runtime: remove the started field from sweepdata

[gostls13.git] / src / runtime / mgcsweep.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.

// Garbage collector: sweeping

// The sweeper consists of two different algorithms:
//
// * The object reclaimer finds and frees unmarked slots in spans. It
//   can free a whole span if none of the objects are marked, but that
//   isn't its goal. This can be driven either synchronously by
//   mcentral.cacheSpan for mcentral spans, or asynchronously by
//   sweepone, which looks at all the mcentral lists.
//
// * The span reclaimer looks for spans that contain no marked objects
//   and frees whole spans. This is a separate algorithm because
//   freeing whole spans is the hardest task for the object reclaimer,
//   but is critical when allocating new spans. The entry point for
//   this is mheap_.reclaim and it's driven by a sequential scan of
//   the page marks bitmap in the heap arenas.
//
// Both algorithms ultimately call mspan.sweep, which sweeps a single
// heap span.

package runtime

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

var sweep sweepdata

// State of background sweep.
type sweepdata struct {
        lock   mutex
        g      *g
        parked bool

        nbgsweep    uint32
        npausesweep uint32

        // active tracks outstanding sweepers and the sweep
        // termination condition.
        active activeSweep

        // centralIndex is the current unswept span class.
        // It represents an index into the mcentral span
        // sets. Accessed and updated via its load and
        // update methods. Not protected by a lock.
        //
        // Reset at mark termination.
        // Used by mheap.nextSpanForSweep.
        centralIndex sweepClass
}

// sweepClass is a spanClass and one bit to represent whether we're currently
// sweeping partial or full spans.
type sweepClass uint32

const (
        numSweepClasses            = numSpanClasses * 2
        sweepClassDone  sweepClass = sweepClass(^uint32(0))
)

func (s *sweepClass) load() sweepClass {
        return sweepClass(atomic.Load((*uint32)(s)))
}

func (s *sweepClass) update(sNew sweepClass) {
        // Only update *s if its current value is less than sNew,
        // since *s increases monotonically.
        sOld := s.load()
        for sOld < sNew && !atomic.Cas((*uint32)(s), uint32(sOld), uint32(sNew)) {
                sOld = s.load()
        }
        // TODO(mknyszek): This isn't the only place we have
        // an atomic monotonically increasing counter. It would
        // be nice to have an "atomic max" which is just implemented
        // as the above on most architectures. Some architectures
        // like RISC-V however have native support for an atomic max.
}

func (s *sweepClass) clear() {
        atomic.Store((*uint32)(s), 0)
}

// split returns the underlying span class as well as
// whether we're interested in the full or partial
// unswept lists for that class, indicated as a boolean
// (true means "full").
func (s sweepClass) split() (spc spanClass, full bool) {
        return spanClass(s >> 1), s&1 == 0
}

// nextSpanForSweep finds and pops the next span for sweeping from the
// central sweep buffers. It returns ownership of the span to the caller.
// Returns nil if no such span exists.
func (h *mheap) nextSpanForSweep() *mspan {
        sg := h.sweepgen
        for sc := sweep.centralIndex.load(); sc < numSweepClasses; sc++ {
                spc, full := sc.split()
                c := &h.central[spc].mcentral
                var s *mspan
                if full {
                        s = c.fullUnswept(sg).pop()
                } else {
                        s = c.partialUnswept(sg).pop()
                }
                if s != nil {
                        // Write down that we found something so future sweepers
                        // can start from here.
                        sweep.centralIndex.update(sc)
                        return s
                }
        }
        // Write down that we found nothing.
        sweep.centralIndex.update(sweepClassDone)
        return nil
}

const sweepDrainedMask = 1 << 31

// activeSweep is a type that captures whether sweeping
// is done, and whether there are any outstanding sweepers.
//
// Every potential sweeper must call begin() before they look
// for work, and end() after they've finished sweeping.
type activeSweep struct {
        // state is divided into two parts.
        //
        // The top bit (masked by sweepDrainedMask) is a boolean
        // value indicating whether all the sweep work has been
        // drained from the queue.
        //
        // The rest of the bits are a counter, indicating the
        // number of outstanding concurrent sweepers.
        state atomic.Uint32
}

