src/runtime/mgcsweep.go - go - Git at Google

 // 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

 package runtime

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

 var sweep sweepdata

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

 	spanidx uint32 // background sweeper position

 	nbgsweep    uint32
 	npausesweep uint32
 }

 //go:nowritebarrier
 func finishsweep_m(stw bool) {
 	// 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++
 	}

 	// There may be some other spans being swept concurrently that
 	// we need to wait for. If finishsweep_m is done with the world stopped
 	// this is not required because the STW must have waited for sweeps.
 	//
 	// TODO(austin): As of this writing, we always pass true for stw.
 	// Consider removing this code.
 	if !stw {
 		sg := mheap_.sweepgen
 		for _, s := range work.spans {
 			if s.sweepgen != sg && s.state == _MSpanInUse {
 				s.ensureSwept()
 			}
 		}
 	}
 }

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

 	lock(&sweep.lock)
 	sweep.parked = true
 	c <- 1
 	goparkunlock(&sweep.lock, "GC sweep wait", traceEvGoBlock, 1)

 	for {
 		for gosweepone() != ^uintptr(0) {
 			sweep.nbgsweep++
 			Gosched()
 		}
 		lock(&sweep.lock)
 		if !gosweepdone() {
 			// 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, "GC sweep wait", traceEvGoBlock, 1)
 	}
 }

 // sweeps one span
 // returns number of pages returned to heap, or ^uintptr(0) if there is nothing to sweep
 //go:nowritebarrier
 func sweepone() uintptr {
 	_g_ := 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
 	_g_.m.locks++
 	sg := mheap_.sweepgen
 	for {
 		idx := atomic.Xadd(&sweep.spanidx, 1) - 1
 		if idx >= uint32(len(work.spans)) {
 			mheap_.sweepdone = 1
 			_g_.m.locks--
 			if debug.gcpacertrace > 0 && idx == uint32(len(work.spans)) {
 				print("pacer: sweep done at heap size ", memstats.heap_live>>20, "MB; allocated ", mheap_.spanBytesAlloc>>20, "MB of spans; swept ", mheap_.pagesSwept, " pages\n")
 			}
 			return ^uintptr(0)
 		}
 		s := work.spans[idx]
 		if s.state != mSpanInUse {
 			s.sweepgen = sg
 			continue
 		}
 		if s.sweepgen != sg-2 || !atomic.Cas(&s.sweepgen, sg-2, sg-1) {
 			continue
 		}
 		npages := s.npages
 		if !s.sweep(false) {
 			npages = 0
 		}
 		_g_.m.locks--
 		return npages
 	}
 }

 //go:nowritebarrier
 func gosweepone() uintptr {
 	var ret uintptr
 	systemstack(func() {
 		ret = sweepone()
 	})
 	return ret
 }

 //go:nowritebarrier
 func gosweepdone() bool {
 	return mheap_.sweepdone != 0
 }

 // 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).
 	_g_ := getg()
 	if _g_.m.locks == 0 && _g_.m.mallocing == 0 && _g_ != _g_.m.g0 {
 		throw("MSpan_EnsureSwept: m is not locked")
 	}

 	sg := mheap_.sweepgen
 	if atomic.Load(&s.sweepgen) == sg {
 		return
 	}
 	// The caller must be sure that the span is a MSpanInUse span.
 	if atomic.Cas(&s.sweepgen, sg-2, sg-1) {
 		s.sweep(false)
 		return
 	}
 	// unfortunate condition, and we don't have efficient means to wait
 	for atomic.Load(&s.sweepgen) != sg {
 		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.
 //TODO go:nowritebarrier
 func (s *mspan) 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.
 	_g_ := getg()
 	if _g_.m.locks == 0 && _g_.m.mallocing == 0 && _g_ != _g_.m.g0 {
 		throw("MSpan_Sweep: m is not locked")
 	}
 	sweepgen := mheap_.sweepgen
 	if s.state != mSpanInUse || s.sweepgen != sweepgen-1 {
 		print("MSpan_Sweep: state=", s.state, " sweepgen=", s.sweepgen, " mheap.sweepgen=", sweepgen, "\n")
 		throw("MSpan_Sweep: bad span state")
 	}

 	if trace.enabled {
 		traceGCSweepStart()
 	}

 	atomic.Xadd64(&mheap_.pagesSwept, int64(s.npages))

 	cl := s.sizeclass
 	size := s.elemsize
 	res := false
 	nfree := 0

 	var head, end gclinkptr

 	c := _g_.m.mcache
 	freeToHeap := false

 	// Mark any free objects in this span so we don't collect them.
 	sstart := uintptr(s.start << _PageShift)
 	for link := s.freelist; link.ptr() != nil; link = link.ptr().next {
 		if uintptr(link) < sstart || s.limit <= uintptr(link) {
 			// Free list is corrupted.
 			dumpFreeList(s)
 			throw("free list corrupted")
 		}
 		heapBitsForAddr(uintptr(link)).setMarkedNonAtomic()
 	}

