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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"
"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.
//
//go:notinheap
type mcache struct {
// 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 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 = 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() *mcache {
// Grab the mcache, since that's where stats live.
pp := getg().m.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 uintptr(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)
}
// Get a new cached span from the central lists.
s = mheap_.central[spc].mcentral.cacheSpan()
if s == nil {
throw("out of memory")
}
if uintptr(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
// Assume all objects from this span will be allocated in the
// mcache. If it gets uncached, we'll adjust this.
stats := memstats.heapStats.acquire()
atomic.Xadduintptr(&stats.smallAllocCount[spc.sizeclass()], uintptr(s.nelems)-uintptr(s.allocCount))
memstats.heapStats.release()
// Update heap_live with the same assumption.
usedBytes := uintptr(s.allocCount) * s.elemsize
atomic.Xadd64(&memstats.heap_live, int64(s.npages*pageSize)-int64(usedBytes))
// Flush tinyAllocs.
if spc == tinySpanClass {
atomic.Xadd64(&memstats.tinyallocs, int64(c.tinyAllocs))
c.tinyAllocs = 0
}
// While we're here, flush scanAlloc, since we have to call
// revise anyway.
atomic.Xadd64(&memstats.heap_scan, int64(c.scanAlloc))
c.scanAlloc = 0
if trace.enabled {
// heap_live changed.
traceHeapAlloc()
}
if gcBlackenEnabled != 0 {
// heap_live and heap_scan changed.
gcController.revise()
}
c.alloc[spc] = s
}
// allocLarge allocates a span for a large object.
func (c *mcache) allocLarge(size uintptr, needzero bool, 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, needzero)
if s == nil {
throw("out of memory")
}
stats := memstats.heapStats.acquire()
atomic.Xadduintptr(&stats.largeAlloc, npages*pageSize)
atomic.Xadduintptr(&stats.largeAllocCount, 1)
memstats.heapStats.release()
// Update heap_live and revise pacing if needed.
atomic.Xadd64(&memstats.heap_live, int64(npages*pageSize))
if trace.enabled {
// Trace that a heap alloc occurred because heap_live changed.
traceHeapAlloc()
}
if gcBlackenEnabled != 0 {
gcController.revise()
}
// 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
heapBitsForAddr(s.base()).initSpan(s)
return s
}
func (c *mcache) releaseAll() {
// Take this opportunity to flush scanAlloc.
atomic.Xadd64(&memstats.heap_scan, int64(c.scanAlloc))
c.scanAlloc = 0
sg := mheap_.sweepgen
for i := range c.alloc {
s := c.alloc[i]
if s != &emptymspan {
// Adjust nsmallalloc in case the span wasn't fully allocated.
n := uintptr(s.nelems) - uintptr(s.allocCount)
stats := memstats.heapStats.acquire()
atomic.Xadduintptr(&stats.smallAllocCount[spanClass(i).sizeclass()], -n)
memstats.heapStats.release()
if s.sweepgen != sg+1 {
// refill 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))
}
// 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
atomic.Xadd64(&memstats.tinyallocs, int64(c.tinyAllocs))
c.tinyAllocs = 0
// Updated heap_scan and possible heap_live.
if gcBlackenEnabled != 0 {
gcController.revise()
}
}
// 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
if c.flushGen == sg {
return
} else if c.flushGen != sg-2 {
println("bad flushGen", c.flushGen, "in prepareForSweep; sweepgen", sg)
throw("bad flushGen")
}
c.releaseAll()
stackcache_clear(c)
atomic.Store(&c.flushGen, mheap_.sweepgen) // Synchronizes with gcStart
}