src/runtime/mcache.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.

 package runtime

 import (
 	"runtime/internal/atomic"
 	"runtime/internal/sys"
 	"unsafe"
 )

 // Per-thread (in Go, per-P) cache for small objects.
 // This includes a small object cache and local allocation stats.
 // No locking needed because it is per-thread (per-P).
 //
 // mcaches are allocated from non-GC'd memory, so any heap pointers
 // must be specially handled.
 type mcache struct {
 	_ sys.NotInHeap

 	// The following members are accessed on every malloc,
 	// so they are grouped here for better caching.
 	nextSample uintptr // trigger heap sample after allocating this many bytes
 	scanAlloc  uintptr // bytes of scannable heap allocated

 	// Allocator cache for tiny objects w/o pointers.
 	// See "Tiny allocator" comment in malloc.go.

 	// tiny points to the beginning of the current tiny block, or
 	// nil if there is no current tiny block.
 	//
 	// tiny is a heap pointer. Since mcache is in non-GC'd memory,
 	// we handle it by clearing it in releaseAll during mark
 	// termination.
 	//
 	// tinyAllocs is the number of tiny allocations performed
 	// by the P that owns this mcache.
 	tiny       uintptr
 	tinyoffset uintptr
 	tinyAllocs uintptr

 	// The rest is not accessed on every malloc.

 	alloc [numSpanClasses]*mspan // spans to allocate from, indexed by spanClass

 	stackcache [_NumStackOrders]stackfreelist

 	// flushGen indicates the sweepgen during which this mcache
 	// was last flushed. If flushGen != mheap_.sweepgen, the spans
 	// in this mcache are stale and need to the flushed so they
 	// can be swept. This is done in acquirep.
 	flushGen atomic.Uint32
 }

 // A gclink is a node in a linked list of blocks, like mlink,
 // but it is opaque to the garbage collector.
 // The GC does not trace the pointers during collection,
 // and the compiler does not emit write barriers for assignments
 // of gclinkptr values. Code should store references to gclinks
 // as gclinkptr, not as *gclink.
 type gclink struct {
 	next gclinkptr
 }

 // A gclinkptr is a pointer to a gclink, but it is opaque
 // to the garbage collector.
 type gclinkptr uintptr

 // ptr returns the *gclink form of p.
 // The result should be used for accessing fields, not stored
 // in other data structures.
 func (p gclinkptr) ptr() *gclink {
 	return (*gclink)(unsafe.Pointer(p))
 }

 type stackfreelist struct {
 	list gclinkptr // linked list of free stacks
 	size uintptr   // total size of stacks in list
 }

 // dummy mspan that contains no free objects.
 var emptymspan mspan

 func allocmcache() *mcache {
 	var c *mcache
 	systemstack(func() {
 		lock(&mheap_.lock)
 		c = (*mcache)(mheap_.cachealloc.alloc())
 		c.flushGen.Store(mheap_.sweepgen)
 		unlock(&mheap_.lock)
 	})
 	for i := range c.alloc {
 		c.alloc[i] = &emptymspan
 	}
 	c.nextSample = nextSample()
 	return c
 }

 // freemcache releases resources associated with this
 // mcache and puts the object onto a free list.
 //
 // In some cases there is no way to simply release
 // resources, such as statistics, so donate them to
 // a different mcache (the recipient).
 func freemcache(c *mcache) {
 	systemstack(func() {
 		c.releaseAll()
 		stackcache_clear(c)

 		// NOTE(rsc,rlh): If gcworkbuffree comes back, we need to coordinate
 		// with the stealing of gcworkbufs during garbage collection to avoid
 		// a race where the workbuf is double-freed.
 		// gcworkbuffree(c.gcworkbuf)

 		lock(&mheap_.lock)
 		mheap_.cachealloc.free(unsafe.Pointer(c))
 		unlock(&mheap_.lock)
 	})
 }

 // getMCache is a convenience function which tries to obtain an mcache.
 //
 // Returns nil if we're not bootstrapping or we don't have a P. The caller's
 // P must not change, so we must be in a non-preemptible state.
 func getMCache(mp *m) *mcache {
 	// Grab the mcache, since that's where stats live.
 	pp := mp.p.ptr()
 	var c *mcache
 	if pp == nil {
 		// We will be called without a P while bootstrapping,
 		// in which case we use mcache0, which is set in mallocinit.
 		// mcache0 is cleared when bootstrapping is complete,
 		// by procresize.
 		c = mcache0
 	} else {
 		c = pp.mcache
 	}
 	return c
 }

