauseNS)
        memstats.pause_end[memstats.numgc%uint32(len(memstats.pause_end))] = uint64(unixNow)
        memstats.pause_total_ns += uint64(work.pauseNS)

        // Update work.totaltime.
        sweepTermCpu := int64(work.stwprocs) * (work.tMark - work.tSweepTerm)
        // We report idle marking time below, but omit it from the
        // overall utilization here since it's "free".
        markCpu := gcController.assistTime + gcController.dedicatedMarkTime + gcController.fractionalMarkTime
        markTermCpu := int64(work.stwprocs) * (work.tEnd - work.tMarkTerm)
        cycleCpu := sweepTermCpu + markCpu + markTermCpu
        work.totaltime += cycleCpu

        // Compute overall GC CPU utilization.
        totalCpu := sched.totaltime + (now-sched.procresizetime)*int64(gomaxprocs)
        memstats.gc_cpu_fraction = float64(work.totaltime) / float64(totalCpu)

        memstats.numgc++

        // Reset sweep state.
        sweep.nbgsweep = 0
        sweep.npausesweep = 0

        systemstack(startTheWorldWithSema)

        // Free stack spans. This must be done between GC cycles.
        systemstack(freeStackSpans)

        // Print gctrace before dropping worldsema. As soon as we drop
        // worldsema another cycle could start and smash the stats
        // we're trying to print.
        if debug.gctrace > 0 {
                util := int(memstats.gc_cpu_fraction * 100)

                var sbuf [24]byte
                printlock()
                print("gc ", memstats.numgc,
                        " @", string(itoaDiv(sbuf[:], uint64(work.tSweepTerm-runtimeInitTime)/1e6, 3)), "s ",
                        util, "%: ")
                prev := work.tSweepTerm
                for i, ns := range []int64{work.tMark, work.tMarkTerm, work.tEnd} {
                        if i != 0 {
                                print("+")
                        }
                        print(string(fmtNSAsMS(sbuf[:], uint64(ns-prev))))
                        prev = ns
                }
                print(" ms clock, ")
                for i, ns := range []int64{sweepTermCpu, gcController.assistTime, gcController.dedicatedMarkTime + gcController.fractionalMarkTime, gcController.idleMarkTime, markTermCpu} {
                        if i == 2 || i == 3 {
                                // Separate mark time components with /.
                                print("/")
                        } else if i != 0 {
                                print("+")
                        }
                        print(string(fmtNSAsMS(sbuf[:], uint64(ns))))
                }
                print(" ms cpu, ",
                        work.heap0>>20, "->", work.heap1>>20, "->", work.heap2>>20, " MB, ",
                        work.heapGoal>>20, " MB goal, ",
                        work.maxprocs, " P")
                if work.mode != gcBackgroundMode {
                        print(" (forced)")
                }
                print("\n")
                printunlock()
        }

        semrelease(&worldsema)
        // Careful: another GC cycle may start now.

        releasem(mp)
        mp = nil

        // now that gc is done, kick off finalizer thread if needed
        if !concurrentSweep {
                // give the queued finalizers, if any, a chance to run
                Gosched()
        }
}

// gcBgMarkStartWorkers prepares background mark worker goroutines.
// These goroutines will not run until the mark phase, but they must
// be started while the work is not stopped and from a regular G
// stack. The caller must hold worldsema.
func gcBgMarkStartWorkers() {
        // Background marking is performed by per-P G's. Ensure that
        // each P has a background GC G.
        for _, p := range &allp {
                if p == nil || p.status == _Pdead {
                        break
                }
                if p.gcBgMarkWorker == 0 {
                        go gcBgMarkWorker(p)
                        notetsleepg(&work.bgMarkReady, -1)
                        noteclear(&work.bgMarkReady)
                }
        }
}