// begin registers a new sweeper. Returns a sweepLocker
// for acquiring spans for sweeping. Any outstanding sweeper blocks
// sweep termination.
//
// If the sweepLocker is invalid, the caller can be sure that all
// outstanding sweep work has been drained, so there is nothing left
// to sweep. Note that there may be sweepers currently running, so
// this does not indicate that all sweeping has completed.
//
// Even if the sweepLocker is invalid, its sweepGen is always valid.
func (a *activeSweep) begin() sweepLocker {
        for {
                state := a.state.Load()
                if state&sweepDrainedMask != 0 {
                        return sweepLocker{mheap_.sweepgen, false}
                }
                if a.state.CompareAndSwap(state, state+1) {
                        return sweepLocker{mheap_.sweepgen, true}
                }
        }
}

// end deregisters a sweeper. Must be called once for each time
// begin is called if the sweepLocker is valid.
func (a *activeSweep) end(sl sweepLocker) {
        if sl.sweepGen != mheap_.sweepgen {
                throw("sweeper left outstanding across sweep generations")
        }
        for {
                state := a.state.Load()
                if (state&^sweepDrainedMask)-1 >= sweepDrainedMask {
                        throw("mismatched begin/end of activeSweep")
                }
                if a.state.CompareAndSwap(state, state-1) {
                        if state != sweepDrainedMask {
                                return
                        }
                        if debug.gcpacertrace > 0 {
                                live := gcController.heapLive.Load()
                                print("pacer: sweep done at heap size ", live>>20, "MB; allocated ", (live-mheap_.sweepHeapLiveBasis)>>20, "MB during sweep; swept ", mheap_.pagesSwept.Load(), " pages at ", mheap_.sweepPagesPerByte, " pages/byte\n")
                        }
                        return
                }
        }
}

// markDrained marks the active sweep cycle as having drained
// all remaining work. This is safe to be called concurrently
// with all other methods of activeSweep, though may race.
//
// Returns true if this call was the one that actually performed
// the mark.
func (a *activeSweep) markDrained() bool {
        for {
                state := a.state.Load()
                if state&sweepDrainedMask != 0 {
                        return false
                }
                if a.state.CompareAndSwap(state, state|sweepDrainedMask) {
                        return true
                }
        }
}

// sweepers returns the current number of active sweepers.
func (a *activeSweep) sweepers() uint32 {
        return a.state.Load() &^ sweepDrainedMask
}

// isDone returns true if all sweep work has been drained and no more
// outstanding sweepers exist. That is, when the sweep phase is
// completely done.
func (a *activeSweep) isDone() bool {
        return a.state.Load() == sweepDrainedMask
}

// reset sets up the activeSweep for the next sweep cycle.
//
// The world must be stopped.
func (a *activeSweep) reset() {
        assertWorldStopped()
        a.state.Store(0)
}

// finishsweep_m ensures that all spans are swept.
//
// The world must be stopped. This ensures there are no sweeps in
// progress.
//
//go:nowritebarrier
func finishsweep_m() {
        assertWorldStopped()

        // Sweeping must be complete before marking commences, so
        // sweep any unswept spans. If this is a concurrent GC, there
        // shouldn't be any spans left to sweep, so this should finish
        // instantly. If GC was forced before the concurrent sweep
        // finished, there may be spans to sweep.
        for sweepone() != ^uintptr(0) {
                sweep.npausesweep++
        }

        // Make sure there aren't any outstanding sweepers left.
        // At this point, with the world stopped, it means one of two
        // things. Either we were able to preempt a sweeper, or that
        // a sweeper didn't call sweep.active.end when it should have.
        // Both cases indicate a bug, so throw.
        if sweep.active.sweepers() != 0 {
                throw("active sweepers found at start of mark phase")
        }

        // Reset all the unswept buffers, which should be empty.
        // Do this in sweep termination as opposed to mark termination
        // so that we can catch unswept spans and reclaim blocks as
        // soon as possible.
        sg := mheap_.sweepgen
        for i := range mheap_.central {
                c := &mheap_.central[i].mcentral
                c.partialUnswept(sg).reset()
                c.fullUnswept(sg).reset()
        }