 // refill acquires a new span of span class spc for c. This span will
 // have at least one free object. The current span in c must be full.
 //
 // Must run in a non-preemptible context since otherwise the owner of
 // c could change.
 func (c *mcache) refill(spc spanClass) {
 	// Return the current cached span to the central lists.
 	s := c.alloc[spc]

 	if s.allocCount != s.nelems {
 		throw("refill of span with free space remaining")
 	}
 	if s != &emptymspan {
 		// Mark this span as no longer cached.
 		if s.sweepgen != mheap_.sweepgen+3 {
 			throw("bad sweepgen in refill")
 		}
 		mheap_.central[spc].mcentral.uncacheSpan(s)

 		// Count up how many slots were used and record it.
 		stats := memstats.heapStats.acquire()
 		slotsUsed := int64(s.allocCount) - int64(s.allocCountBeforeCache)
 		atomic.Xadd64(&stats.smallAllocCount[spc.sizeclass()], slotsUsed)

 		// Flush tinyAllocs.
 		if spc == tinySpanClass {
 			atomic.Xadd64(&stats.tinyAllocCount, int64(c.tinyAllocs))
 			c.tinyAllocs = 0
 		}
 		memstats.heapStats.release()

 		// Count the allocs in inconsistent, internal stats.
 		bytesAllocated := slotsUsed * int64(s.elemsize)
 		gcController.totalAlloc.Add(bytesAllocated)

 		// Clear the second allocCount just to be safe.
 		s.allocCountBeforeCache = 0
 	}

 	// Get a new cached span from the central lists.
 	s = mheap_.central[spc].mcentral.cacheSpan()
 	if s == nil {
 		throw("out of memory")
 	}

 	if s.allocCount == s.nelems {
 		throw("span has no free space")
 	}

 	// Indicate that this span is cached and prevent asynchronous
 	// sweeping in the next sweep phase.
 	s.sweepgen = mheap_.sweepgen + 3

 	// Store the current alloc count for accounting later.
 	s.allocCountBeforeCache = s.allocCount

 	// Update heapLive and flush scanAlloc.
 	//
 	// We have not yet allocated anything new into the span, but we
 	// assume that all of its slots will get used, so this makes
 	// heapLive an overestimate.
 	//
 	// When the span gets uncached, we'll fix up this overestimate
 	// if necessary (see releaseAll).
 	//
 	// We pick an overestimate here because an underestimate leads
 	// the pacer to believe that it's in better shape than it is,
 	// which appears to lead to more memory used. See #53738 for
 	// more details.
 	usedBytes := uintptr(s.allocCount) * s.elemsize
 	gcController.update(int64(s.npages*pageSize)-int64(usedBytes), int64(c.scanAlloc))
 	c.scanAlloc = 0

 	c.alloc[spc] = s
 }

 // allocLarge allocates a span for a large object.
 func (c *mcache) allocLarge(size uintptr, noscan bool) *mspan {
 	if size+_PageSize < size {
 		throw("out of memory")
 	}
 	npages := size >> _PageShift
 	if size&_PageMask != 0 {
 		npages++
 	}

 	// Deduct credit for this span allocation and sweep if
 	// necessary. mHeap_Alloc will also sweep npages, so this only
 	// pays the debt down to npage pages.
 	deductSweepCredit(npages*_PageSize, npages)

 	spc := makeSpanClass(0, noscan)
 	s := mheap_.alloc(npages, spc)
 	if s == nil {
 		throw("out of memory")
 	}

 	// Count the alloc in consistent, external stats.
 	stats := memstats.heapStats.acquire()
 	atomic.Xadd64(&stats.largeAlloc, int64(npages*pageSize))
 	atomic.Xadd64(&stats.largeAllocCount, 1)
 	memstats.heapStats.release()

 	// Count the alloc in inconsistent, internal stats.
 	gcController.totalAlloc.Add(int64(npages * pageSize))

 	// Update heapLive.
 	gcController.update(int64(s.npages*pageSize), 0)

 	// Put the large span in the mcentral swept list so that it's
 	// visible to the background sweeper.
 	mheap_.central[spc].mcentral.fullSwept(mheap_.sweepgen).push(s)
 	s.limit = s.base() + size
 	s.initHeapBits(false)
 	return s
 }

 func (c *mcache) releaseAll() {
 	// Take this opportunity to flush scanAlloc.
 	scanAlloc := int64(c.scanAlloc)
 	c.scanAlloc = 0

 	sg := mheap_.sweepgen
 	dHeapLive := int64(0)
 	for i := range c.alloc {
 		s := c.alloc[i]
 		if s != &emptymspan {
 			slotsUsed := int64(s.allocCount) - int64(s.allocCountBeforeCache)
 			s.allocCountBeforeCache = 0

