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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.
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
}