// gcBgMarkPrepare sets up state for background marking.
// Mutator assists must not yet be enabled.
func gcBgMarkPrepare() {
        // Background marking will stop when the work queues are empty
        // and there are no more workers (note that, since this is
        // concurrent, this may be a transient state, but mark
        // termination will clean it up). Between background workers
        // and assists, we don't really know how many workers there
        // will be, so we pretend to have an arbitrarily large number
        // of workers, almost all of which are "waiting". While a
        // worker is working it decrements nwait. If nproc == nwait,
        // there are no workers.
        work.nproc = ^uint32(0)
        work.nwait = ^uint32(0)
}

func gcBgMarkWorker(_p_ *p) {
        type parkInfo struct {
                m      *m // Release this m on park.
                attach *p // If non-nil, attach to this p on park.
        }
        var park parkInfo

        gp := getg()
        park.m = acquirem()
        park.attach = _p_
        // Inform gcBgMarkStartWorkers that this worker is ready.
        // After this point, the background mark worker is scheduled
        // cooperatively by gcController.findRunnable. Hence, it must
        // never be preempted, as this would put it into _Grunnable
        // and put it on a run queue. Instead, when the preempt flag
        // is set, this puts itself into _Gwaiting to be woken up by
        // gcController.findRunnable at the appropriate time.
        notewakeup(&work.bgMarkReady)

        for {
                // Go to sleep until woken by gcContoller.findRunnable.
                // We can't releasem yet since even the call to gopark
                // may be preempted.
                gopark(func(g *g, parkp unsafe.Pointer) bool {
                        park := (*parkInfo)(parkp)

                        // The worker G is no longer running, so it's
                        // now safe to allow preemption.
                        releasem(park.m)

                        // If the worker isn't attached to its P,
                        // attach now. During initialization and after
                        // a phase change, the worker may have been
                        // running on a different P. As soon as we
                        // attach, the owner P may schedule the
                        // worker, so this must be done after the G is
                        // stopped.
                        if park.attach != nil {
                                p := park.attach
                                park.attach = nil
                                // cas the worker because we may be
                                // racing with a new worker starting
                                // on this P.
                                if !p.gcBgMarkWorker.cas(0, guintptr(unsafe.Pointer(g))) {
                                        // The P got a new worker.
                                        // Exit this worker.
                                        return false
                                }
                        }
                        return true
                }, noescape(unsafe.Pointer(&park)), "GC worker (idle)", traceEvGoBlock, 0)

                // Loop until the P dies and disassociates this
                // worker (the P may later be reused, in which case
                // it will get a new worker) or we failed to associate.
                if _p_.gcBgMarkWorker.ptr() != gp {
                        break
                }

                // Disable preemption so we can use the gcw. If the
                // scheduler wants to preempt us, we'll stop draining,
                // dispose the gcw, and then preempt.
                park.m = acquirem()

                if gcBlackenEnabled == 0 {
                        throw("gcBgMarkWorker: blackening not enabled")
                }

                startTime := nanotime()

                decnwait := atomic.Xadd(&work.nwait, -1)
                if decnwait == work.nproc {
                        println("runtime: work.nwait=", decnwait, "work.nproc=", work.nproc)
                        throw("work.nwait was > work.nproc")
                }

                switch _p_.gcMarkWorkerMode {
                default:
                        throw("gcBgMarkWorker: unexpected gcMarkWorkerMode")
                case gcMarkWorkerDedicatedMode:
                        gcDrain(&_p_.gcw, gcDrainNoBlock|gcDrainFlushBgCredit)
                case gcMarkWorkerFractionalMode, gcMarkWorkerIdleMode:
                        gcDrain(&_p_.gcw, gcDrainUntilPreempt|gcDrainFlushBgCredit)
                }

                // If we are nearing the end of mark, dispose
                // of the cache promptly. We must do this
                // before signaling that we're no longer
                // working so that other workers can't observe
                // no workers and no work while we have this
                // cached, and before we compute done.
                if gcBlackenPromptly {
                        _p_.gcw.dispose()
                }

                // Account for time.
                duration := nanotime() - startTime
                switch _p_.gcMarkWorkerMode {
                case gcMarkWorkerDedicatedMode:
                        atomic.Xaddint64(&gcController.dedicatedMarkTime, duration)
                        atomic.Xaddint64(&gcController.dedicatedMarkWorkersNeeded, 1)
                case gcMarkWorkerFractionalMode:
                        atomic.Xaddint64(&gcController.fractionalMarkTime, duration)
                        atomic.Xaddint64(&gcController.fractionalMarkWorkersNeeded, 1)
                case gcMarkWorkerIdleMode:
                        atomic.Xaddint64(&gcController.idleMarkTime, duration)
                }

