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// 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
started bool
nbgsweep uint32
npausesweep uint32
// 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
}
// 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++
}
// 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.
wakeScavenger()
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 {
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_.nextSpanForSweep()
if s == nil {
atomic.Store(&mheap_.sweepdone, 1)
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 == sg || s.sweepgen == sg+3) {
print("runtime: bad span s.state=", 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 {
// Since the sweeper is done, move the scavenge gen forward (signalling
// 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 cyle. 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.
systemstack(func() {
lock(&mheap_.lock)
mheap_.pages.scavengeStartGen()
unlock(&mheap_.lock)
})
// Since we might sweep in an allocation path, it's not possible
// for us to wake the scavenger directly via wakeScavenger, since
// it could allocate. Ask sysmon to do it for us instead.
readyForScavenger()
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.
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 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)
}
atomic.Xadd64(&mheap_.pagesSwept, 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
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 hadSpecials && s.specials == nil {
spanHasNoSpecials(s)
}
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()
}
}
// 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 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.Xadduintptr(&stats.smallFreeCount[spc.sizeclass()], uintptr(nfreed))
memstats.heapStats.release()
}
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)
}
stats := memstats.heapStats.acquire()
atomic.Xadduintptr(&stats.largeFreeCount, 1)
atomic.Xadduintptr(&stats.largeFree, size)
memstats.heapStats.release()
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 := 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
}
}