// 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 from the list of all in-use spans in mheap_.sweepSpans.
//
// * 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
started bool
nbgsweep uint32
npausesweep uint32
}
// 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() {
// 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++
}
nextMarkBitArenaEpoch()
}
func bgsweep(c chan int) {
sweep.g = getg()
lock(&sweep.lock)
sweep.parked = true
c <- 1
goparkunlock(&sweep.lock, waitReasonGCSweepWait, traceEvGoBlock, 1)
for {
for sweepone() != ^uintptr(0) {
sweep.nbgsweep++
Gosched()
}
for freeSomeWbufs(true) {
Gosched()
}
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)
}
}
// 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 {
_g_ := getg()
sweepRatio := mheap_.sweepPagesPerByte // For debugging
// 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++
if atomic.Load(&mheap_.sweepdone) != 0 {
_g_.m.locks--
return ^uintptr(0)
}
atomic.Xadd(&mheap_.sweepers, +1)
// Find a span to sweep.
var s *mspan
sg := mheap_.sweepgen
for {
s = mheap_.sweepSpans[1-sg/2%2].pop()
if s == nil {
atomic.Store(&mheap_.sweepdone, 1)
break
}
if s.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 == sg || s.sweepgen == sg+3) {
print("runtime: bad span s.state=", s.state, " s.sweepgen=", s.sweepgen, " sweepgen=", sg, "\n")
throw("non in-use span in unswept list")
}
continue
}
if s.sweepgen == sg-2 && atomic.Cas(&s.sweepgen, sg-2, sg-1) {
break
}
}
// Sweep the span we found.
npages := ^uintptr(0)
if s != nil {
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.
atomic.Xadduintptr(&mheap_.reclaimCredit, 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
}
}
// Decrement the number of active sweepers and if this is the
// last one print trace information.
if atomic.Xadd(&mheap_.sweepers, -1) == 0 && atomic.Load(&mheap_.sweepdone) != 0 {
if debug.gcpacertrace > 0 {
print("pacer: sweep done at heap size ", memstats.heap_live>>20, "MB; allocated ", (memstats.heap_live-mheap_.sweepHeapLiveBasis)>>20, "MB during sweep; swept ", mheap_.pagesSwept, " pages at ", sweepRatio, " pages/byte\n")
}
}
_g_.m.locks--
return npages
}
// isSweepDone reports whether all spans are swept or currently being 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 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
spangen := atomic.Load(&s.sweepgen)
if spangen == sg || spangen == sg+3 {
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 {
spangen := atomic.Load(&s.sweepgen)
if spangen == sg || spangen == sg+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.
//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 {
traceGCSweepSpan(s.npages * _PageSize)
}
atomic.Xadd64(&mheap_.pagesSwept, int64(s.npages))
spc := s.spanclass
size := s.elemsize
res := false
c := _g_.m.mcache
freeToHeap := false
// 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.
specialp := &s.specials
special := *specialp
for special != nil {
// A finalizer can be set for an inner byte of an object, find object beginning.
objIndex := uintptr(special.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 := special; 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 special != nil && uintptr(special.offset) < endOffset {
// Find the exact byte for which the special was setup
// (as opposed to object beginning).
p := s.base() + 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
}
}
if debug.allocfreetrace != 0 || debug.clobberfree != 0 || raceenabled || msanenabled {
// 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)
}
if raceenabled {
racefree(unsafe.Pointer(x), size)
}
if msanenabled {
msanfree(unsafe.Pointer(x), size)
}
}
mbits.advance()
abits.advance()
}
}
// Count the number of free objects in this span.
nalloc := uint16(s.countAlloc())
if spc.sizeclass() == 0 && nalloc == 0 {
s.needzero = 1
freeToHeap = true
}
nfreed := s.allocCount - nalloc
if nalloc > s.allocCount {
print("runtime: nelems=", s.nelems, " nalloc=", nalloc, " previous allocCount=", s.allocCount, " nfreed=", nfreed, "\n")
throw("sweep increased allocation count")
}
s.allocCount = nalloc
wasempty := s.nextFreeIndex() == s.nelems
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)
// 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 || nfreed == 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")
}
// 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 nfreed > 0 && spc.sizeclass() != 0 {
c.local_nsmallfree[spc.sizeclass()] += uintptr(nfreed)
res = mheap_.central[spc].mcentral.freeSpan(s, preserve, wasempty)
// 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(s.base()), size)
} else {
mheap_.freeSpan(s, true)
}
c.local_nlargefree++
c.local_largefree += size
res = true
}
if !res {
// The span has been swept and is still in-use, so put
// it on the swept in-use list.
mheap_.sweepSpans[sweepgen/2%2].push(s)
}
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.
//
// 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 := atomic.Load64(&mheap_.pagesSweptBasis)
// Fix debt if necessary.
newHeapLive := uintptr(atomic.Load64(&memstats.heap_live)-mheap_.sweepHeapLiveBasis) + spanBytes
pagesTarget := int64(mheap_.sweepPagesPerByte*float64(newHeapLive)) - int64(callerSweepPages)
for pagesTarget > int64(atomic.Load64(&mheap_.pagesSwept)-sweptBasis) {
if sweepone() == ^uintptr(0) {
mheap_.sweepPagesPerByte = 0
break
}
if atomic.Load64(&mheap_.pagesSweptBasis) != 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
}
}