 	// 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.
 	specialp := &s.specials
 	special := *specialp
 	for special != nil {
 		// A finalizer can be set for an inner byte of an object, find object beginning.
 		p := uintptr(s.start<<_PageShift) + uintptr(special.offset)/size*size
 		hbits := heapBitsForAddr(p)
 		if !hbits.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 - uintptr(s.start<<_PageShift) + size
 			for tmp := special; tmp != nil && uintptr(tmp.offset) < endOffset; tmp = tmp.next {
 				if tmp.kind == _KindSpecialFinalizer {
 					// Stop freeing of object if it has a finalizer.
 					hbits.setMarkedNonAtomic()
 					hasFin = true
 					break
 				}
 			}
 			// Pass 2: queue all finalizers _or_ handle profile record.
 			for special != nil && uintptr(special.offset) < endOffset {
 				// Find the exact byte for which the special was setup
 				// (as opposed to object beginning).
 				p := uintptr(s.start<<_PageShift) + uintptr(special.offset)
 				if special.kind == _KindSpecialFinalizer || !hasFin {
 					// Splice out special record.
 					y := special
 					special = special.next
 					*specialp = special
 					freespecial(y, unsafe.Pointer(p), size)
 				} else {
 					// This is profile record, but the object has finalizers (so kept alive).
 					// Keep special record.
 					specialp = &special.next
 					special = *specialp
 				}
 			}
 		} else {
 			// object is still live: keep special record
 			specialp = &special.next
 			special = *specialp
 		}
 	}

 	// Sweep through n objects of given size starting at p.
 	// This thread owns the span now, so it can manipulate
 	// the block bitmap without atomic operations.

 	size, n, _ := s.layout()
 	heapBitsSweepSpan(s.base(), size, n, func(p uintptr) {
 		// At this point we know that we are looking at garbage object
 		// that needs to be collected.
 		if debug.allocfreetrace != 0 {
 			tracefree(unsafe.Pointer(p), size)
 		}
 		if msanenabled {
 			msanfree(unsafe.Pointer(p), size)
 		}

 		// Reset to allocated+noscan.
 		if cl == 0 {
 			// Free large span.
 			if preserve {
 				throw("can't preserve large span")
 			}
 			heapBitsForSpan(p).initSpan(s.layout())
 			s.needzero = 1

 			// Free the span after heapBitsSweepSpan
 			// returns, since it's not done with the span.
 			freeToHeap = true
 		} else {
 			// Free small object.
 			if size > 2*sys.PtrSize {
 				*(*uintptr)(unsafe.Pointer(p + sys.PtrSize)) = uintptrMask & 0xdeaddeaddeaddead // mark as "needs to be zeroed"
 			} else if size > sys.PtrSize {
 				*(*uintptr)(unsafe.Pointer(p + sys.PtrSize)) = 0
 			}
 			if head.ptr() == nil {
 				head = gclinkptr(p)
 			} else {
 				end.ptr().next = gclinkptr(p)
 			}
 			end = gclinkptr(p)
 			end.ptr().next = gclinkptr(0x0bade5)
 			nfree++
 		}
 	})

 	// 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.
 	if freeToHeap || nfree == 0 {
 		// The span must be in our exclusive ownership until we update sweepgen,
 		// check for potential races.
 		if s.state != mSpanInUse || s.sweepgen != sweepgen-1 {
 			print("MSpan_Sweep: state=", s.state, " sweepgen=", s.sweepgen, " mheap.sweepgen=", sweepgen, "\n")
 			throw("MSpan_Sweep: bad span state after sweep")
 		}
 		atomic.Store(&s.sweepgen, sweepgen)
 	}
 	if nfree > 0 {
 		c.local_nsmallfree[cl] += uintptr(nfree)
 		res = mheap_.central[cl].mcentral.freeSpan(s, int32(nfree), head, end, preserve)
 		// MCentral_FreeSpan updates sweepgen
 	} else if freeToHeap {
 		// Free large 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(uintptr(s.start<<_PageShift)), size)
 		} else {
 			mheap_.freeSpan(s, 1)
 		}
 		c.local_nlargefree++
 		c.local_largefree += size
 		res = true
 	}
 	if trace.enabled {
 		traceGCSweepDone()
 	}
 	return res
 }

