一、tcmalloc介绍<参考资源>
go的内存管理和tcmalloc（thread-caching malloc）很像，先看一下tcmalloc的实现。
1.1 简介
tcmalloc是google推出的一种内存分配器，常见的内存分配器还有glibc的ptmalloc和google的jemalloc。相比于ptmalloc，tcmalloc性能更好，特别适用于高并发场景。
1.2 tcmalloc算法策略
tcmalloc分配的内存主要来自两个地方：全局缓存堆和进程的私有缓存。对于一些小容量的内存申请使用进程的私有缓存，私有缓存不足的时候可以再从全局缓存申请一部分作为私有缓存。对于大容量的内存申请则需要从全局缓存中进行申请。而大小容量的边界就是32k。缓存的组织方式是一个单链表数组，数组的每个元素是一个单链表，链表中的每个元素具有相同的大小。
1.2.1 Small Object Allocation
小对象内存分配默认会分配86个不同大小的块，而这些块的大小并没有明确说明，需要查一下源码。每种大小的块的数组的长度都采用使用了才初始化，有点类似于lazy-initialize。
Small Object Allocation
1.2.2 Big Object Allocation
对于大于32k的内存申请，使用全局内存来分配。全局内存的组织也是单链表数组，数组长度为256，分别对用1 page大小, 2 page大小（1 page=4k）。
Big Object Allocation
1.2.3 Span
tcmalloc使用span来管理内存分页，一个span可以包含几个连续分页。span的状态只有未分配、作为大对象分配、作为小对象分配。
Span
1.3 golang对比
golang的内存分配并不是和tcmalloc一模一样。
- 局部缓存并不是分配给进程或者线程，而是分配给P（这个还需要说一下go的goroutine实现）
- go的GC是stop the world，并不是每个进程单独进行GC。
- span的管理更有效率
二、基本知识
2.1 golang运行调度
在 Golang 里面有三个基本的概念：G, M, P。
- G（Goroutine）：我们所说的协程，为用户级的轻量级线程，每个Goroutine对象中的sched保存着其上下文信息
- M（Machine）：对内核级线程的封装，数量对应真实的CPU数（真正干活的对象）
- P（Processor）：即为G和M的调度对象，用来调度G和M之间的关联关系，其数量可通过GOMAXPROCS()来设置，默认为核心数。
一个 Goroutine 的运行需要 G + P + M 三部分结合起来。
2.2 逃逸分析（escape analysis）
逃逸分析请戳 GoLang-逃逸分析
三、关键数据结构
几个关键的地方：
- mcache: per-P cache，可以认为是 local cache。
- mcentral: 全局 cache，mcache 不够用的时候向 mcentral 申请。
- mheap: 当 mcentral 也不够用的时候，通过 mheap 向操作系统申请。
可以将其看成多级内存分配器。
3.1 mcache
我们知道每个 Gorontine 的运行都是绑定到一个 P 上面，mcache 是每个 P 的 cache。这么做的好处是分配内存时不需要加锁。mcache 结构如下。
//go version 1.10.8
//file runtime/mcache.go
// Per-thread (in Go, per-P) cache for small objects.
// 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.
next_sample int32 // trigger heap sample after allocating this many bytes
local_scan 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.
tiny uintptr
tinyoffset uintptr
local_tinyallocs uintptr // number of tiny allocs not counted in other stats
// The rest is not accessed on every malloc.
alloc [numSpanClasses]*mspan // spans to allocate from, indexed by spanClass
stackcache [_NumStackOrders]stackfreelist
// Local allocator stats, flushed during GC.
local_nlookup uintptr // number of pointer lookups
local_largefree uintptr // bytes freed for large objects (>maxsmallsize)
local_nlargefree uintptr // number of frees for large objects (>maxsmallsize)
local_nsmallfree [_NumSizeClasses]uintptr // number of frees for small objects (<=maxsmallsize)
}
我们可以暂时只关注 alloc [_NumSizeClasses]*mspan，这是一个大小为 67 的指针（指针指向 mspan ）数组（_NumSizeClasses = 67），每个数组元素用来包含特定大小的块。当要分配内存大小时，为 object 在 alloc 数组中选择合适的元素来分配。67 种块大小为 0，8 byte, 16 byte, … ，这个和 tcmalloc 稍有区别。
//go version 1.10.8
//file runtime/sizeclasses.go
var class_to_size = [_NumSizeClasses]uint16{0, 8, 16, 32, 48, 64, 80, 96, 112, 128, 144, 160, 176, 192, 208, 224, 240, 256, 288, 320, 352, 384, 416, 448, 480, 512, 576, 640, 704, 768, 896, 1024, 1152, 1280, 1408, 1536, 1792, 2048, 2304, 2688, 3072, 3200, 3456, 4096, 4864, 5376, 6144, 6528, 6784, 6912, 8192, 9472, 9728, 10240, 10880, 12288, 13568, 14336, 16384, 18432, 19072, 20480, 21760, 24576, 27264, 28672, 32768}
上面的 alloc 类似内存池的 freelist 数组或者链表，正常实现每个数组元素是一个链表，链表由特定大小的块串起来。但是这里统一使用了 mspan 结构，那么只有一种可能，就是 mspan 中记录了需要分配的块大小。我们来看一下 mspan 的结构。
3.2 mspan
span 在 tcmalloc 中作为一种管理内存的基本单位而存在。Golang 的 mspan 的结构如下。
//go version 1.10.8
//file runtime/mheap.go
//go:notinheap
type mspan struct {
next *mspan // next span in list, or nil if none
prev *mspan // previous span in list, or nil if none
list *mSpanList // For debugging. TODO: Remove.
startAddr uintptr // address of first byte of span aka s.base()
npages uintptr // number of pages in span
manualFreeList gclinkptr // list of free objects in _MSpanManual spans
// freeindex is the slot index between 0 and nelems at which to begin scanning
// for the next free object in this span.
