Source: mcache.go in package runtime


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

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.

Grab the mcache, since that's where stats live.

	pp := getg().m.p.ptr()
	var c *mcache

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.

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")
	}

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

heap_live changed.

		traceHeapAlloc()
	}

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))

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
}

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]

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()

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.

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