 // 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. The caller should call reimburseSweepCredit if that
 // turns out not to be the case once the span is allocated.
 //
 // 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
 	}

 	// Account for this span allocation.
 	spanBytesAlloc := atomic.Xadd64(&mheap_.spanBytesAlloc, int64(spanBytes))

 	// Fix debt if necessary.
 	pagesOwed := int64(mheap_.sweepPagesPerByte * float64(spanBytesAlloc))
 	for pagesOwed-int64(atomic.Load64(&mheap_.pagesSwept)) > int64(callerSweepPages) {
 		if gosweepone() == ^uintptr(0) {
 			mheap_.sweepPagesPerByte = 0
 			break
 		}
 	}
 }

 // reimburseSweepCredit records that unusableBytes bytes of a
 // just-allocated span are not available for object allocation. This
 // offsets the worst-case charge performed by deductSweepCredit.
 func reimburseSweepCredit(unusableBytes uintptr) {
 	if mheap_.sweepPagesPerByte == 0 {
 		// Nobody cares about the credit. Avoid the atomic.
 		return
 	}
 	if int64(atomic.Xadd64(&mheap_.spanBytesAlloc, -int64(unusableBytes))) < 0 {
 		throw("spanBytesAlloc underflow")
 	}
 }

 func dumpFreeList(s *mspan) {
 	printlock()
 	print("runtime: free list of span ", s, ":\n")
 	sstart := uintptr(s.start << _PageShift)
 	link := s.freelist
 	for i := 0; i < int(s.npages*_PageSize/s.elemsize); i++ {
 		if i != 0 {
 			print(" -> ")
 		}
 		print(hex(link))
 		if link.ptr() == nil {
 			break
 		}
 		if uintptr(link) < sstart || s.limit <= uintptr(link) {
 			// Bad link. Stop walking before we crash.
 			print(" (BAD)")
 			break
 		}
 		link = link.ptr().next
 	}
 	print("\n")
 	printunlock()
 }
	// 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

	package runtime

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

	var sweep sweepdata

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

	spanidx uint32 // background sweeper position

	nbgsweep uint32
	npausesweep uint32
	}

	//go:nowritebarrier
	func finishsweep_m(stw bool) {
	// 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++
	}

	// There may be some other spans being swept concurrently that
	// we need to wait for. If finishsweep_m is done with the world stopped
	// this is not required because the STW must have waited for sweeps.
	//
	// TODO(austin): As of this writing, we always pass true for stw.
	// Consider removing this code.
	if !stw {
	sg := mheap_.sweepgen
	for _, s := range work.spans {
	if s.sweepgen != sg && s.state == _MSpanInUse {
	s.ensureSwept()
	}
	}
	}
	}

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

	lock(&sweep.lock)
	sweep.parked = true
	c <- 1
	goparkunlock(&sweep.lock, "GC sweep wait", traceEvGoBlock, 1)

	for {
	for gosweepone() != ^uintptr(0) {
	sweep.nbgsweep++
	Gosched()
	}
	lock(&sweep.lock)
	if !gosweepdone() {
	// 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, "GC sweep wait", traceEvGoBlock, 1)
	}
	}

	// sweeps one span
	// returns number of pages returned to heap, or ^uintptr(0) if there is nothing to sweep
	//go:nowritebarrier
	func sweepone() uintptr {
	_g_ := 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
	_g_.m.locks++
	sg := mheap_.sweepgen
	for {
	idx := atomic.Xadd(&sweep.spanidx, 1) - 1
	if idx >= uint32(len(work.spans)) {
	mheap_.sweepdone = 1
	_g_.m.locks--
	if debug.gcpacertrace > 0 && idx == uint32(len(work.spans)) {
	print("pacer: sweep done at heap size ", memstats.heap_live>>20, "MB; allocated ", mheap_.spanBytesAlloc>>20, "MB of spans; swept ", mheap_.pagesSwept, " pages\n")
	}
	return ^uintptr(0)
	}
	s := work.spans[idx]
	if s.state != mSpanInUse {
	s.sweepgen = sg
	continue
	}
	if s.sweepgen != sg-2 \|\| !atomic.Cas(&s.sweepgen, sg-2, sg-1) {
	continue
	}
	npages := s.npages
	if !s.sweep(false) {
	npages = 0
	}
	_g_.m.locks--
	return npages
	}
	}