// Each allocation scans allocBits starting at freeindex until it encounters a 0
// indicating a free object. freeindex is then adjusted so that subsequent scans begin
// just past the newly discovered free object.
//
// If freeindex == nelem, this span has no free objects.
//
// allocBits is a bitmap of objects in this span.
// If n >= freeindex and allocBits[n/8] & (1<<(n%8)) is 0
// then object n is free;
// otherwise, object n is allocated. Bits starting at nelem are
// undefined and should never be referenced.
//
// Object n starts at address n*elemsize + (start << pageShift).
freeindex uintptr
// TODO: Look up nelems from sizeclass and remove this field if it
// helps performance.
nelems uintptr // number of object in the span.
// Cache of the allocBits at freeindex. allocCache is shifted
// such that the lowest bit corresponds to the bit freeindex.
// allocCache holds the complement of allocBits, thus allowing
// ctz (count trailing zero) to use it directly.
// allocCache may contain bits beyond s.nelems; the caller must ignore
// these.
allocCache uint64
// allocBits and gcmarkBits hold pointers to a span's mark and
// allocation bits. The pointers are 8 byte aligned.
// There are three arenas where this data is held.
// free: Dirty arenas that are no longer accessed
// and can be reused.
// next: Holds information to be used in the next GC cycle.
// current: Information being used during this GC cycle.
// previous: Information being used during the last GC cycle.
// A new GC cycle starts with the call to finishsweep_m.
// finishsweep_m moves the previous arena to the free arena,
// the current arena to the previous arena, and
// the next arena to the current arena.
// The next arena is populated as the spans request
// memory to hold gcmarkBits for the next GC cycle as well
// as allocBits for newly allocated spans.
//
// The pointer arithmetic is done "by hand" instead of using
// arrays to avoid bounds checks along critical performance
// paths.
// The sweep will free the old allocBits and set allocBits to the
// gcmarkBits. The gcmarkBits are replaced with a fresh zeroed
// out memory.
allocBits *gcBits
gcmarkBits *gcBits
// sweep generation:
// if sweepgen == h->sweepgen - 2, the span needs sweeping
// if sweepgen == h->sweepgen - 1, the span is currently being swept
// if sweepgen == h->sweepgen, the span is swept and ready to use
// h->sweepgen is incremented by 2 after every GC
sweepgen uint32
divMul uint16 // for divide by elemsize - divMagic.mul
baseMask uint16 // if non-0, elemsize is a power of 2, & this will get object allocation base
allocCount uint16 // number of allocated objects
spanclass spanClass // size class and noscan (uint8)
incache bool // being used by an mcache
state mSpanState // mspaninuse etc
needzero uint8 // needs to be zeroed before allocation
divShift uint8 // for divide by elemsize - divMagic.shift
divShift2 uint8 // for divide by elemsize - divMagic.shift2
elemsize uintptr // computed from sizeclass or from npages
unusedsince int64 // first time spotted by gc in mspanfree state
npreleased uintptr // number of pages released to the os
limit uintptr // end of data in span
speciallock mutex // guards specials list
specials *special // linked list of special records sorted by offset.
}
从上面的结构可以看出：
- next, prev: 指针域，因为 mspan 一般都是以链表形式使用。
- npages: mspan 的大小为 page 大小的整数倍。
- spanclass(size class and noscan (uint8)): 0 ~ _NumSizeClasses 之间的一个值，这个解释了我们的疑问。比如，spanclass = 3，那么这个 mspan 被分割成 32 byte 的块。
- elemsize: 通过 sizeclass 或者 npages 可以计算出来。比如 sizeclass = 3, elemsize = 32 byte。对于大于 32Kb 的内存分配，都是分配整数页，elemsize = page_size * npages。
- nelems: span 中包块的总数目。
- freeindex: 0 ~ nelemes-1，表示分配到第几个块。
3.3 mcentral
上面说到当 mcache 不够用的时候，会从 mcentral 申请。那我们下面就来介绍一下 mcentral。
//go version 1.10.8
//file runtime/mcentral.go
// Central list of free objects of a given size.
//
//go:notinheap
type mcentral struct {
lock mutex
spanclass spanClass
nonempty mSpanList // list of spans with a free object, ie a nonempty free list
empty mSpanList // list of spans with no free objects (or cached in an mcache)
// nmalloc is the cumulative count of objects allocated from
// this mcentral, assuming all spans in mcaches are
// fully-allocated. Written atomically, read under STW.
nmalloc uint64
}
// mSpanList heads a linked list of spans.