        // Sweeping is done, so if the scavenger isn't already awake,
        // wake it up. There's definitely work for it to do at this
        // point.
        scavenger.wake()

        nextMarkBitArenaEpoch()
}

func bgsweep(c chan int) {
        sweep.g = getg()

        lockInit(&sweep.lock, lockRankSweep)
        lock(&sweep.lock)
        sweep.parked = true
        c <- 1
        goparkunlock(&sweep.lock, waitReasonGCSweepWait, traceEvGoBlock, 1)

        for {
                // bgsweep attempts to be a "low priority" goroutine by intentionally
                // yielding time. It's OK if it doesn't run, because goroutines allocating
                // memory will sweep and ensure that all spans are swept before the next
                // GC cycle. We really only want to run when we're idle.
                //
                // However, calling Gosched after each span swept produces a tremendous
                // amount of tracing events, sometimes up to 50% of events in a trace. It's
                // also inefficient to call into the scheduler so much because sweeping a
                // single span is in general a very fast operation, taking as little as 30 ns
                // on modern hardware. (See #54767.)
                //
                // As a result, bgsweep sweeps in batches, and only calls into the scheduler
                // at the end of every batch. Furthermore, it only yields its time if there
                // isn't spare idle time available on other cores. If there's available idle
                // time, helping to sweep can reduce allocation latencies by getting ahead of
                // the proportional sweeper and having spans ready to go for allocation.
                const sweepBatchSize = 10
                nSwept := 0
                for sweepone() != ^uintptr(0) {
                        sweep.nbgsweep++
                        nSwept++
                        if nSwept%sweepBatchSize == 0 {
                                goschedIfBusy()
                        }
                }
                for freeSomeWbufs(true) {
                        // N.B. freeSomeWbufs is already batched internally.
                        goschedIfBusy()
                }
                lock(&sweep.lock)
                if !isSweepDone() {
                        // This can happen if a GC runs between
                        // gosweepone returning ^0 above
                        // and the lock being acquired.
                        unlock(&sweep.lock)
                        continue
                }
                sweep.parked = true
                goparkunlock(&sweep.lock, waitReasonGCSweepWait, traceEvGoBlock, 1)
        }
}

// sweepLocker acquires sweep ownership of spans.
type sweepLocker struct {
        // sweepGen is the sweep generation of the heap.
        sweepGen uint32
        valid    bool
}

// sweepLocked represents sweep ownership of a span.
type sweepLocked struct {
        *mspan
}

// tryAcquire attempts to acquire sweep ownership of span s. If it
// successfully acquires ownership, it blocks sweep completion.
func (l *sweepLocker) tryAcquire(s *mspan) (sweepLocked, bool) {
        if !l.valid {
                throw("use of invalid sweepLocker")
        }
        // Check before attempting to CAS.
        if atomic.Load(&s.sweepgen) != l.sweepGen-2 {
                return sweepLocked{}, false
        }
        // Attempt to acquire sweep ownership of s.
        if !atomic.Cas(&s.sweepgen, l.sweepGen-2, l.sweepGen-1) {
                return sweepLocked{}, false
        }
        return sweepLocked{s}, true
}

// sweepone sweeps some unswept heap span and returns the number of pages returned
// to the heap, or ^uintptr(0) if there was nothing to sweep.
func sweepone() uintptr {
        gp := getg()

        // Increment locks to ensure that the goroutine is not preempted
        // in the middle of sweep thus leaving the span in an inconsistent state for next GC
        gp.m.locks++

        // TODO(austin): sweepone is almost always called in a loop;
        // lift the sweepLocker into its callers.
        sl := sweep.active.begin()
        if !sl.valid {
                gp.m.locks--
                return ^uintptr(0)
        }

