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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.
// Central free lists.
//
// See malloc.go for an overview.
//
// The mcentral doesn't actually contain the list of free objects; the mspan does.
// Each mcentral is two lists of mspans: those with free objects (c->nonempty)
// and those that are completely allocated (c->empty).
package runtime
import "runtime/internal/atomic"
// Central list of free objects of a given size.
//
//go:notinheap
type mcentral struct {
lock mutex
spanclass spanClass
// For !go115NewMCentralImpl.
nonempty mSpanList // list of spans with a free object, ie a nonempty free list
empty mSpanList // list of spans with no free objects (or cached in an mcache)
// partial and full contain two mspan sets: one of swept in-use
// spans, and one of unswept in-use spans. These two trade
// roles on each GC cycle. The unswept set is drained either by
// allocation or by the background sweeper in every GC cycle,
// so only two roles are necessary.
//
// sweepgen is increased by 2 on each GC cycle, so the swept
// spans are in partial[sweepgen/2%2] and the unswept spans are in
// partial[1-sweepgen/2%2]. Sweeping pops spans from the
// unswept set and pushes spans that are still in-use on the
// swept set. Likewise, allocating an in-use span pushes it
// on the swept set.
//
// Some parts of the sweeper can sweep arbitrary spans, and hence
// can't remove them from the unswept set, but will add the span
// to the appropriate swept list. As a result, the parts of the
// sweeper and mcentral that do consume from the unswept list may
// encounter swept spans, and these should be ignored.
partial [2]spanSet // list of spans with a free object
full [2]spanSet // list of spans with no free objects
// nmalloc is the cumulative count of objects allocated from
// this mcentral, assuming all spans in mcaches are
// fully-allocated. Written atomically, read under STW.
nmalloc uint64
}
// Initialize a single central free list.
func (c *mcentral) init(spc spanClass) {
c.spanclass = spc
if go115NewMCentralImpl {
lockInit(&c.partial[0].spineLock, lockRankSpanSetSpine)
lockInit(&c.partial[1].spineLock, lockRankSpanSetSpine)
lockInit(&c.full[0].spineLock, lockRankSpanSetSpine)
lockInit(&c.full[1].spineLock, lockRankSpanSetSpine)
} else {
c.nonempty.init()
c.empty.init()
lockInit(&c.lock, lockRankMcentral)
}
}
// partialUnswept returns the spanSet which holds partially-filled
// unswept spans for this sweepgen.
func (c *mcentral) partialUnswept(sweepgen uint32) *spanSet {
return &c.partial[1-sweepgen/2%2]
}
// partialSwept returns the spanSet which holds partially-filled
// swept spans for this sweepgen.
func (c *mcentral) partialSwept(sweepgen uint32) *spanSet {
return &c.partial[sweepgen/2%2]
}
// fullUnswept returns the spanSet which holds unswept spans without any
// free slots for this sweepgen.
func (c *mcentral) fullUnswept(sweepgen uint32) *spanSet {
return &c.full[1-sweepgen/2%2]
}
// fullSwept returns the spanSet which holds swept spans without any
// free slots for this sweepgen.
func (c *mcentral) fullSwept(sweepgen uint32) *spanSet {
return &c.full[sweepgen/2%2]
}
// Allocate a span to use in an mcache.
func (c *mcentral) cacheSpan() *mspan {
if !go115NewMCentralImpl {
return c.oldCacheSpan()
}
// Deduct credit for this span allocation and sweep if necessary.
spanBytes := uintptr(class_to_allocnpages[c.spanclass.sizeclass()]) * _PageSize
deductSweepCredit(spanBytes, 0)
sg := mheap_.sweepgen
traceDone := false
if trace.enabled {
traceGCSweepStart()
}
// If we sweep spanBudget spans without finding any free
// space, just allocate a fresh span. This limits the amount
// of time we can spend trying to find free space and
// amortizes the cost of small object sweeping over the
// benefit of having a full free span to allocate from. By
// setting this to 100, we limit the space overhead to 1%.