//
//go:notinheap
type mSpanList struct {
first *mspan // first span in list, or nil if none
last *mspan // last span in list, or nil if none
}
mcentral 分析：
- spanclass: 也有成员 spanclass，那么 mcentral 是不是也有 67 个呢？是的。
- lock: 因为会有多个 P 过来竞争。
- nonempty: mspan 的双向链表，当前 mcentral 中可用的 mspan list。
- empty: 已经被使用的，可以认为是一种对所有 mspan 的 track。
问题来了，mcentral 存在于什么地方？虽然在上面我们将 mcentral 和 mheap 作为两个部分来讲，但是作为全局的结构，这两部分是可以定义在一起的。实际上也是这样，mcentral 包含在 mheap 中。
3.4 mheap
Golang 中的 mheap 结构定义如下。
//go version 1.10.8
//file runtime/mheap.go
// Main malloc heap.
// The heap itself is the "free[]" and "large" arrays,
// but all the other global data is here too.
//
// mheap must not be heap-allocated because it contains mSpanLists,
// which must not be heap-allocated.
//
//go:notinheap
type mheap struct {
lock mutex
free [_MaxMHeapList]mSpanList // free lists of given length up to _MaxMHeapList
freelarge mTreap // free treap of length >= _MaxMHeapList
busy [_MaxMHeapList]mSpanList // busy lists of large spans of given length
busylarge mSpanList // busy lists of large spans length >= _MaxMHeapList
sweepgen uint32 // sweep generation, see comment in mspan
sweepdone uint32 // all spans are swept
sweepers uint32 // number of active sweepone calls
// allspans is a slice of all mspans ever created. Each mspan
// appears exactly once.
//
// The memory for allspans is manually managed and can be
// reallocated and move as the heap grows.
//
// In general, allspans is protected by mheap_.lock, which
// prevents concurrent access as well as freeing the backing
// store. Accesses during STW might not hold the lock, but
// must ensure that allocation cannot happen around the
// access (since that may free the backing store).
allspans []*mspan // all spans out there
// spans is a lookup table to map virtual address page IDs to *mspan.
// For allocated spans, their pages map to the span itself.
// For free spans, only the lowest and highest pages map to the span itself.
// Internal pages map to an arbitrary span.
// For pages that have never been allocated, spans entries are nil.
//
// Modifications are protected by mheap.lock. Reads can be
// performed without locking, but ONLY from indexes that are
// known to contain in-use or stack spans. This means there
// must not be a safe-point between establishing that an
// address is live and looking it up in the spans array.
//
// This is backed by a reserved region of the address space so
// it can grow without moving. The memory up to len(spans) is
// mapped. cap(spans) indicates the total reserved memory.
spans []*mspan
// sweepSpans contains two mspan stacks: one of swept in-use
// spans, and one of unswept in-use spans. These two trade
// roles on each GC cycle. Since the sweepgen increases by 2
// on each cycle, this means the swept spans are in
// sweepSpans[sweepgen/2%2] and the unswept spans are in
// sweepSpans[1-sweepgen/2%2]. Sweeping pops spans from the
// unswept stack and pushes spans that are still in-use on the
// swept stack. Likewise, allocating an in-use span pushes it
// on the swept stack.
sweepSpans [2]gcSweepBuf
_ uint32 // align uint64 fields on 32-bit for atomics
// Proportional sweep
//
// These parameters represent a linear function from heap_live
// to page sweep count. The proportional sweep system works to
// stay in the black by keeping the current page sweep count
// above this line at the current heap_live.
//
// The line has slope sweepPagesPerByte and passes through a
// basis point at (sweepHeapLiveBasis, pagesSweptBasis). At
// any given time, the system is at (memstats.heap_live,
// pagesSwept) in this space.
//
// It's important that the line pass through a point we
// control rather than simply starting at a (0,0) origin
// because that lets us adjust sweep pacing at any time while
// accounting for current progress. If we could only adjust
// the slope, it would create a discontinuity in debt if any
// progress has already been made.
pagesInUse uint64 // pages of spans in stats _MSpanInUse; R/W with mheap.lock
pagesSwept uint64 // pages swept this cycle; updated atomically
pagesSweptBasis uint64 // pagesSwept to use as the origin of the sweep ratio; updated atomically
sweepHeapLiveBasis uint64 // value of heap_live to use as the origin of sweep ratio; written with lock, read without
sweepPagesPerByte float64 // proportional sweep ratio; written with lock, read without
// TODO(austin): pagesInUse should be a uintptr, but the 386
// compiler can't 8-byte align fields.
// Malloc stats.
largealloc uint64 // bytes allocated for large objects
nlargealloc uint64 // number of large object allocations
largefree uint64 // bytes freed for large objects (>maxsmallsize)
nlargefree uint64 // number of frees for large objects (>maxsmallsize)
nsmallfree [_NumSizeClasses]uint64 // number of frees for small objects (<=maxsmallsize)
// range of addresses we might see in the heap
bitmap uintptr // Points to one byte past the end of the bitmap
bitmap_mapped uintptr
// The arena_* fields indicate the addresses of the Go heap.
//
// The maximum range of the Go heap is
// [arena_start, arena_start+_MaxMem+1).
//
// The range of the current Go heap is
// [arena_start, arena_used). Parts of this range may not be
// mapped, but the metadata structures are always mapped for
// the full range.
arena_start uintptr
arena_used uintptr // Set with setArenaUsed.