        // Find a span to sweep.
        npages := ^uintptr(0)
        var noMoreWork bool
        for {
                s := mheap_.nextSpanForSweep()
                if s == nil {
                        noMoreWork = sweep.active.markDrained()
                        break
                }
                if state := s.state.get(); state != mSpanInUse {
                        // This can happen if direct sweeping already
                        // swept this span, but in that case the sweep
                        // generation should always be up-to-date.
                        if !(s.sweepgen == sl.sweepGen || s.sweepgen == sl.sweepGen+3) {
                                print("runtime: bad span s.state=", state, " s.sweepgen=", s.sweepgen, " sweepgen=", sl.sweepGen, "\n")
                                throw("non in-use span in unswept list")
                        }
                        continue
                }
                if s, ok := sl.tryAcquire(s); ok {
                        // Sweep the span we found.
                        npages = s.npages
                        if s.sweep(false) {
                                // Whole span was freed. Count it toward the
                                // page reclaimer credit since these pages can
                                // now be used for span allocation.
                                mheap_.reclaimCredit.Add(npages)
                        } else {
                                // Span is still in-use, so this returned no
                                // pages to the heap and the span needs to
                                // move to the swept in-use list.
                                npages = 0
                        }
                        break
                }
        }
        sweep.active.end(sl)

        if noMoreWork {
                // The sweep list is empty. There may still be
                // concurrent sweeps running, but we're at least very
                // close to done sweeping.

                // Move the scavenge gen forward (signaling
                // that there's new work to do) and wake the scavenger.
                //
                // The scavenger is signaled by the last sweeper because once
                // sweeping is done, we will definitely have useful work for
                // the scavenger to do, since the scavenger only runs over the
                // heap once per GC cycle. This update is not done during sweep
                // termination because in some cases there may be a long delay
                // between sweep done and sweep termination (e.g. not enough
                // allocations to trigger a GC) which would be nice to fill in
                // with scavenging work.
                if debug.scavtrace > 0 {
                        systemstack(func() {
                                lock(&mheap_.lock)
                                released := atomic.Loaduintptr(&mheap_.pages.scav.released)
                                printScavTrace(released, false)
                                atomic.Storeuintptr(&mheap_.pages.scav.released, 0)
                                unlock(&mheap_.lock)
                        })
                }
                scavenger.ready()
        }

        gp.m.locks--
        return npages
}

// isSweepDone reports whether all spans are swept.
//
// Note that this condition may transition from false to true at any
// time as the sweeper runs. It may transition from true to false if a
// GC runs; to prevent that the caller must be non-preemptible or must
// somehow block GC progress.
func isSweepDone() bool {
        return sweep.active.isDone()
}

// Returns only when span s has been swept.
//
//go:nowritebarrier
func (s *mspan) ensureSwept() {
        // Caller must disable preemption.
        // Otherwise when this function returns the span can become unswept again
        // (if GC is triggered on another goroutine).
        gp := getg()
        if gp.m.locks == 0 && gp.m.mallocing == 0 && gp != gp.m.g0 {
                throw("mspan.ensureSwept: m is not locked")
        }

        // If this operation fails, then that means that there are
        // no more spans to be swept. In this case, either s has already
        // been swept, or is about to be acquired for sweeping and swept.
        sl := sweep.active.begin()
        if sl.valid {
                // The caller must be sure that the span is a mSpanInUse span.
                if s, ok := sl.tryAcquire(s); ok {
                        s.sweep(false)
                        sweep.active.end(sl)
                        return
                }
                sweep.active.end(sl)
        }

        // Unfortunately we can't sweep the span ourselves. Somebody else
        // got to it first. We don't have efficient means to wait, but that's
        // OK, it will be swept fairly soon.
        for {
                spangen := atomic.Load(&s.sweepgen)
                if spangen == sl.sweepGen || spangen == sl.sweepGen+3 {
                        break
                }
                osyield()
        }
}

// Sweep frees or collects finalizers for blocks not marked in the mark phase.
// It clears the mark bits in preparation for the next GC round.
// Returns true if the span was returned to heap.
// If preserve=true, don't return it to heap nor relink in mcentral lists;
// caller takes care of it.
func (sl *sweepLocked) sweep(preserve bool) bool {
        // It's critical that we enter this function with preemption disabled,
        // GC must not start while we are in the middle of this function.
        gp := getg()
        if gp.m.locks == 0 && gp.m.mallocing == 0 && gp != gp.m.g0 {
                throw("mspan.sweep: m is not locked")
        }

