src/runtime/mcentral.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.

 // 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 {
 	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
 	lockInit(&c.partial[0].spineLock, lockRankSpanSetSpine)
 	lockInit(&c.partial[1].spineLock, lockRankSpanSetSpine)
 	lockInit(&c.full[0].spineLock, lockRankSpanSetSpine)
 	lockInit(&c.full[1].spineLock, lockRankSpanSetSpine)
 }

 // 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 {
 	// 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
 }

 // 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 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)
 		}
 	}
 }

 // 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
 }
	// 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 {
	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
	lockInit(&c.partial[0].spineLock, lockRankSpanSetSpine)
	lockInit(&c.partial[1].spineLock, lockRankSpanSetSpine)
	lockInit(&c.full[0].spineLock, lockRankSpanSetSpine)
	lockInit(&c.full[1].spineLock, lockRankSpanSetSpine)
	}

	// 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 {
	// 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
	}

	// 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 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)
	}
	}
	}

	// 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
	}