	//go:nowritebarrier
	func gosweepone() uintptr {
	var ret uintptr
	systemstack(func() {
	ret = sweepone()
	})
	return ret
	}

	//go:nowritebarrier
	func gosweepdone() bool {
	return mheap_.sweepdone != 0
	}

	// 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).
	_g_ := getg()
	if _g_.m.locks == 0 && _g_.m.mallocing == 0 && _g_ != _g_.m.g0 {
	throw("MSpan_EnsureSwept: m is not locked")
	}

	sg := mheap_.sweepgen
	if atomic.Load(&s.sweepgen) == sg {
	return
	}
	// The caller must be sure that the span is a MSpanInUse span.
	if atomic.Cas(&s.sweepgen, sg-2, sg-1) {
	s.sweep(false)
	return
	}
	// unfortunate condition, and we don't have efficient means to wait
	for atomic.Load(&s.sweepgen) != sg {
	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.
	//TODO go:nowritebarrier
	func (s *mspan) 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.
	_g_ := getg()
	if _g_.m.locks == 0 && _g_.m.mallocing == 0 && _g_ != _g_.m.g0 {
	throw("MSpan_Sweep: m is not locked")
	}
	sweepgen := mheap_.sweepgen
	if s.state != mSpanInUse \|\| s.sweepgen != sweepgen-1 {
	print("MSpan_Sweep: state=", s.state, " sweepgen=", s.sweepgen, " mheap.sweepgen=", sweepgen, "\n")
	throw("MSpan_Sweep: bad span state")
	}

	if trace.enabled {
	traceGCSweepStart()
	}

	atomic.Xadd64(&mheap_.pagesSwept, int64(s.npages))

	cl := s.sizeclass
	size := s.elemsize
	res := false
	nfree := 0

	var head, end gclinkptr

	c := _g_.m.mcache
	freeToHeap := false

	// Mark any free objects in this span so we don't collect them.
	sstart := uintptr(s.start << _PageShift)
	for link := s.freelist; link.ptr() != nil; link = link.ptr().next {
	if uintptr(link) < sstart \|\| s.limit <= uintptr(link) {
	// Free list is corrupted.
	dumpFreeList(s)
	throw("free list corrupted")
	}
	heapBitsForAddr(uintptr(link)).setMarkedNonAtomic()
	}

	// 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.
	specialp := &s.specials
	special := *specialp
	for special != nil {
	// A finalizer can be set for an inner byte of an object, find object beginning.
	p := uintptr(s.start<<_PageShift) + uintptr(special.offset)/size*size
	hbits := heapBitsForAddr(p)
	if !hbits.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 - uintptr(s.start<<_PageShift) + size
	for tmp := special; tmp != nil && uintptr(tmp.offset) < endOffset; tmp = tmp.next {
	if tmp.kind == _KindSpecialFinalizer {
	// Stop freeing of object if it has a finalizer.
	hbits.setMarkedNonAtomic()
	hasFin = true
	break
	}
	}
	// Pass 2: queue all finalizers _or_ handle profile record.
	for special != nil && uintptr(special.offset) < endOffset {
	// Find the exact byte for which the special was setup
	// (as opposed to object beginning).
	p := uintptr(s.start<<_PageShift) + uintptr(special.offset)
	if special.kind == _KindSpecialFinalizer \|\| !hasFin {
	// Splice out special record.
	y := special
	special = special.next
	*specialp = special
	freespecial(y, unsafe.Pointer(p), size)
	} else {
	// This is profile record, but the object has finalizers (so kept alive).
	// Keep special record.
	specialp = &special.next
	special = *specialp
	}
	}
	} else {
	// object is still live: keep special record
	specialp = &special.next
	special = *specialp
	}
	}

	// Sweep through n objects of given size starting at p.
	// This thread owns the span now, so it can manipulate
	// the block bitmap without atomic operations.