        s := sl.mspan
        if !preserve {
                // We'll release ownership of this span. Nil it out to
                // prevent the caller from accidentally using it.
                sl.mspan = nil
        }

        sweepgen := mheap_.sweepgen
        if state := s.state.get(); state != mSpanInUse || s.sweepgen != sweepgen-1 {
                print("mspan.sweep: state=", state, " sweepgen=", s.sweepgen, " mheap.sweepgen=", sweepgen, "\n")
                throw("mspan.sweep: bad span state")
        }

        if trace.enabled {
                traceGCSweepSpan(s.npages * _PageSize)
        }

        mheap_.pagesSwept.Add(int64(s.npages))

        spc := s.spanclass
        size := s.elemsize

        // The allocBits indicate which unmarked objects don't need to be
        // processed since they were free at the end of the last GC cycle
        // and were not allocated since then.
        // If the allocBits index is >= s.freeindex and the bit
        // is not marked then the object remains unallocated
        // since the last GC.
        // This situation is analogous to being on a freelist.

        // Unlink & free special records for any objects we're about to free.
        // Two complications here:
        // 1. An object can have both finalizer and profile special records.
        //    In such case we need to queue finalizer for execution,
        //    mark the object as live and preserve the profile special.
        // 2. A tiny object can have several finalizers setup for different offsets.
        //    If such object is not marked, we need to queue all finalizers at once.
        // Both 1 and 2 are possible at the same time.
        hadSpecials := s.specials != nil
        siter := newSpecialsIter(s)
        for siter.valid() {
                // A finalizer can be set for an inner byte of an object, find object beginning.
                objIndex := uintptr(siter.s.offset) / size
                p := s.base() + objIndex*size
                mbits := s.markBitsForIndex(objIndex)
                if !mbits.isMarked() {
                        // This object is not marked and has at least one special record.
                        // Pass 1: see if it has at least one finalizer.
                        hasFin := false
                        endOffset := p - s.base() + size
                        for tmp := siter.s; tmp != nil && uintptr(tmp.offset) < endOffset; tmp = tmp.next {
                                if tmp.kind == _KindSpecialFinalizer {
                                        // Stop freeing of object if it has a finalizer.
                                        mbits.setMarkedNonAtomic()
                                        hasFin = true
                                        break
                                }
                        }
                        // Pass 2: queue all finalizers _or_ handle profile record.
                        for siter.valid() && uintptr(siter.s.offset) < endOffset {
                                // Find the exact byte for which the special was setup
                                // (as opposed to object beginning).
                                special := siter.s
                                p := s.base() + uintptr(special.offset)
                                if special.kind == _KindSpecialFinalizer || !hasFin {
                                        siter.unlinkAndNext()
                                        freeSpecial(special, unsafe.Pointer(p), size)
                                } else {
                                        // The object has finalizers, so we're keeping it alive.
                                        // All other specials only apply when an object is freed,
                                        // so just keep the special record.
                                        siter.next()
                                }
                        }
                } else {
                        // object is still live
                        if siter.s.kind == _KindSpecialReachable {
                                special := siter.unlinkAndNext()
                                (*specialReachable)(unsafe.Pointer(special)).reachable = true
                                freeSpecial(special, unsafe.Pointer(p), size)
                        } else {
                                // keep special record
                                siter.next()
                        }
                }
        }
        if hadSpecials && s.specials == nil {
                spanHasNoSpecials(s)
        }

        if debug.allocfreetrace != 0 || debug.clobberfree != 0 || raceenabled || msanenabled || asanenabled {
                // Find all newly freed objects. This doesn't have to
                // efficient; allocfreetrace has massive overhead.
                mbits := s.markBitsForBase()
                abits := s.allocBitsForIndex(0)
                for i := uintptr(0); i < s.nelems; i++ {
                        if !mbits.isMarked() && (abits.index < s.freeindex || abits.isMarked()) {
                                x := s.base() + i*s.elemsize
                                if debug.allocfreetrace != 0 {
                                        tracefree(unsafe.Pointer(x), size)
                                }
                                if debug.clobberfree != 0 {
                                        clobberfree(unsafe.Pointer(x), size)
                                }
                                // User arenas are handled on explicit free.
                                if raceenabled && !s.isUserArenaChunk {
                                        racefree(unsafe.Pointer(x), size)
                                }
                                if msanenabled && !s.isUserArenaChunk {
                                        msanfree(unsafe.Pointer(x), size)
                                }
                                if asanenabled && !s.isUserArenaChunk {
                                        asanpoison(unsafe.Pointer(x), size)
                                }
                        }
                        mbits.advance()
                        abits.advance()
                }
        }