//
// TODO(austin,mknyszek): This still has bad worst-case
// throughput. For example, this could find just one free slot
// on the 100th swept span. That limits allocation latency, but
// still has very poor throughput. We could instead keep a
// running free-to-used budget and switch to fresh span
// allocation if the budget runs low.
spanBudget := 100
var s *mspan
// Try partial swept spans first.
if s = c.partialSwept(sg).pop(); s != nil {
goto havespan
}
// Now try partial unswept spans.
for ; spanBudget >= 0; spanBudget-- {
s = c.partialUnswept(sg).pop()
if s == nil {
break
}
if atomic.Load(&s.sweepgen) == sg-2 && atomic.Cas(&s.sweepgen, sg-2, sg-1) {
// We got ownership of the span, so let's sweep it and use it.
s.sweep(true)
goto havespan
}
// We failed to get ownership of the span, which means it's being or
// has been swept by an asynchronous sweeper that just couldn't remove it
// from the unswept list. That sweeper took ownership of the span and
// responsibility for either freeing it to the heap or putting it on the
// right swept list. Either way, we should just ignore it (and it's unsafe
// for us to do anything else).
}
// Now try full unswept spans, sweeping them and putting them into the
// right list if we fail to get a span.
for ; spanBudget >= 0; spanBudget-- {
s = c.fullUnswept(sg).pop()
if s == nil {
break
}
if atomic.Load(&s.sweepgen) == sg-2 && atomic.Cas(&s.sweepgen, sg-2, sg-1) {
// We got ownership of the span, so let's sweep it.
s.sweep(true)
// Check if there's any free space.
freeIndex := s.nextFreeIndex()
if freeIndex != s.nelems {
s.freeindex = freeIndex
goto havespan
}
// Add it to the swept list, because sweeping didn't give us any free space.
c.fullSwept(sg).push(s)
}
// See comment for partial unswept spans.
}
if trace.enabled {
traceGCSweepDone()
traceDone = true
}
// We failed to get a span from the mcentral so get one from mheap.
s = c.grow()
if s == nil {
return nil
}
// At this point s is a span that should have free slots.
havespan:
if trace.enabled && !traceDone {
traceGCSweepDone()
}
n := int(s.nelems) - int(s.allocCount)
if n == 0 || s.freeindex == s.nelems || uintptr(s.allocCount) == s.nelems {
throw("span has no free objects")
}
// Assume all objects from this span will be allocated in the
// mcache. If it gets uncached, we'll adjust this.
atomic.Xadd64(&c.nmalloc, int64(n))
usedBytes := uintptr(s.allocCount) * s.elemsize
atomic.Xadd64(&memstats.heap_live, int64(spanBytes)-int64(usedBytes))
if trace.enabled {
// heap_live changed.
traceHeapAlloc()
}
if gcBlackenEnabled != 0 {
// heap_live changed.
gcController.revise()
}
freeByteBase := s.freeindex &^ (64 - 1)
whichByte := freeByteBase / 8
// Init alloc bits cache.
s.refillAllocCache(whichByte)
// Adjust the allocCache so that s.freeindex corresponds to the low bit in
// s.allocCache.
s.allocCache >>= s.freeindex % 64
return s
}
// Allocate a span to use in an mcache.
//
// For !go115NewMCentralImpl.
func (c *mcentral) oldCacheSpan() *mspan {
// Deduct credit for this span allocation and sweep if necessary.