// The heap is grown using a linear allocator that allocates
// from the block [arena_alloc, arena_end). arena_alloc is
// often, but *not always* equal to arena_used.
arena_alloc uintptr
arena_end uintptr
// arena_reserved indicates that the memory [arena_alloc,
// arena_end) is reserved (e.g., mapped PROT_NONE). If this is
// false, we have to be careful not to clobber existing
// mappings here. If this is true, then we own the mapping
// here and *must* clobber it to use it.
arena_reserved bool
_ uint32 // ensure 64-bit alignment
// central free lists for small size classes.
// the padding makes sure that the MCentrals are
// spaced CacheLineSize bytes apart, so that each MCentral.lock
// gets its own cache line.
// central is indexed by spanClass.
central [numSpanClasses]struct {
mcentral mcentral
pad [sys.CacheLineSize - unsafe.Sizeof(mcentral{})%sys.CacheLineSize]byte
}
spanalloc fixalloc // allocator for span*
cachealloc fixalloc // allocator for mcache*
treapalloc fixalloc // allocator for treapNodes* used by large objects
specialfinalizeralloc fixalloc // allocator for specialfinalizer*
specialprofilealloc fixalloc // allocator for specialprofile*
speciallock mutex // lock for special record allocators.
unused *specialfinalizer // never set, just here to force the specialfinalizer type into DWARF
}
var mheap_ mheap
mheap_ 是一个全局变量，会在系统初始化的时候初始化（在函数 mallocinit() 中）。我们先看一下 mheap 具体结构。
- allspans []*mspan: 所有的 spans 都是通过 mheap_ 申请，所有申请过的 mspan 都会记录在 allspans。结构体中的 lock 就是用来保证并发安全的。注释中有关于 STW 的说明，与Golang 的 GC 有关，这里不做详细说明。
- central [_NumSizeClasses]…: 这个就是之前介绍的 mcentral ，每种大小的块对应一个 mcentral。mcentral 上面介绍过了。pad 可以认为是一个字节填充，为了避免伪共享（false sharing）问题的。False Sharing 可以参考 False Sharing - wikipedia，这里就不细说了。
- sweepgen, sweepdone: GC 相关。（Golang 的 GC 策略是 Mark & Sweep, 这里是用来表示 sweep 的，这里就不再深入了。）
- free [_MaxMHeapList]mSpanList: 这是一个 SpanList 数组，每个 SpanList 里面的 mspan 由 1 ~ 127 (_MaxMHeapList - 1) 个 page 组成。比如 free[3] 是由包含 3 个 page 的 mspan 组成的链表。free 表示的是 free list，也就是未分配的。对应的还有 busy list。
- freelarge: mspan 组成的链表，每个元素（也就是 mspan）的 page 个数大于 127。对应的还有 busylarge。
- spans []*mspan: 记录 arena 区域页号（page number）和 mspan 的映射关系。
-
arena_start, arena_end, arena_used: 要解释这几个变量之前要解释一下 arena。arena 是 Golang 中用于分配内存的连续虚拟地址区域。由 mheap 管理，堆上申请的所有内存都来自 arena。那么如何标志内存可用呢？操作系统的常见做法用两种：一种是用链表将所有的可用内存都串起来；另一种是使用位图来标志内存块是否可用。结合上面一条 spans，内存的布局是下面这样的。
golang内存布局
- spanalloc, cachealloc fixalloc: fixalloc 是 free-list，用来分配特定大小的块。
- 剩下的是一些统计信息和 GC 相关的信息，这里暂且按住不表。
四、初始化
在系统初始化阶段，上面介绍的几个结构会被进行初始化，我们直接看一下初始化代码：mallocinit()。
//go version 1.10.8
//file runtime/malloc.go
func mallocinit() {
if class_to_size[_TinySizeClass] != _TinySize {
throw("bad TinySizeClass")
}
testdefersizes()
// Copy class sizes out for statistics table.
for i := range class_to_size {
memstats.by_size[i].size = uint32(class_to_size[i])
}
// Check physPageSize.
if physPageSize == 0 {
// The OS init code failed to fetch the physical page size.
throw("failed to get system page size")
}
if physPageSize < minPhysPageSize {
print("system page size (", physPageSize, ") is smaller than minimum page size (", minPhysPageSize, ")\n")
throw("bad system page size")
}
if physPageSize&(physPageSize-1) != 0 {
print("system page size (", physPageSize, ") must be a power of 2\n")
throw("bad system page size")
}
// The auxiliary regions start at p and are laid out in the
// following order: spans, bitmap, arena.
var p, pSize uintptr
var reserved bool
// The spans array holds one *mspan per _PageSize of arena.
var spansSize uintptr = (_MaxMem + 1) / _PageSize * sys.PtrSize
spansSize = round(spansSize, _PageSize)
// The bitmap holds 2 bits per word of arena.
var bitmapSize uintptr = (_MaxMem + 1) / (sys.PtrSize * 8 / 2)
bitmapSize = round(bitmapSize, _PageSize)
// Set up the allocation arena, a contiguous area of memory where
// allocated data will be found.
if sys.PtrSize == 8 {
// On a 64-bit machine, allocate from a single contiguous reservation.
// 512 GB (MaxMem) should be big enough for now.
//
// The code will work with the reservation at any address, but ask
// SysReserve to use 0x0000XXc000000000 if possible (XX=00...7f).
// Allocating a 512 GB region takes away 39 bits, and the amd64
// doesn't let us choose the top 17 bits, so that leaves the 9 bits
// in the middle of 0x00c0 for us to choose. Choosing 0x00c0 means
// that the valid memory addresses will begin 0x00c0, 0x00c1, ..., 0x00df.