        // Check for zombie objects.
        if s.freeindex < s.nelems {
                // Everything < freeindex is allocated and hence
                // cannot be zombies.
                //
                // Check the first bitmap byte, where we have to be
                // careful with freeindex.
                obj := s.freeindex
                if (*s.gcmarkBits.bytep(obj / 8)&^*s.allocBits.bytep(obj / 8))>>(obj%8) != 0 {
                        s.reportZombies()
                }
                // Check remaining bytes.
                for i := obj/8 + 1; i < divRoundUp(s.nelems, 8); i++ {
                        if *s.gcmarkBits.bytep(i)&^*s.allocBits.bytep(i) != 0 {
                                s.reportZombies()
                        }
                }
        }

        // Count the number of free objects in this span.
        nalloc := uint16(s.countAlloc())
        nfreed := s.allocCount - nalloc
        if nalloc > s.allocCount {
                // The zombie check above should have caught this in
                // more detail.
                print("runtime: nelems=", s.nelems, " nalloc=", nalloc, " previous allocCount=", s.allocCount, " nfreed=", nfreed, "\n")
                throw("sweep increased allocation count")
        }

        s.allocCount = nalloc
        s.freeindex = 0 // reset allocation index to start of span.
        if trace.enabled {
                getg().m.p.ptr().traceReclaimed += uintptr(nfreed) * s.elemsize
        }

        // gcmarkBits becomes the allocBits.
        // get a fresh cleared gcmarkBits in preparation for next GC
        s.allocBits = s.gcmarkBits
        s.gcmarkBits = newMarkBits(s.nelems)

        // Initialize alloc bits cache.
        s.refillAllocCache(0)

        // The span must be in our exclusive ownership until we update sweepgen,
        // check for potential races.
        if state := s.state.get(); state != mSpanInUse || s.sweepgen != sweepgen-1 {
                print("mspan.sweep: state=", state, " sweepgen=", s.sweepgen, " mheap.sweepgen=", sweepgen, "\n")
                throw("mspan.sweep: bad span state after sweep")
        }
        if s.sweepgen == sweepgen+1 || s.sweepgen == sweepgen+3 {
                throw("swept cached span")
        }

        // We need to set s.sweepgen = h.sweepgen only when all blocks are swept,
        // because of the potential for a concurrent free/SetFinalizer.
        //
        // But we need to set it before we make the span available for allocation
        // (return it to heap or mcentral), because allocation code assumes that a
        // span is already swept if available for allocation.
        //
        // Serialization point.
        // At this point the mark bits are cleared and allocation ready
        // to go so release the span.
        atomic.Store(&s.sweepgen, sweepgen)

        if s.isUserArenaChunk {
                if preserve {
                        // This is a case that should never be handled by a sweeper that
                        // preserves the span for reuse.
                        throw("sweep: tried to preserve a user arena span")
                }
                if nalloc > 0 {
                        // There still exist pointers into the span or the span hasn't been
                        // freed yet. It's not ready to be reused. Put it back on the
                        // full swept list for the next cycle.
                        mheap_.central[spc].mcentral.fullSwept(sweepgen).push(s)
                        return false
                }

                // It's only at this point that the sweeper doesn't actually need to look
                // at this arena anymore, so subtract from pagesInUse now.
                mheap_.pagesInUse.Add(-s.npages)
                s.state.set(mSpanDead)