	size, n, _ := s.layout()
	heapBitsSweepSpan(s.base(), size, n, func(p uintptr) {
	// At this point we know that we are looking at garbage object
	// that needs to be collected.
	if debug.allocfreetrace != 0 {
	tracefree(unsafe.Pointer(p), size)
	}
	if msanenabled {
	msanfree(unsafe.Pointer(p), size)
	}

	// Reset to allocated+noscan.
	if cl == 0 {
	// Free large span.
	if preserve {
	throw("can't preserve large span")
	}
	heapBitsForSpan(p).initSpan(s.layout())
	s.needzero = 1

	// Free the span after heapBitsSweepSpan
	// returns, since it's not done with the span.
	freeToHeap = true
	} else {
	// Free small object.
	if size > 2*sys.PtrSize {
	(uintptr)(unsafe.Pointer(p + sys.PtrSize)) = uintptrMask & 0xdeaddeaddeaddead // mark as "needs to be zeroed"
	} else if size > sys.PtrSize {
	(uintptr)(unsafe.Pointer(p + sys.PtrSize)) = 0
	}
	if head.ptr() == nil {
	head = gclinkptr(p)
	} else {
	end.ptr().next = gclinkptr(p)
	}
	end = gclinkptr(p)
	end.ptr().next = gclinkptr(0x0bade5)
	nfree++
	}
	})

	// 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.
	if freeToHeap \|\| nfree == 0 {
	// The span must be in our exclusive ownership until we update sweepgen,
	// check for potential races.
	if s.state != mSpanInUse \|\| s.sweepgen != sweepgen-1 {
	print("MSpan_Sweep: state=", s.state, " sweepgen=", s.sweepgen, " mheap.sweepgen=", sweepgen, "\n")
	throw("MSpan_Sweep: bad span state after sweep")
	}
	atomic.Store(&s.sweepgen, sweepgen)
	}
	if nfree > 0 {
	c.local_nsmallfree[cl] += uintptr(nfree)
	res = mheap_.central[cl].mcentral.freeSpan(s, int32(nfree), head, end, preserve)
	// MCentral_FreeSpan updates sweepgen
	} else if freeToHeap {
	// Free large 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(uintptr(s.start<<_PageShift)), size)
	} else {
	mheap_.freeSpan(s, 1)
	}
	c.local_nlargefree++
	c.local_largefree += size
	res = true
	}
	if trace.enabled {
	traceGCSweepDone()
	}
	return res
	}

	// 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. The caller should call reimburseSweepCredit if that
	// turns out not to be the case once the span is allocated.
	//
	// 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
	}

	// Account for this span allocation.
	spanBytesAlloc := atomic.Xadd64(&mheap_.spanBytesAlloc, int64(spanBytes))

	// Fix debt if necessary.
	pagesOwed := int64(mheap_.sweepPagesPerByte * float64(spanBytesAlloc))
	for pagesOwed-int64(atomic.Load64(&mheap_.pagesSwept)) > int64(callerSweepPages) {
	if gosweepone() == ^uintptr(0) {
	mheap_.sweepPagesPerByte = 0
	break
	}
	}
	}

	// reimburseSweepCredit records that unusableBytes bytes of a
	// just-allocated span are not available for object allocation. This
	// offsets the worst-case charge performed by deductSweepCredit.
	func reimburseSweepCredit(unusableBytes uintptr) {
	if mheap_.sweepPagesPerByte == 0 {
	// Nobody cares about the credit. Avoid the atomic.
	return
	}
	if int64(atomic.Xadd64(&mheap_.spanBytesAlloc, -int64(unusableBytes))) < 0 {
	throw("spanBytesAlloc underflow")
	}
	}

	func dumpFreeList(s *mspan) {
	printlock()
	print("runtime: free list of span ", s, ":\n")
	sstart := uintptr(s.start << _PageShift)
	link := s.freelist
	for i := 0; i < int(s.npages*_PageSize/s.elemsize); i++ {
	if i != 0 {
	print(" -> ")
	}
	print(hex(link))
	if link.ptr() == nil {
	break
	}
	if uintptr(link) < sstart \|\| s.limit <= uintptr(link) {
	// Bad link. Stop walking before we crash.
	print(" (BAD)")
	break
	}
	link = link.ptr().next
	}
	print("\n")
	printunlock()
	}