// In little-endian, that's c0 00, c1 00, ..., df 00. None of those are valid
// UTF-8 sequences, and they are otherwise as far away from
// ff (likely a common byte) as possible. If that fails, we try other 0xXXc0
// addresses. An earlier attempt to use 0x11f8 caused out of memory errors
// on OS X during thread allocations. 0x00c0 causes conflicts with
// AddressSanitizer which reserves all memory up to 0x0100.
// These choices are both for debuggability and to reduce the
// odds of a conservative garbage collector (as is still used in gccgo)
// not collecting memory because some non-pointer block of memory
// had a bit pattern that matched a memory address.
//
// Actually we reserve 544 GB (because the bitmap ends up being 32 GB)
// but it hardly matters: e0 00 is not valid UTF-8 either.
//
// If this fails we fall back to the 32 bit memory mechanism
//
// However, on arm64, we ignore all this advice above and slam the
// allocation at 0x40 << 32 because when using 4k pages with 3-level
// translation buffers, the user address space is limited to 39 bits
// On darwin/arm64, the address space is even smaller.
arenaSize := round(_MaxMem, _PageSize)
pSize = bitmapSize + spansSize + arenaSize + _PageSize
for i := 0; i <= 0x7f; i++ {
switch {
case GOARCH == "arm64" && GOOS == "darwin":
p = uintptr(i)<<40 | uintptrMask&(0x0013<<28)
case GOARCH == "arm64":
p = uintptr(i)<<40 | uintptrMask&(0x0040<<32)
default:
p = uintptr(i)<<40 | uintptrMask&(0x00c0<<32)
}
p = uintptr(sysReserve(unsafe.Pointer(p), pSize, &reserved))
if p != 0 {
break
}
}
}
if p == 0 {
// On a 32-bit machine, we can't typically get away
// with a giant virtual address space reservation.
// Instead we map the memory information bitmap
// immediately after the data segment, large enough
// to handle the entire 4GB address space (256 MB),
// along with a reservation for an initial arena.
// When that gets used up, we'll start asking the kernel
// for any memory anywhere.
// We want to start the arena low, but if we're linked
// against C code, it's possible global constructors
// have called malloc and adjusted the process' brk.
// Query the brk so we can avoid trying to map the
// arena over it (which will cause the kernel to put
// the arena somewhere else, likely at a high
// address).
procBrk := sbrk0()
// If we fail to allocate, try again with a smaller arena.
// This is necessary on Android L where we share a process
// with ART, which reserves virtual memory aggressively.
// In the worst case, fall back to a 0-sized initial arena,
// in the hope that subsequent reservations will succeed.
arenaSizes := []uintptr{
512 << 20,
256 << 20,
128 << 20,
0,
}
for _, arenaSize := range arenaSizes {
// SysReserve treats the address we ask for, end, as a hint,
// not as an absolute requirement. If we ask for the end
// of the data segment but the operating system requires
// a little more space before we can start allocating, it will
// give out a slightly higher pointer. Except QEMU, which
// is buggy, as usual: it won't adjust the pointer upward.
// So adjust it upward a little bit ourselves: 1/4 MB to get
// away from the running binary image and then round up
// to a MB boundary.
p = round(firstmoduledata.end+(1<<18), 1<<20)
pSize = bitmapSize + spansSize + arenaSize + _PageSize
if p <= procBrk && procBrk < p+pSize {
// Move the start above the brk,
// leaving some room for future brk
// expansion.
p = round(procBrk+(1<<20), 1<<20)
}
p = uintptr(sysReserve(unsafe.Pointer(p), pSize, &reserved))
if p != 0 {
break
}
}
if p == 0 {
throw("runtime: cannot reserve arena virtual address space")
}
}
// PageSize can be larger than OS definition of page size,
// so SysReserve can give us a PageSize-unaligned pointer.
// To overcome this we ask for PageSize more and round up the pointer.
p1 := round(p, _PageSize)
pSize -= p1 - p
spansStart := p1
p1 += spansSize
mheap_.bitmap = p1 + bitmapSize
p1 += bitmapSize
if sys.PtrSize == 4 {
// Set arena_start such that we can accept memory
// reservations located anywhere in the 4GB virtual space.
mheap_.arena_start = 0
} else {
mheap_.arena_start = p1
}
mheap_.arena_end = p + pSize
mheap_.arena_used = p1
mheap_.arena_alloc = p1
mheap_.arena_reserved = reserved
if mheap_.arena_start&(_PageSize-1) != 0 {
println("bad pagesize", hex(p), hex(p1), hex(spansSize), hex(bitmapSize), hex(_PageSize), "start", hex(mheap_.arena_start))
throw("misrounded allocation in mallocinit")
}
// Initialize the rest of the allocator.
mheap_.init(spansStart, spansSize)
_g_ := getg()
_g_.m.mcache = allocmcache()
}
4.1 arena 相关
// The spans array holds one *mspan per _PageSize of arena.
var spansSize uintptr = (_MaxMem + 1) / _PageSize * sys.PtrSize
spansSize = round(spansSize, _PageSize)
// The bitmap holds 2 bits per word of arena.
var bitmapSize uintptr = (_MaxMem + 1) / (sys.PtrSize * 8 / 2)
bitmapSize = round(bitmapSize, _PageSize)
arenaSize := round(_MaxMem, _PageSize)
// _MaxMem is the maximum heap arena size minus 1.
