转自[http://skoo.me/go/2013/10/13/go-memory-manage-system-alloc ]
这个拿来主义虽然不太好, 但总比不拿强..
吃水不忘挖井人,感谢原文作者分享.
内存布局结构图
我把整个核心代码的逻辑给抽象绘制出了这个内存布局图，它基本展示了Go语言内存分配器的整体结构以及部分细节（这结构图应该同样适用于tcmalloc）。从此结构图来看，内存分配器还是有一点小复杂的，但根据具体的逻辑层次可以拆成三个大模块——cache，central，heap，然后一个一个的模块分析下去，逻辑就显得特别清晰明了了。
位于结构图最下边的Cache就是cache模块部分；
central模块对应深蓝色部分的MCentral，central模块的逻辑结构很简单，所以结构图就没有详细的绘制了；
Heap是结构图中的核心结构，对应heap模块，也可以看出来central是直接被Heap管理起来的，属于Heap的子模块。
在分析内存分配器这部分源码的时候，首先需要明确的是所有内存分配的入口，有了入口就可以从这里作为起点一条线的看下去，不会有太大的障碍。这个入口就是/src/runtime/malloc.go源文件中的runtime·mallocgc函数，这个入口函数的主要工作就是分配内存以及触发gc(本文将只介绍内存分配)，在进入真正的分配内存之前，此入口函数还会判断请求的是小内存分配还是大内存分配(32k作为分界线)；小内存分配将调用mcache.alloc函数从Cache获取，而大内存分配调用mheap.alloc直接从Heap获取。入口函数过后，就会真正的进入到具体的内存分配过程中去了。
先上一大段的代码吧.
其中小内存分配走tiny,大内存分配走large…
// Allocate an object of size bytes.
// Small objects are allocated from the per-P cache's free lists.
// Large objects (> 32 kB) are allocated straight from the heap.
func mallocgc(size uintptr, typ *_type, needzero bool) unsafe.Pointer {
if gcphase == _GCmarktermination {
throw("mallocgc called with gcphase == _GCmarktermination")
}
if size == 0 {
return unsafe.Pointer(&zerobase)
}
if debug.sbrk != 0 {
align := uintptr(16)
if typ != nil {
align = uintptr(typ.align)
}
return persistentalloc(size, align, &memstats.other_sys)
}
// assistG is the G to charge for this allocation, or nil if
// GC is not currently active.
var assistG *g
if gcBlackenEnabled != 0 {
// Charge the current user G for this allocation.
assistG = getg()
if assistG.m.curg != nil {
assistG = assistG.m.curg
}
// Charge the allocation against the G. We'll account
// for internal fragmentation at the end of mallocgc.
assistG.gcAssistBytes -= int64(size)
if assistG.gcAssistBytes < 0 {
// This G is in debt. Assist the GC to correct
// this before allocating. This must happen
// before disabling preemption.
gcAssistAlloc(assistG)
}
}
// Set mp.mallocing to keep from being preempted by GC.
mp := acquirem()
if mp.mallocing != 0 {
throw("malloc deadlock")
}
if mp.gsignal == getg() {
throw("malloc during signal")
}
mp.mallocing = 1
shouldhelpgc := false
dataSize := size
c := gomcache()
var x unsafe.Pointer
noscan := typ == nil || typ.kind&kindNoPointers != 0
if size <= maxSmallSize {
if noscan && size < maxTinySize {
// Tiny allocator.
//
// Tiny allocator combines several tiny allocation requests
// into a single memory block. The resulting memory block
// is freed when all subobjects are unreachable. The subobjects
// must be noscan (don't have pointers), this ensures that
// the amount of potentially wasted memory is bounded.
//
// Size of the memory block used for combining (maxTinySize) is tunable.
// Current setting is 16 bytes, which relates to 2x worst case memory
// wastage (when all but one subobjects are unreachable).
// 8 bytes would result in no wastage at all, but provides less
// opportunities for combining.
// 32 bytes provides more opportunities for combining,
// but can lead to 4x worst case wastage.
// The best case winning is 8x regardless of block size.
//
// Objects obtained from tiny allocator must not be freed explicitly.
// So when an object will be freed explicitly, we ensure that
// its size >= maxTinySize.
//
// SetFinalizer has a special case for objects potentially coming
// from tiny allocator, it such case it allows to set finalizers
// for an inner byte of a memory block.