                // Was this the last worker and did we run out
                // of work?
                incnwait := atomic.Xadd(&work.nwait, +1)
                if incnwait > work.nproc {
                        println("runtime: p.gcMarkWorkerMode=", _p_.gcMarkWorkerMode,
                                "work.nwait=", incnwait, "work.nproc=", work.nproc)
                        throw("work.nwait > work.nproc")
                }

                // If this worker reached a background mark completion
                // point, signal the main GC goroutine.
                if incnwait == work.nproc && !gcMarkWorkAvailable(nil) {
                        // Make this G preemptible and disassociate it
                        // as the worker for this P so
                        // findRunnableGCWorker doesn't try to
                        // schedule it.
                        _p_.gcBgMarkWorker.set(nil)
                        releasem(park.m)

                        gcMarkDone()

                        // Disable preemption and prepare to reattach
                        // to the P.
                        //
                        // We may be running on a different P at this
                        // point, so we can't reattach until this G is
                        // parked.
                        park.m = acquirem()
                        park.attach = _p_
                }
        }
}

// gcMarkWorkAvailable returns true if executing a mark worker
// on p is potentially useful. p may be nil, in which case it only
// checks the global sources of work.
func gcMarkWorkAvailable(p *p) bool {
        if p != nil && !p.gcw.empty() {
                return true
        }
        if atomic.Load64(&work.full) != 0 {
                return true // global work available
        }
        if work.markrootNext < work.markrootJobs {
                return true // root scan work available
        }
        return false
}

// gcFlushGCWork disposes the gcWork caches of all Ps. The world must
// be stopped.
//go:nowritebarrier
func gcFlushGCWork() {
        // Gather all cached GC work. All other Ps are stopped, so
        // it's safe to manipulate their GC work caches.
        for i := 0; i < int(gomaxprocs); i++ {
                allp[i].gcw.dispose()
        }
}

// gcMark runs the mark (or, for concurrent GC, mark termination)
// STW is in effect at this point.
//TODO go:nowritebarrier
func gcMark(start_time int64) {
        if debug.allocfreetrace > 0 {
                tracegc()
        }

        if gcphase != _GCmarktermination {
                throw("in gcMark expecting to see gcphase as _GCmarktermination")
        }
        work.tstart = start_time

        gcCopySpans() // TODO(rlh): should this be hoisted and done only once? Right now it is done for normal marking and also for checkmarking.

        // Make sure the per-P gcWork caches are empty. During mark
        // termination, these caches can still be used temporarily,
        // but must be disposed to the global lists immediately.
        gcFlushGCWork()

        // Queue root marking jobs.
        gcMarkRootPrepare()

        work.nwait = 0
        work.ndone = 0
        work.nproc = uint32(gcprocs())

        if trace.enabled {
                traceGCScanStart()
        }

        if work.nproc > 1 {
                noteclear(&work.alldone)
                helpgc(int32(work.nproc))
        }

        gchelperstart()

        gcw := &getg().m.p.ptr().gcw
        gcDrain(gcw, gcDrainBlock)
        gcw.dispose()

        gcMarkRootCheck()
        if work.full != 0 {
                throw("work.full != 0")
        }

        if work.nproc > 1 {
                notesleep(&work.alldone)
        }

        // markroot is done now, so record that objects with
        // finalizers have been scanned.
        work.finalizersDone = true

        for i := 0; i < int(gomaxprocs); i++ {
                if !allp[i].gcw.empty() {
                        throw("P has cached GC work at end of mark termination")
                }
        }

        if trace.enabled {
                traceGCScanDone()
        }

        cachestats()

        // Compute the reachable heap size at the beginning of the
        // cycle. This is approximately the marked heap size at the
        // end (which we know) minus the amount of marked heap that
        // was allocated after marking began (which we don't know, but
        // is approximately the amount of heap that was allocated
        // since marking began).
        allocatedDuringCycle := memstats.heap_live - work.initialHeapLive
        if memstats.heap_live < work.initialHeapLive {
                // This can happen if mCentral_UncacheSpan tightens
                // the heap_live approximation.
                allocatedDuringCycle = 0
        }
        if work.bytesMarked >= allocatedDuringCycle {
                memstats.heap_reachable = work.bytesMarked - allocatedDuringCycle
        } else {
                // This can happen if most of the allocation during
                // the cycle never became reachable from the heap.
                // Just set the reachable heap approximation to 0 and
                // let the heapminimum kick in below.
                memstats.heap_reachable = 0
        }