//
// On 32-bit, this is also the maximum heap pointer value,
// since the arena starts at address 0.
_MaxMem = 1<<_MHeapMap_TotalBits - 1
首先解释一下变量 _MaxMem ，里面还有一个变量就不再列出来了。简单来说 _MaxMem 就是系统为 arena 区分配的大小：64 位系统分配 512 G；对于 Windows 64 位系统，arena 区分配 32 G。round 是一个对齐操作，向上取 _PageSize 的倍数。实现也很有意思，代码如下。
// round n up to a multiple of a. a must be a power of 2.
func round(n, a uintptr) uintptr {
return (n + a - 1) &^ (a - 1)
}
bitmap 用两个 bit 表示一个字的可用状态，那么算下来 bitmap 的大小为 16 G。spans 记录的 arena 区的块页号和对应的 mspan 指针的对应关系。比如 arena 区内存地址 x，对应的页号就是 page_num = (x - arena_start) / page_size，那么 spans 就会记录 spans[page_num] = x。如果 arena 为 512 G的话，spans 区的大小为 512 G / 8K * 8 = 512 M。这里值得注意的是 Golang 的内存管理虚拟地址页大小为 8k。
_PageSize = 1 << _PageShift
_PageMask = _PageSize - 1
所以这一段连续的的虚拟内存布局（64 位）如下：
golang虚拟内存布局
4.2 虚拟地址申请
主要是下面这段代码。
pSize = bitmapSize + spansSize + arenaSize + _PageSize
for i := 0; i <= 0x7f; i++ {
switch {
case GOARCH == "arm64" && GOOS == "darwin":
p = uintptr(i)<<40 | uintptrMask&(0x0013<<28)
case GOARCH == "arm64":
p = uintptr(i)<<40 | uintptrMask&(0x0040<<32)
default:
p = uintptr(i)<<40 | uintptrMask&(0x00c0<<32)
}
p = uintptr(sysReserve(unsafe.Pointer(p), pSize, &reserved))
if p != 0 {
break
}
}
初始化的时候，Golang 向操作系统申请一段连续的地址空间，就是上面的 spans + bitmap + arena。p 就是这段连续地址空间的开始地址，不同平台的 p 取值不一样。向 OS 申请的时候视不同的 OS 版本，调用不同的系统调用，比如 Unix 系统调用 mmap (mmap 向操作系统内核申请新的虚拟地址区间，可指定起始地址和长度)，Windows 系统调用 VirtualAlloc （类似 mmap）。
//来源于互联网
//bsd
func sysReserve(v unsafe.Pointer, n uintptr, reserved *bool) unsafe.Pointer {
if sys.PtrSize == 8 && uint64(n) > 1<<32 || sys.GoosNacl != 0 {
*reserved = false
return v
}
p := mmap(v, n, _PROT_NONE, _MAP_ANON|_MAP_PRIVATE, -1, 0)
if uintptr(p) < 4096 {
return nil
}
*reserved = true
return p
}
//darwin
func sysReserve(v unsafe.Pointer, n uintptr, reserved *bool) unsafe.Pointer {
*reserved = true
p := mmap(v, n, _PROT_NONE, _MAP_ANON|_MAP_PRIVATE, -1, 0)
if uintptr(p) < 4096 {
return nil
}
return p
}
//linux
func sysReserve(v unsafe.Pointer, n uintptr, reserved *bool) unsafe.Pointer {
...
p := mmap(v, n, _PROT_NONE, _MAP_ANON|_MAP_PRIVATE, -1, 0)
if uintptr(p) < 4096 {
return nil
}
*reserved = true
return p
}
//windows
func sysReserve(v unsafe.Pointer, n uintptr, reserved *bool) unsafe.Pointer {
*reserved = true
// v is just a hint.
// First try at v.
v = unsafe.Pointer(stdcall4(_VirtualAlloc, uintptr(v), n, _MEM_RESERVE, _PAGE_READWRITE))
if v != nil {
return v
}
// Next let the kernel choose the address.
return unsafe.Pointer(stdcall4(_VirtualAlloc, 0, n, _MEM_RESERVE, _PAGE_READWRITE))
}
4.3 mheap 初始化
我们上面介绍 mheap 结构的时候知道 spans, bitmap, arena 都是存在于 mheap 中的，从操作系统申请完地址之后就是初始化 mheap 了。
// PageSize can be larger than OS definition of page size,
// so SysReserve can give us a PageSize-unaligned pointer.