spanBytes := uintptr(class_to_allocnpages[c.spanclass.sizeclass()]) * _PageSize
deductSweepCredit(spanBytes, 0)
lock(&c.lock)
traceDone := false
if trace.enabled {
traceGCSweepStart()
}
sg := mheap_.sweepgen
retry:
var s *mspan
for s = c.nonempty.first; s != nil; s = s.next {
if s.sweepgen == sg-2 && atomic.Cas(&s.sweepgen, sg-2, sg-1) {
c.nonempty.remove(s)
c.empty.insertBack(s)
unlock(&c.lock)
s.sweep(true)
goto havespan
}
if s.sweepgen == sg-1 {
// the span is being swept by background sweeper, skip
continue
}
// we have a nonempty span that does not require sweeping, allocate from it
c.nonempty.remove(s)
c.empty.insertBack(s)
unlock(&c.lock)
goto havespan
}
for s = c.empty.first; s != nil; s = s.next {
if s.sweepgen == sg-2 && atomic.Cas(&s.sweepgen, sg-2, sg-1) {
// we have an empty span that requires sweeping,
// sweep it and see if we can free some space in it
c.empty.remove(s)
// swept spans are at the end of the list
c.empty.insertBack(s)
unlock(&c.lock)
s.sweep(true)
freeIndex := s.nextFreeIndex()
if freeIndex != s.nelems {
s.freeindex = freeIndex
goto havespan
}
lock(&c.lock)
// the span is still empty after sweep
// it is already in the empty list, so just retry
goto retry
}
if s.sweepgen == sg-1 {
// the span is being swept by background sweeper, skip
continue
}
// already swept empty span,
// all subsequent ones must also be either swept or in process of sweeping
break
}
if trace.enabled {
traceGCSweepDone()
traceDone = true
}
unlock(&c.lock)
// Replenish central list if empty.
s = c.grow()
if s == nil {
return nil
}
lock(&c.lock)
c.empty.insertBack(s)
unlock(&c.lock)
// At this point s is a non-empty span, queued at the end of the empty list,
// c is unlocked.
havespan:
if trace.enabled && !traceDone {
traceGCSweepDone()
}
n := int(s.nelems) - int(s.allocCount)
if n == 0 || s.freeindex == s.nelems || uintptr(s.allocCount) == s.nelems {
throw("span has no free objects")
}
// Assume all objects from this span will be allocated in the
// mcache. If it gets uncached, we'll adjust this.
atomic.Xadd64(&c.nmalloc, int64(n))
usedBytes := uintptr(s.allocCount) * s.elemsize
atomic.Xadd64(&memstats.heap_live, int64(spanBytes)-int64(usedBytes))
if trace.enabled {
// heap_live changed.
traceHeapAlloc()
}
if gcBlackenEnabled != 0 {
// heap_live changed.
gcController.revise()
}
freeByteBase := s.freeindex &^ (64 - 1)
whichByte := freeByteBase / 8
// Init alloc bits cache.
s.refillAllocCache(whichByte)
// Adjust the allocCache so that s.freeindex corresponds to the low bit in
// s.allocCache.
s.allocCache >>= s.freeindex % 64
return s
}
// Return span from an mcache.
//
// s must have a span class corresponding to this
// mcentral and it must not be empty.
func (c *mcentral) uncacheSpan(s *mspan) {
if !go115NewMCentralImpl {
c.oldUncacheSpan(s)
return
}
if s.allocCount == 0 {
throw("uncaching span but s.allocCount == 0")
}
sg := mheap_.sweepgen
stale := s.sweepgen == sg+1
// Fix up sweepgen.
if stale {
// Span was cached before sweep began. It's our
// responsibility to sweep it.
//
// Set sweepgen to indicate it's not cached but needs
// sweeping and can't be allocated from. sweep will
// set s.sweepgen to indicate s is swept.
atomic.Store(&s.sweepgen, sg-1)
} else {
// Indicate that s is no longer cached.
atomic.Store(&s.sweepgen, sg)
}
n := int(s.nelems) - int(s.allocCount)
// Fix up statistics.
if n > 0 {
// cacheSpan updated alloc assuming all objects on s
// were going to be allocated. Adjust for any that
// weren't. We must do this before potentially
// sweeping the span.
atomic.Xadd64(&c.nmalloc, -int64(n))
if !stale {
// (*mcentral).cacheSpan conservatively counted
// unallocated slots in heap_live. Undo this.