 			// Adjust smallAllocCount for whatever was allocated.
 			stats := memstats.heapStats.acquire()
 			atomic.Xadd64(&stats.smallAllocCount[spanClass(i).sizeclass()], slotsUsed)
 			memstats.heapStats.release()

 			// Adjust the actual allocs in inconsistent, internal stats.
 			// We assumed earlier that the full span gets allocated.
 			gcController.totalAlloc.Add(slotsUsed * int64(s.elemsize))

 			if s.sweepgen != sg+1 {
 				// refill conservatively counted unallocated slots in gcController.heapLive.
 				// Undo this.
 				//
 				// If this span was cached before sweep, then gcController.heapLive was totally
 				// recomputed since caching this span, so we don't do this for stale spans.
 				dHeapLive -= int64(s.nelems-s.allocCount) * int64(s.elemsize)
 			}

 			// Release the span to the mcentral.
 			mheap_.central[i].mcentral.uncacheSpan(s)
 			c.alloc[i] = &emptymspan
 		}
 	}
 	// Clear tinyalloc pool.
 	c.tiny = 0
 	c.tinyoffset = 0

 	// Flush tinyAllocs.
 	stats := memstats.heapStats.acquire()
 	atomic.Xadd64(&stats.tinyAllocCount, int64(c.tinyAllocs))
 	c.tinyAllocs = 0
 	memstats.heapStats.release()

 	// Update heapLive and heapScan.
 	gcController.update(dHeapLive, scanAlloc)
 }

 // prepareForSweep flushes c if the system has entered a new sweep phase
 // since c was populated. This must happen between the sweep phase
 // starting and the first allocation from c.
 func (c *mcache) prepareForSweep() {
 	// Alternatively, instead of making sure we do this on every P
 	// between starting the world and allocating on that P, we
 	// could leave allocate-black on, allow allocation to continue
 	// as usual, use a ragged barrier at the beginning of sweep to
 	// ensure all cached spans are swept, and then disable
 	// allocate-black. However, with this approach it's difficult
 	// to avoid spilling mark bits into the *next* GC cycle.
 	sg := mheap_.sweepgen
 	flushGen := c.flushGen.Load()
 	if flushGen == sg {
 		return
 	} else if flushGen != sg-2 {
 		println("bad flushGen", flushGen, "in prepareForSweep; sweepgen", sg)
 		throw("bad flushGen")
 	}
 	c.releaseAll()
 	stackcache_clear(c)
 	c.flushGen.Store(mheap_.sweepgen) // Synchronizes with gcStart
 }
	// 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.

	package runtime

	import (
	"runtime/internal/atomic"
	"runtime/internal/sys"
	"unsafe"
	)

	// Per-thread (in Go, per-P) cache for small objects.
	// This includes a small object cache and local allocation stats.
	// No locking needed because it is per-thread (per-P).
	//
	// mcaches are allocated from non-GC'd memory, so any heap pointers
	// must be specially handled.
	type mcache struct {
	_ sys.NotInHeap

	// The following members are accessed on every malloc,
	// so they are grouped here for better caching.
	nextSample uintptr // trigger heap sample after allocating this many bytes
	scanAlloc uintptr // bytes of scannable heap allocated

	// Allocator cache for tiny objects w/o pointers.
	// See "Tiny allocator" comment in malloc.go.

	// tiny points to the beginning of the current tiny block, or
	// nil if there is no current tiny block.
	//
	// tiny is a heap pointer. Since mcache is in non-GC'd memory,
	// we handle it by clearing it in releaseAll during mark
	// termination.
	//
	// tinyAllocs is the number of tiny allocations performed
	// by the P that owns this mcache.
	tiny uintptr
	tinyoffset uintptr
	tinyAllocs uintptr

	// The rest is not accessed on every malloc.

	alloc [numSpanClasses]*mspan // spans to allocate from, indexed by spanClass

	stackcache [_NumStackOrders]stackfreelist

	// flushGen indicates the sweepgen during which this mcache
	// was last flushed. If flushGen != mheap_.sweepgen, the spans
	// in this mcache are stale and need to the flushed so they
	// can be swept. This is done in acquirep.
	flushGen atomic.Uint32
	}

	// A gclink is a node in a linked list of blocks, like mlink,
	// but it is opaque to the garbage collector.
	// The GC does not trace the pointers during collection,
	// and the compiler does not emit write barriers for assignments
	// of gclinkptr values. Code should store references to gclinks
	// as gclinkptr, not as *gclink.
	type gclink struct {
	next gclinkptr
	}