                // The arena is ready to be recycled. Remove it from the quarantine list
                // and place it on the ready list. Don't add it back to any sweep lists.
                systemstack(func() {
                        // It's the arena code's responsibility to get the chunk on the quarantine
                        // list by the time all references to the chunk are gone.
                        if s.list != &mheap_.userArena.quarantineList {
                                throw("user arena span is on the wrong list")
                        }
                        lock(&mheap_.lock)
                        mheap_.userArena.quarantineList.remove(s)
                        mheap_.userArena.readyList.insert(s)
                        unlock(&mheap_.lock)
                })
                return false
        }

        if spc.sizeclass() != 0 {
                // Handle spans for small objects.
                if nfreed > 0 {
                        // Only mark the span as needing zeroing if we've freed any
                        // objects, because a fresh span that had been allocated into,
                        // wasn't totally filled, but then swept, still has all of its
                        // free slots zeroed.
                        s.needzero = 1
                        stats := memstats.heapStats.acquire()
                        atomic.Xadd64(&stats.smallFreeCount[spc.sizeclass()], int64(nfreed))
                        memstats.heapStats.release()

                        // Count the frees in the inconsistent, internal stats.
                        gcController.totalFree.Add(int64(nfreed) * int64(s.elemsize))
                }
                if !preserve {
                        // The caller may not have removed this span from whatever
                        // unswept set its on but taken ownership of the span for
                        // sweeping by updating sweepgen. If this span still is in
                        // an unswept set, then the mcentral will pop it off the
                        // set, check its sweepgen, and ignore it.
                        if nalloc == 0 {
                                // Free totally free span directly back to the heap.
                                mheap_.freeSpan(s)
                                return true
                        }
                        // Return span back to the right mcentral list.
                        if uintptr(nalloc) == s.nelems {
                                mheap_.central[spc].mcentral.fullSwept(sweepgen).push(s)
                        } else {
                                mheap_.central[spc].mcentral.partialSwept(sweepgen).push(s)
                        }
                }
        } else if !preserve {
                // Handle spans for large objects.
                if nfreed != 0 {
                        // Free large object span to heap.

                        // NOTE(rsc,dvyukov): The original implementation of efence
                        // in CL 22060046 used sysFree instead of sysFault, so that
                        // the operating system would eventually give the memory
                        // back to us again, so that an efence program could run
                        // longer without running out of memory. Unfortunately,
                        // calling sysFree here without any kind of adjustment of the
                        // heap data structures means that when the memory does
                        // come back to us, we have the wrong metadata for it, either in
                        // the mspan structures or in the garbage collection bitmap.
                        // Using sysFault here means that the program will run out of
                        // memory fairly quickly in efence mode, but at least it won't
                        // have mysterious crashes due to confused memory reuse.
                        // It should be possible to switch back to sysFree if we also
                        // implement and then call some kind of mheap.deleteSpan.
                        if debug.efence > 0 {
                                s.limit = 0 // prevent mlookup from finding this span
                                sysFault(unsafe.Pointer(s.base()), size)
                        } else {
                                mheap_.freeSpan(s)
                        }

                        // Count the free in the consistent, external stats.
                        stats := memstats.heapStats.acquire()
                        atomic.Xadd64(&stats.largeFreeCount, 1)
                        atomic.Xadd64(&stats.largeFree, int64(size))
                        memstats.heapStats.release()

                        // Count the free in the inconsistent, internal stats.
                        gcController.totalFree.Add(int64(size))

                        return true
                }

                // Add a large span directly onto the full+swept list.
                mheap_.central[spc].mcentral.fullSwept(sweepgen).push(s)
        }
        return false
}

// reportZombies reports any marked but free objects in s and throws.
//
// This generally means one of the following:
//
// 1. User code converted a pointer to a uintptr and then back
// unsafely, and a GC ran while the uintptr was the only reference to
// an object.
//
// 2. User code (or a compiler bug) constructed a bad pointer that
// points to a free slot, often a past-the-end pointer.
//
// 3. The GC two cycles ago missed a pointer and freed a live object,
// but it was still live in the last cycle, so this GC cycle found a
// pointer to that object and marked it.
func (s *mspan) reportZombies() {
        printlock()
        print("runtime: marked free object in span ", s, ", elemsize=", s.elemsize, " freeindex=", s.freeindex, " (bad use of unsafe.Pointer? try -d=checkptr)\n")
        mbits := s.markBitsForBase()
        abits := s.allocBitsForIndex(0)
        for i := uintptr(0); i < s.nelems; i++ {
                addr := s.base() + i*s.elemsize
                print(hex(addr))
                alloc := i < s.freeindex || abits.isMarked()
                if alloc {
                        print(" alloc")
                } else {
                        print(" free ")
                }
                if mbits.isMarked() {
                        print(" marked  ")
                } else {
                        print(" unmarked")
                }
                zombie := mbits.isMarked() && !alloc
                if zombie {
                        print(" zombie")
                }
                print("\n")
                if zombie {
                        length := s.elemsize
                        if length > 1024 {
                                length = 1024
                        }
                        hexdumpWords(addr, addr+length, nil)
                }
                mbits.advance()
                abits.advance()
        }
        throw("found pointer to free object")
}