        // Trigger the next GC cycle when the allocated heap has grown
        // by triggerRatio over the reachable heap size. Assume that
        // we're in steady state, so the reachable heap size is the
        // same now as it was at the beginning of the GC cycle.
        memstats.next_gc = uint64(float64(memstats.heap_reachable) * (1 + gcController.triggerRatio))
        if memstats.next_gc < heapminimum {
                memstats.next_gc = heapminimum
        }
        if int64(memstats.next_gc) < 0 {
                print("next_gc=", memstats.next_gc, " bytesMarked=", work.bytesMarked, " heap_live=", memstats.heap_live, " initialHeapLive=", work.initialHeapLive, "\n")
                throw("next_gc underflow")
        }

        // Update other GC heap size stats. This must happen after
        // cachestats (which flushes local statistics to these) and
        // flushallmcaches (which modifies heap_live).
        memstats.heap_live = work.bytesMarked
        memstats.heap_marked = work.bytesMarked
        memstats.heap_scan = uint64(gcController.scanWork)

        minNextGC := memstats.heap_live + sweepMinHeapDistance*uint64(gcpercent)/100
        if memstats.next_gc < minNextGC {
                // The allocated heap is already past the trigger.
                // This can happen if the triggerRatio is very low and
                // the reachable heap estimate is less than the live
                // heap size.
                //
                // Concurrent sweep happens in the heap growth from
                // heap_live to next_gc, so bump next_gc up to ensure
                // that concurrent sweep has some heap growth in which
                // to perform sweeping before we start the next GC
                // cycle.
                memstats.next_gc = minNextGC
        }

        if trace.enabled {
                traceHeapAlloc()
                traceNextGC()
        }
}

func gcSweep(mode gcMode) {
        if gcphase != _GCoff {
                throw("gcSweep being done but phase is not GCoff")
        }
        gcCopySpans()

        lock(&mheap_.lock)
        mheap_.sweepgen += 2
        mheap_.sweepdone = 0
        sweep.spanidx = 0
        unlock(&mheap_.lock)

        if !_ConcurrentSweep || mode == gcForceBlockMode {
                // Special case synchronous sweep.
                // Record that no proportional sweeping has to happen.
                lock(&mheap_.lock)
                mheap_.sweepPagesPerByte = 0
                mheap_.pagesSwept = 0
                unlock(&mheap_.lock)
                // Sweep all spans eagerly.
                for sweepone() != ^uintptr(0) {
                        sweep.npausesweep++
                }
                // Do an additional mProf_GC, because all 'free' events are now real as well.
                mProf_GC()
                mProf_GC()
                return
        }

        // Concurrent sweep needs to sweep all of the in-use pages by
        // the time the allocated heap reaches the GC trigger. Compute
        // the ratio of in-use pages to sweep per byte allocated.
        heapDistance := int64(memstats.next_gc) - int64(memstats.heap_live)
        // Add a little margin so rounding errors and concurrent
        // sweep are less likely to leave pages unswept when GC starts.
        heapDistance -= 1024 * 1024
        if heapDistance < _PageSize {
                // Avoid setting the sweep ratio extremely high
                heapDistance = _PageSize
        }
        lock(&mheap_.lock)
        mheap_.sweepPagesPerByte = float64(mheap_.pagesInUse) / float64(heapDistance)
        mheap_.pagesSwept = 0
        mheap_.spanBytesAlloc = 0
        unlock(&mheap_.lock)

        // Background sweep.
        lock(&sweep.lock)
        if sweep.parked {
                sweep.parked = false
                ready(sweep.g, 0)
        }
        unlock(&sweep.lock)
        mProf_GC()
}

func gcCopySpans() {
        // Cache runtime.mheap_.allspans in work.spans to avoid conflicts with
        // resizing/freeing allspans.
        // New spans can be created while GC progresses, but they are not garbage for
        // this round:
        //  - new stack spans can be created even while the world is stopped.
        //  - new malloc spans can be created during the concurrent sweep
        // Even if this is stop-the-world, a concurrent exitsyscall can allocate a stack from heap.
        lock(&mheap_.lock)
        // Free the old cached mark array if necessary.
        if work.spans != nil && &work.spans[0] != &h_allspans[0] {
                sysFree(unsafe.Pointer(&work.spans[0]), uintptr(len(work.spans))*unsafe.Sizeof(work.spans[0]), &memstats.other_sys)
        }
        // Cache the current array for sweeping.
        mheap_.gcspans = mheap_.allspans
        work.spans = h_allspans
        unlock(&mheap_.lock)
}