	// A gclinkptr is a pointer to a gclink, but it is opaque
	// to the garbage collector.
	type gclinkptr uintptr

	// ptr returns the *gclink form of p.
	// The result should be used for accessing fields, not stored
	// in other data structures.
	func (p gclinkptr) ptr() *gclink {
	return (*gclink)(unsafe.Pointer(p))
	}

	type stackfreelist struct {
	list gclinkptr // linked list of free stacks
	size uintptr // total size of stacks in list
	}

	// dummy mspan that contains no free objects.
	var emptymspan mspan

	func allocmcache() *mcache {
	var c *mcache
	systemstack(func() {
	lock(&mheap_.lock)
	c = (*mcache)(mheap_.cachealloc.alloc())
	c.flushGen.Store(mheap_.sweepgen)
	unlock(&mheap_.lock)
	})
	for i := range c.alloc {
	c.alloc[i] = &emptymspan
	}
	c.nextSample = nextSample()
	return c
	}

	// freemcache releases resources associated with this
	// mcache and puts the object onto a free list.
	//
	// In some cases there is no way to simply release
	// resources, such as statistics, so donate them to
	// a different mcache (the recipient).
	func freemcache(c *mcache) {
	systemstack(func() {
	c.releaseAll()
	stackcache_clear(c)

	// NOTE(rsc,rlh): If gcworkbuffree comes back, we need to coordinate
	// with the stealing of gcworkbufs during garbage collection to avoid
	// a race where the workbuf is double-freed.
	// gcworkbuffree(c.gcworkbuf)

	lock(&mheap_.lock)
	mheap_.cachealloc.free(unsafe.Pointer(c))
	unlock(&mheap_.lock)
	})
	}

	// getMCache is a convenience function which tries to obtain an mcache.
	//
	// Returns nil if we're not bootstrapping or we don't have a P. The caller's
	// P must not change, so we must be in a non-preemptible state.
	func getMCache(mp m) mcache {
	// Grab the mcache, since that's where stats live.
	pp := mp.p.ptr()
	var c *mcache
	if pp == nil {
	// We will be called without a P while bootstrapping,
	// in which case we use mcache0, which is set in mallocinit.
	// mcache0 is cleared when bootstrapping is complete,
	// by procresize.
	c = mcache0
	} else {
	c = pp.mcache
	}
	return c
	}

	// refill acquires a new span of span class spc for c. This span will
	// have at least one free object. The current span in c must be full.
	//
	// Must run in a non-preemptible context since otherwise the owner of
	// c could change.
	func (c *mcache) refill(spc spanClass) {
	// Return the current cached span to the central lists.
	s := c.alloc[spc]

	if s.allocCount != s.nelems {
	throw("refill of span with free space remaining")
	}
	if s != &emptymspan {
	// Mark this span as no longer cached.
	if s.sweepgen != mheap_.sweepgen+3 {
	throw("bad sweepgen in refill")
	}
	mheap_.central[spc].mcentral.uncacheSpan(s)

	// Count up how many slots were used and record it.
	stats := memstats.heapStats.acquire()
	slotsUsed := int64(s.allocCount) - int64(s.allocCountBeforeCache)
	atomic.Xadd64(&stats.smallAllocCount[spc.sizeclass()], slotsUsed)

	// Flush tinyAllocs.
	if spc == tinySpanClass {
	atomic.Xadd64(&stats.tinyAllocCount, int64(c.tinyAllocs))
	c.tinyAllocs = 0
	}
	memstats.heapStats.release()

	// Count the allocs in inconsistent, internal stats.
	bytesAllocated := slotsUsed * int64(s.elemsize)
	gcController.totalAlloc.Add(bytesAllocated)

	// Clear the second allocCount just to be safe.
	s.allocCountBeforeCache = 0
	}

	// Get a new cached span from the central lists.
	s = mheap_.central[spc].mcentral.cacheSpan()
	if s == nil {
	throw("out of memory")
	}

	if s.allocCount == s.nelems {
	throw("span has no free space")
	}

	// Indicate that this span is cached and prevent asynchronous
	// sweeping in the next sweep phase.
	s.sweepgen = mheap_.sweepgen + 3

	// Store the current alloc count for accounting later.
	s.allocCountBeforeCache = s.allocCount

