// 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 "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() {
// The world is stopped so we should be able to complete the sweeps
// quickly.
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 code is not required.
sg := mheap_.sweepgen
for _, s := range work.spans {
if s.sweepgen != sg && s.state == _MSpanInUse {
mSpan_EnsureSwept(s)
}
}
}
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 := xadd(&sweep.spanidx, 1) - 1
if idx >= uint32(len(work.spans)) {
mheap_.sweepdone = 1
_g_.m.locks--
return ^uintptr(0)
}
s := work.spans[idx]
if s.state != mSpanInUse {
s.sweepgen = sg
continue
}
if s.sweepgen != sg-2 || !cas(&s.sweepgen, sg-2, sg-1) {
continue
}
npages := s.npages
if !mSpan_Sweep(s, 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 mSpan_EnsureSwept(s *mspan) {
// 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 atomicload(&s.sweepgen) == sg {
return
}
// The caller must be sure that the span is a MSpanInUse span.
if cas(&s.sweepgen, sg-2, sg-1) {
mSpan_Sweep(s, false)
return
}
// unfortunate condition, and we don't have efficient means to wait
for atomicload(&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 mSpan_Sweep(s *mspan, 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()
}
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.
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() {
// Find the exact byte for which the special was setup
// (as opposed to object beginning).
p := uintptr(s.start<<_PageShift) + uintptr(special.offset)
// about to free object: splice out special record
y := special
special = special.next
*specialp = special
if !freespecial(y, unsafe.Pointer(p), size, false) {
// stop freeing of object if it has a finalizer
hbits.setMarkedNonAtomic()
}
} 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)
}
// 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
// important to set sweepgen before returning it to heap
atomicstore(&s.sweepgen, sweepgen)
// Free the span after heapBitsSweepSpan
// returns, since it's not done with the span.
freeToHeap = true
} else {
// Free small object.
if size > 2*ptrSize {
*(*uintptr)(unsafe.Pointer(p + ptrSize)) = uintptrMask & 0xdeaddeaddeaddead // mark as "needs to be zeroed"
} else if size > ptrSize {
*(*uintptr)(unsafe.Pointer(p + 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.
//
// TODO(austin): Clean this up by consolidating atomicstore in
// large span path above with this.
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")
}
atomicstore(&s.sweepgen, sweepgen)
}
if nfree > 0 {
c.local_nsmallfree[cl] += uintptr(nfree)
res = mCentral_FreeSpan(&mheap_.central[cl].mcentral, 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_Free(&mheap_, 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 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 := xadd64(&mheap_.spanBytesAlloc, int64(spanBytes))
// Fix debt if necessary.
pagesOwed := int64(mheap_.sweepPagesPerByte * float64(spanBytesAlloc))
for pagesOwed-int64(atomicload64(&mheap_.pagesSwept)) > int64(callerSweepPages) {
if gosweepone() == ^uintptr(0) {
mheap_.sweepPagesPerByte = 0
break
}
}
}
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()
}