// To overcome this we ask for PageSize more and round up the pointer.
p1 := round(p, _PageSize)
pSize -= p1 - p
spansStart := p1
p1 += spansSize
mheap_.bitmap = p1 + bitmapSize
p1 += bitmapSize
if sys.PtrSize == 4 {
// Set arena_start such that we can accept memory
// reservations located anywhere in the 4GB virtual space.
mheap_.arena_start = 0
} else {
mheap_.arena_start = p1
}
mheap_.arena_end = p + pSize
mheap_.arena_used = p1
mheap_.arena_alloc = p1
mheap_.arena_reserved = reserved
if mheap_.arena_start&(_PageSize-1) != 0 {
println("bad pagesize", hex(p), hex(p1), hex(spansSize), hex(bitmapSize), hex(_PageSize), "start", hex(mheap_.arena_start))
throw("misrounded allocation in mallocinit")
}
// Initialize the rest of the allocator.
mheap_.init(spansStart, spansSize)
_g_ := getg()
_g_.m.mcache = allocmcache()
p 是从连续虚拟地址的起始地址，先进行对齐，然后初始化 arena，bitmap，spans 地址。mheap_.init()会初始化 fixalloc 等相关的成员，还有 mcentral 的初始化。
// Initialize the heap.
func (h *mheap) init(spansStart, spansBytes uintptr) {
h.treapalloc.init(unsafe.Sizeof(treapNode{}), nil, nil, &memstats.other_sys)
h.spanalloc.init(unsafe.Sizeof(mspan{}), recordspan, unsafe.Pointer(h), &memstats.mspan_sys)
h.cachealloc.init(unsafe.Sizeof(mcache{}), nil, nil, &memstats.mcache_sys)
h.specialfinalizeralloc.init(unsafe.Sizeof(specialfinalizer{}), nil, nil, &memstats.other_sys)
h.specialprofilealloc.init(unsafe.Sizeof(specialprofile{}), nil, nil, &memstats.other_sys)
// Don't zero mspan allocations. Background sweeping can
// inspect a span concurrently with allocating it, so it's
// important that the span's sweepgen survive across freeing
// and re-allocating a span to prevent background sweeping
// from improperly cas'ing it from 0.
//
// This is safe because mspan contains no heap pointers.
h.spanalloc.zero = false
// h->mapcache needs no init
for i := range h.free {
h.free[i].init()
h.busy[i].init()
}
h.busylarge.init()
for i := range h.central {
h.central[i].mcentral.init(spanClass(i))
}
sp := (*slice)(unsafe.Pointer(&h.spans))
sp.array = unsafe.Pointer(spansStart)
sp.len = 0
sp.cap = int(spansBytes / sys.PtrSize)
// Map metadata structures. But don't map race detector memory
// since we're not actually growing the arena here (and TSAN
// gets mad if you map 0 bytes).
h.setArenaUsed(h.arena_used, false)
}
mheap 初始化之后，对当前的线程也就是 M 进行初始化。
//获取当前 G
_g_ := getg()
// 获取 G 上绑定的 M 的 mcache
_g_.m.mcache = allocmcache()
4.4 per-P mcache 初始化
上面好像并没有说到针对 P 的 mcache 初始化，因为这个时候还没有初始化 P。我们看一下 bootstrap 的代码。
func schedinit() {
...
mallocinit()
...
if procs > _MaxGomaxprocs {
procs = _MaxGomaxprocs
}
if procresize(procs) != nil {
...
}
}
其中 mallocinit() 上面说过了。对 P 的初始化在函数 procresize() 中执行，我们下面只看内存相关的部分。
//go version 1.10.8
//file runtime/proc.go
func procresize(nprocs int32) *p {
...
// initialize new P's
for i := int32(0); i < nprocs; i++ {
pp := allp[i]
if pp == nil {
pp = new(p)
pp.id = i
pp.status = _Pgcstop
pp.sudogcache = pp.sudogbuf[:0]
for i := range pp.deferpool {
pp.deferpool[i] = pp.deferpoolbuf[i][:0]
}
atomicstorep(unsafe.Pointer(&allp[i]), unsafe.Pointer(pp))
}
// P mcache 初始化
if pp.mcache == nil {
if old == 0 && i == 0 {
if getg().m.mcache == nil {
throw("missing mcache?")
}
// P[0] 分配给主 Goroutine
pp.mcache = getg().m.mcache // bootstrap
} else {
// P[0] 之外的 P 申请 mcache
pp.mcache = allocmcache()
}
}
...
}
...