	// Update heapLive and flush scanAlloc.
	//
	// We have not yet allocated anything new into the span, but we
	// assume that all of its slots will get used, so this makes
	// heapLive an overestimate.
	//
	// When the span gets uncached, we'll fix up this overestimate
	// if necessary (see releaseAll).
	//
	// We pick an overestimate here because an underestimate leads
	// the pacer to believe that it's in better shape than it is,
	// which appears to lead to more memory used. See #53738 for
	// more details.
	usedBytes := uintptr(s.allocCount) * s.elemsize
	gcController.update(int64(s.npages*pageSize)-int64(usedBytes), int64(c.scanAlloc))
	c.scanAlloc = 0

	c.alloc[spc] = s
	}

	// allocLarge allocates a span for a large object.
	func (c mcache) allocLarge(size uintptr, noscan bool) mspan {
	if size+_PageSize < size {
	throw("out of memory")
	}
	npages := size >> _PageShift
	if size&_PageMask != 0 {
	npages++
	}

	// Deduct credit for this span allocation and sweep if
	// necessary. mHeap_Alloc will also sweep npages, so this only
	// pays the debt down to npage pages.
	deductSweepCredit(npages*_PageSize, npages)

	spc := makeSpanClass(0, noscan)
	s := mheap_.alloc(npages, spc)
	if s == nil {
	throw("out of memory")
	}

	// Count the alloc in consistent, external stats.
	stats := memstats.heapStats.acquire()
	atomic.Xadd64(&stats.largeAlloc, int64(npages*pageSize))
	atomic.Xadd64(&stats.largeAllocCount, 1)
	memstats.heapStats.release()

	// Count the alloc in inconsistent, internal stats.
	gcController.totalAlloc.Add(int64(npages * pageSize))

	// Update heapLive.
	gcController.update(int64(s.npages*pageSize), 0)

	// Put the large span in the mcentral swept list so that it's
	// visible to the background sweeper.
	mheap_.central[spc].mcentral.fullSwept(mheap_.sweepgen).push(s)
	s.limit = s.base() + size
	s.initHeapBits(false)
	return s
	}

	func (c *mcache) releaseAll() {
	// Take this opportunity to flush scanAlloc.
	scanAlloc := int64(c.scanAlloc)
	c.scanAlloc = 0

	sg := mheap_.sweepgen
	dHeapLive := int64(0)
	for i := range c.alloc {
	s := c.alloc[i]
	if s != &emptymspan {
	slotsUsed := int64(s.allocCount) - int64(s.allocCountBeforeCache)
	s.allocCountBeforeCache = 0

	// Adjust smallAllocCount for whatever was allocated.
	stats := memstats.heapStats.acquire()
	atomic.Xadd64(&stats.smallAllocCount[spanClass(i).sizeclass()], slotsUsed)
	memstats.heapStats.release()

	// Adjust the actual allocs in inconsistent, internal stats.
	// We assumed earlier that the full span gets allocated.
	gcController.totalAlloc.Add(slotsUsed * int64(s.elemsize))

	if s.sweepgen != sg+1 {
	// refill conservatively counted unallocated slots in gcController.heapLive.
	// Undo this.
	//
	// If this span was cached before sweep, then gcController.heapLive was totally
	// recomputed since caching this span, so we don't do this for stale spans.
	dHeapLive -= int64(s.nelems-s.allocCount) * int64(s.elemsize)
	}

	// Release the span to the mcentral.
	mheap_.central[i].mcentral.uncacheSpan(s)
	c.alloc[i] = &emptymspan
	}
	}
	// Clear tinyalloc pool.
	c.tiny = 0
	c.tinyoffset = 0

	// Flush tinyAllocs.
	stats := memstats.heapStats.acquire()
	atomic.Xadd64(&stats.tinyAllocCount, int64(c.tinyAllocs))
	c.tinyAllocs = 0
	memstats.heapStats.release()

	// Update heapLive and heapScan.
	gcController.update(dHeapLive, scanAlloc)
	}

	// prepareForSweep flushes c if the system has entered a new sweep phase
	// since c was populated. This must happen between the sweep phase
	// starting and the first allocation from c.
	func (c *mcache) prepareForSweep() {
	// Alternatively, instead of making sure we do this on every P
	// between starting the world and allocating on that P, we
	// could leave allocate-black on, allow allocation to continue
	// as usual, use a ragged barrier at the beginning of sweep to
	// ensure all cached spans are swept, and then disable
	// allocate-black. However, with this approach it's difficult
	// to avoid spilling mark bits into the next GC cycle.
	sg := mheap_.sweepgen
	flushGen := c.flushGen.Load()
	if flushGen == sg {
	return
	} else if flushGen != sg-2 {
	println("bad flushGen", flushGen, "in prepareForSweep; sweepgen", sg)
	throw("bad flushGen")
	}
	c.releaseAll()
	stackcache_clear(c)
	c.flushGen.Store(mheap_.sweepgen) // Synchronizes with gcStart
	}