//
// The main targets of tiny allocator are small strings and
// standalone escaping variables. On a json benchmark
// the allocator reduces number of allocations by ~12% and
// reduces heap size by ~20%.
off := c.tinyoffset
// Align tiny pointer for required (conservative) alignment.
if size&7 == 0 {
off = round(off, 8)
} else if size&3 == 0 {
off = round(off, 4)
} else if size&1 == 0 {
off = round(off, 2)
}
if off+size <= maxTinySize && c.tiny != 0 {
// The object fits into existing tiny block.
x = unsafe.Pointer(c.tiny + off)
c.tinyoffset = off + size
c.local_tinyallocs++
mp.mallocing = 0
releasem(mp)
return x
}
// Allocate a new maxTinySize block.
span := c.alloc[tinySpanClass]
v := nextFreeFast(span)
if v == 0 {
v, _, shouldhelpgc = c.nextFree(tinySpanClass)
}
x = unsafe.Pointer(v)
(*[2]uint64)(x)[0] = 0
(*[2]uint64)(x)[1] = 0
// See if we need to replace the existing tiny block with the new one
// based on amount of remaining free space.
if size < c.tinyoffset || c.tiny == 0 {
c.tiny = uintptr(x)
c.tinyoffset = size
}
size = maxTinySize
} else {
var sizeclass uint8
if size <= smallSizeMax-8 {
sizeclass = size_to_class8[(size+smallSizeDiv-1)/smallSizeDiv]
} else {
sizeclass = size_to_class128[(size-smallSizeMax+largeSizeDiv-1)/largeSizeDiv]
}
size = uintptr(class_to_size[sizeclass])
spc := makeSpanClass(sizeclass, noscan)
span := c.alloc[spc]
v := nextFreeFast(span)
if v == 0 {
v, span, shouldhelpgc = c.nextFree(spc)
}
x = unsafe.Pointer(v)
if needzero && span.needzero != 0 {
memclrNoHeapPointers(unsafe.Pointer(v), size)
}
}
} else {
var s *mspan
shouldhelpgc = true
systemstack(func() {
s = largeAlloc(size, needzero, noscan)
})
s.freeindex = 1
s.allocCount = 1
x = unsafe.Pointer(s.base())
size = s.elemsize
}
var scanSize uintptr
if !noscan {
// If allocating a defer+arg block, now that we've picked a malloc size
// large enough to hold everything, cut the "asked for" size down to
// just the defer header, so that the GC bitmap will record the arg block
// as containing nothing at all (as if it were unused space at the end of
// a malloc block caused by size rounding).
// The defer arg areas are scanned as part of scanstack.
if typ == deferType {
dataSize = unsafe.Sizeof(_defer{})
}
heapBitsSetType(uintptr(x), size, dataSize, typ)
if dataSize > typ.size {
// Array allocation. If there are any
// pointers, GC has to scan to the last
// element.
if typ.ptrdata != 0 {
scanSize = dataSize - typ.size + typ.ptrdata
}
} else {
scanSize = typ.ptrdata
}
c.local_scan += scanSize
}
// Ensure that the stores above that initialize x to
// type-safe memory and set the heap bits occur before
// the caller can make x observable to the garbage
// collector. Otherwise, on weakly ordered machines,
// the garbage collector could follow a pointer to x,
// but see uninitialized memory or stale heap bits.
publicationBarrier()
// Allocate black during GC.
// All slots hold nil so no scanning is needed.
// This may be racing with GC so do it atomically if there can be
// a race marking the bit.
if gcphase != _GCoff {
gcmarknewobject(uintptr(x), size, scanSize)
}
if raceenabled {
racemalloc(x, size)
}
if msanenabled {
msanmalloc(x, size)
}
mp.mallocing = 0
releasem(mp)
if debug.allocfreetrace != 0 {
tracealloc(x, size, typ)
}
if rate := MemProfileRate; rate > 0 {
if size < uintptr(rate) && int32(size) < c.next_sample {
c.next_sample -= int32(size)
} else {
mp := acquirem()
profilealloc(mp, x, size)
releasem(mp)
}
}
if assistG != nil {
// Account for internal fragmentation in the assist
// debt now that we know it.
assistG.gcAssistBytes -= int64(size - dataSize)
}
if shouldhelpgc {
if t := (gcTrigger{kind: gcTriggerHeap}); t.test() {
gcStart(gcBackgroundMode, t)
}
}
return x
}ok, 今天就到这里, 5.1假期到了, 上半年的假的还是比较多的, 不过每个假期其实都很累啊…