// gcResetMarkState resets global state prior to marking (concurrent
// or STW) and resets the stack scan state of all Gs. Any Gs created
// after this will also be in the reset state.
func gcResetMarkState() {
        // This may be called during a concurrent phase, so make sure
        // allgs doesn't change.
        lock(&allglock)
        for _, gp := range allgs {
                gp.gcscandone = false  // set to true in gcphasework
                gp.gcscanvalid = false // stack has not been scanned
                gp.gcAssistBytes = 0
        }
        unlock(&allglock)

        work.bytesMarked = 0
        work.initialHeapLive = memstats.heap_live
}

// Hooks for other packages

var poolcleanup func()

//go:linkname sync_runtime_registerPoolCleanup sync.runtime_registerPoolCleanup
func sync_runtime_registerPoolCleanup(f func()) {
        poolcleanup = f
}

func clearpools() {
        // clear sync.Pools
        if poolcleanup != nil {
                poolcleanup()
        }

        // Clear central sudog cache.
        // Leave per-P caches alone, they have strictly bounded size.
        // Disconnect cached list before dropping it on the floor,
        // so that a dangling ref to one entry does not pin all of them.
        lock(&sched.sudoglock)
        var sg, sgnext *sudog
        for sg = sched.sudogcache; sg != nil; sg = sgnext {
                sgnext = sg.next
                sg.next = nil
        }
        sched.sudogcache = nil
        unlock(&sched.sudoglock)

        // Clear central defer pools.
        // Leave per-P pools alone, they have strictly bounded size.
        lock(&sched.deferlock)
        for i := range sched.deferpool {
                // disconnect cached list before dropping it on the floor,
                // so that a dangling ref to one entry does not pin all of them.
                var d, dlink *_defer
                for d = sched.deferpool[i]; d != nil; d = dlink {
                        dlink = d.link
                        d.link = nil
                }
                sched.deferpool[i] = nil
        }
        unlock(&sched.deferlock)
}

// Timing

//go:nowritebarrier
func gchelper() {
        _g_ := getg()
        _g_.m.traceback = 2
        gchelperstart()

        if trace.enabled {
                traceGCScanStart()
        }

        // Parallel mark over GC roots and heap
        if gcphase == _GCmarktermination {
                gcw := &_g_.m.p.ptr().gcw
                gcDrain(gcw, gcDrainBlock) // blocks in getfull
                gcw.dispose()
        }

        if trace.enabled {
                traceGCScanDone()
        }

        nproc := work.nproc // work.nproc can change right after we increment work.ndone
        if atomic.Xadd(&work.ndone, +1) == nproc-1 {
                notewakeup(&work.alldone)
        }
        _g_.m.traceback = 0
}

func gchelperstart() {
        _g_ := getg()

        if _g_.m.helpgc < 0 || _g_.m.helpgc >= _MaxGcproc {
                throw("gchelperstart: bad m->helpgc")
        }
        if _g_ != _g_.m.g0 {
                throw("gchelper not running on g0 stack")
        }
}

// itoaDiv formats val/(10**dec) into buf.
func itoaDiv(buf []byte, val uint64, dec int) []byte {
        i := len(buf) - 1
        idec := i - dec
        for val >= 10 || i >= idec {
                buf[i] = byte(val%10 + '0')
                i--
                if i == idec {
                        buf[i] = '.'
                        i--
                }
                val /= 10
        }
        buf[i] = byte(val + '0')
        return buf[i:]
}

// fmtNSAsMS nicely formats ns nanoseconds as milliseconds.
func fmtNSAsMS(buf []byte, ns uint64) []byte {
        if ns >= 10e6 {
                // Format as whole milliseconds.
                return itoaDiv(buf, ns/1e6, 0)
        }
        // Format two digits of precision, with at most three decimal places.
        x := ns / 1e3
        if x == 0 {
                buf[0] = '0'
                return buf[:1]
        }
        dec := 3
        for x >= 100 {
                x /= 10
                dec--
        }
        return itoaDiv(buf, x, dec)
}