//
// If this span was cached before sweep, then
// heap_live was totally recomputed since
// caching this span, so we don't do this for
// stale spans.
atomic.Xadd64(&memstats.heap_live, -int64(n)*int64(s.elemsize))
}
}
// Put the span in the appropriate place.
if stale {
// It's stale, so just sweep it. Sweeping will put it on
// the right list.
s.sweep(false)
} else {
if n > 0 {
// Put it back on the partial swept list.
c.partialSwept(sg).push(s)
} else {
// There's no free space and it's not stale, so put it on the
// full swept list.
c.fullSwept(sg).push(s)
}
}
}
// Return span from an mcache.
//
// For !go115NewMCentralImpl.
func (c *mcentral) oldUncacheSpan(s *mspan) {
if s.allocCount == 0 {
throw("uncaching span but s.allocCount == 0")
}
sg := mheap_.sweepgen
stale := s.sweepgen == sg+1
if stale {
// Span was cached before sweep began. It's our
// responsibility to sweep it.
//
// Set sweepgen to indicate it's not cached but needs
// sweeping and can't be allocated from. sweep will
// set s.sweepgen to indicate s is swept.
atomic.Store(&s.sweepgen, sg-1)
} else {
// Indicate that s is no longer cached.
atomic.Store(&s.sweepgen, sg)
}
n := int(s.nelems) - int(s.allocCount)
if n > 0 {
// cacheSpan updated alloc assuming all objects on s
// were going to be allocated. Adjust for any that
// weren't. We must do this before potentially
// sweeping the span.
atomic.Xadd64(&c.nmalloc, -int64(n))
lock(&c.lock)
c.empty.remove(s)
c.nonempty.insert(s)
if !stale {
// mCentral_CacheSpan conservatively counted
// unallocated slots in heap_live. Undo this.
//
// If this span was cached before sweep, then
// heap_live was totally recomputed since
// caching this span, so we don't do this for
// stale spans.
atomic.Xadd64(&memstats.heap_live, -int64(n)*int64(s.elemsize))
}
unlock(&c.lock)
}
if stale {
// Now that s is in the right mcentral list, we can
// sweep it.
s.sweep(false)
}
}
// freeSpan updates c and s after sweeping s.
// It sets s's sweepgen to the latest generation,
// and, based on the number of free objects in s,
// moves s to the appropriate list of c or returns it
// to the heap.
// freeSpan reports whether s was returned to the heap.
// If preserve=true, it does not move s (the caller
// must take care of it).
//
// For !go115NewMCentralImpl.
func (c *mcentral) freeSpan(s *mspan, preserve bool, wasempty bool) bool {
if sg := mheap_.sweepgen; s.sweepgen == sg+1 || s.sweepgen == sg+3 {
throw("freeSpan given cached span")
}
s.needzero = 1
if preserve {
// preserve is set only when called from (un)cacheSpan above,
// the span must be in the empty list.
if !s.inList() {
throw("can't preserve unlinked span")
}
atomic.Store(&s.sweepgen, mheap_.sweepgen)
return false
}
lock(&c.lock)
// Move to nonempty if necessary.
if wasempty {
c.empty.remove(s)
c.nonempty.insert(s)
}
// delay updating sweepgen until here. This is the signal that
// the span may be used in an mcache, so it must come after the
// linked list operations above (actually, just after the
// lock of c above.)
atomic.Store(&s.sweepgen, mheap_.sweepgen)
if s.allocCount != 0 {
unlock(&c.lock)
return false
}
c.nonempty.remove(s)
unlock(&c.lock)
mheap_.freeSpan(s)
return true
}
// grow allocates a new empty span from the heap and initializes it for c's size class.
func (c *mcentral) grow() *mspan {
npages := uintptr(class_to_allocnpages[c.spanclass.sizeclass()])
size := uintptr(class_to_size[c.spanclass.sizeclass()])
s := mheap_.alloc(npages, c.spanclass, true)
if s == nil {
return nil
}
// Use division by multiplication and shifts to quickly compute:
// n := (npages << _PageShift) / size
n := (npages << _PageShift) >> s.divShift * uintptr(s.divMul) >> s.divShift2
s.limit = s.base() + size*n
heapBitsForAddr(s.base()).initSpan(s)
return s
}