// deductSweepCredit deducts sweep credit for allocating a span of
// size spanBytes. This must be performed *before* the span is
// allocated to ensure the system has enough credit. If necessary, it
// performs sweeping to prevent going in to debt. If the caller will
// also sweep pages (e.g., for a large allocation), it can pass a
// non-zero callerSweepPages to leave that many pages unswept.
//
// deductSweepCredit makes a worst-case assumption that all spanBytes
// bytes of the ultimately allocated span will be available for object
// allocation.
//
// deductSweepCredit is the core of the "proportional sweep" system.
// It uses statistics gathered by the garbage collector to perform
// enough sweeping so that all pages are swept during the concurrent
// sweep phase between GC cycles.
//
// mheap_ must NOT be locked.
func deductSweepCredit(spanBytes uintptr, callerSweepPages uintptr) {
        if mheap_.sweepPagesPerByte == 0 {
                // Proportional sweep is done or disabled.
                return
        }

        if trace.enabled {
                traceGCSweepStart()
        }

retry:
        sweptBasis := mheap_.pagesSweptBasis.Load()

        // Fix debt if necessary.
        newHeapLive := uintptr(gcController.heapLive.Load()-mheap_.sweepHeapLiveBasis) + spanBytes
        pagesTarget := int64(mheap_.sweepPagesPerByte*float64(newHeapLive)) - int64(callerSweepPages)
        for pagesTarget > int64(mheap_.pagesSwept.Load()-sweptBasis) {
                if sweepone() == ^uintptr(0) {
                        mheap_.sweepPagesPerByte = 0
                        break
                }
                if mheap_.pagesSweptBasis.Load() != sweptBasis {
                        // Sweep pacing changed. Recompute debt.
                        goto retry
                }
        }

        if trace.enabled {
                traceGCSweepDone()
        }
}

// clobberfree sets the memory content at x to bad content, for debugging
// purposes.
func clobberfree(x unsafe.Pointer, size uintptr) {
        // size (span.elemsize) is always a multiple of 4.
        for i := uintptr(0); i < size; i += 4 {
                *(*uint32)(add(x, i)) = 0xdeadbeef
        }
}

// gcPaceSweeper updates the sweeper's pacing parameters.
//
// Must be called whenever the GC's pacing is updated.
//
// The world must be stopped, or mheap_.lock must be held.
func gcPaceSweeper(trigger uint64) {
        assertWorldStoppedOrLockHeld(&mheap_.lock)

        // Update sweep pacing.
        if isSweepDone() {
                mheap_.sweepPagesPerByte = 0
        } else {
                // 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, accounting for the fact that
                // some might already be swept.
                heapLiveBasis := gcController.heapLive.Load()
                heapDistance := int64(trigger) - int64(heapLiveBasis)
                // 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
                }
                pagesSwept := mheap_.pagesSwept.Load()
                pagesInUse := mheap_.pagesInUse.Load()
                sweepDistancePages := int64(pagesInUse) - int64(pagesSwept)
                if sweepDistancePages <= 0 {
                        mheap_.sweepPagesPerByte = 0
                } else {
                        mheap_.sweepPagesPerByte = float64(sweepDistancePages) / float64(heapDistance)
                        mheap_.sweepHeapLiveBasis = heapLiveBasis
                        // Write pagesSweptBasis last, since this
                        // signals concurrent sweeps to recompute
                        // their debt.
                        mheap_.pagesSweptBasis.Store(pagesSwept)
                }
        }
}