}
所有的 P 都存放在一个全局数组 allp 中，procresize() 的目的就是将 allp 中用到的 P 进行初始化，同时对多余 P 的资源剥离。
五、内存分配
先说一下给对象 object 分配内存的主要流程：
- object size > 32K，则使用 mheap 直接分配。
- object size < 16 byte，使用 mcache 的小对象分配器 tiny 直接分配。 （其实 tiny 就是一个指针，暂且这么说吧。）
- object size > 16 byte && size <=32K byte 时，先使用 mcache 中对应的 size class 分配。
- 如果 mcache 对应的 size class 的 span 已经没有可用的块，则向 mcentral 请求。
- 如果 mcentral 也没有可用的块，则向 mheap 申请，并切分。
- 如果 mheap 也没有合适的 span，则向操作系统申请。
我们看一下在堆上，也就是 arena 区分配内存的相关函数。
package main
func foo() *int {
x := 1
return &x
}
func main() {
x := foo()
println(*x)
}
根据之前介绍的逃逸分析，foo() 中的 x 会被分配到堆上。把上面代码保存为 main.go 看一下汇编代码。
$ go build -gcflags '-l' -o main main.go
$ go tool objdump -s "main\.foo" main
TEXT main.foo(SB) /Users/xxx/src/heap/main.go
main.go:3 0x104aad0 65488b0c25a0080000 MOVQ GS:0x8a0, CX
main.go:3 0x104aad9 483b6110 CMPQ 0x10(CX), SP
main.go:3 0x104aadd 7639 JBE 0x104ab18
main.go:3 0x104aadf 4883ec18 SUBQ $0x18, SP
main.go:3 0x104aae3 48896c2410 MOVQ BP, 0x10(SP)
main.go:3 0x104aae8 488d6c2410 LEAQ 0x10(SP), BP
main.go:4 0x104aaed 488d052c990000 LEAQ type.*+38976(SB), AX
main.go:4 0x104aaf4 48890424 MOVQ AX, 0(SP)
main.go:4 0x104aaf8 e82304fcff CALL runtime.newobject(SB)
main.go:4 0x104aafd 488b442408 MOVQ 0x8(SP), AX
main.go:4 0x104ab02 48c70001000000 MOVQ $0x1, 0(AX)
main.go:5 0x104ab09 4889442420 MOVQ AX, 0x20(SP)
main.go:5 0x104ab0e 488b6c2410 MOVQ 0x10(SP), BP
main.go:5 0x104ab13 4883c418 ADDQ $0x18, SP
main.go:5 0x104ab17 c3 RET
main.go:3 0x104ab18 e80389ffff CALL runtime.morestack_noctxt(SB)
main.go:3 0x104ab1d ebb1 JMP main.foo(SB)
:-1 0x104ab1f cc INT $0x3
堆上内存分配调用了 runtime 包的 newobject 函数。
//go version 1.10.8
//file runtime/malloc.go
// implementation of new builtin
// compiler (both frontend and SSA backend) knows the signature
// of this function
func newobject(typ *_type) unsafe.Pointer {
return mallocgc(typ.size, typ, true)
}
// 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 {
// object size <= 32K
if noscan && size < maxTinySize {
// 小于 16 byte 的小对象分配
// 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 {
//object size > 32K byte
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
}
整个分配过程可以根据 object size 拆解成三部分：size < 16 byte, 16 byte <= size <= 32 K byte, size > 32 K byte。
5.1 size 小于 16 byte
对于小于 16 byte 的内存块，mcache 有个专门的内存区域 tiny 用来分配，tiny 是指针，指向开始地址。
func mallocgc(...) {
...
off := c.tinyoffset
// 地址对齐
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)
}
// 分配
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
}
// tiny 不够了，为其重新分配一个 16 byte 内存块
span := c.alloc[tinySizeClass]
v := nextFreeFast(span)
if v == 0 {
v, _, shouldhelpgc = c.nextFree(tinySizeClass)
}
x = unsafe.Pointer(v)
//将申请的内存块全置为 0
(*[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.
// 如果申请的内存块用不完，则将剩下的给 tiny，用 tinyoffset 记录分配了多少。
if size < c.tinyoffset || c.tiny == 0 {
c.tiny = uintptr(x)
c.tinyoffset = size
}
size = maxTinySize
}
如上所示，tinyoffset 表示 tiny 当前分配到什么地址了，之后的分配根据 tinyoffset 寻址。先根据要分配的对象大小进行地址对齐，比如 size 是 8 的倍数，tinyoffset 和 8 对齐。然后就是进行分配。如果 tiny 剩余的空间不够用，则重新申请一个 16 byte 的内存块，并分配给 object。如果有结余，则记录在 tiny 上。
5.2 size 大于 32 K byte
对于大于 32 Kb 的内存分配，直接跳过 mcache 和 mcentral，通过 mheap 分配。
func mallocgc(...) {
} else {
var s *mspan
shouldhelpgc = true
systemstack(func() {
s = largeAlloc(size, needzero)
})
s.freeindex = 1
s.allocCount = 1
x = unsafe.Pointer(s.base())
size = s.elemsize
}
...
}
func largeAlloc(size uintptr, needzero bool) *mspan {
...
npages := size >> _PageShift
if size&_PageMask != 0 {
npages++
}
...
s := mheap_.alloc(npages, 0, true, needzero)
if s == nil {
throw("out of memory")
}
s.limit = s.base() + size
heapBitsForSpan(s.base()).initSpan(s)
return s
}
对于大于 32 K 的内存分配都是分配整数页，先右移然后低位与计算需要的页数。
5.3 size 介于 16 和 32K
对于 size 介于 16 ~ 32K byte 的内存分配先计算应该分配的 sizeclass，然后去 mcache 里面 alloc[sizeclass] 申请，如果 mcache.alloc[sizeclass] 不足以申请，则 mcache 向 mcentral 申请，然后再分配。mcentral 给 mcache 分配完之后会判断自己需不需要扩充，如果需要则想 mheap 申请。
func mallocgc(...) {
...
} else {
var sizeclass uint8
//计算 sizeclass
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]
//从对应的 span 里面分配一个 object
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)
}
}
}
我们首先看一下如何计算 sizeclass 的，预先定义了两个数组：size_to_class8 和 size_to_class128。 数组 size_to_class8，其第 i 个值表示地址区间 ( (i-1)8, i8 ] (smallSizeDiv = 8) 对应的 sizeclass，size_to_class128 类似。小于 1024 - 8 = 1016 （smallSizeMax=1024），使用 size_to_class8，否则使用数组 size_to_class128。看一下数组具体的值：0, 1, 2, 3, 3, 4, 4…。举个例子，比如要分配 17 byte 的内存 （16 byte 以下的使用 mcache.tiny 分配），sizeclass = size_to_calss8[(17+7)/8] = size_to_class8[3] = 3。不得不说这种用空间换时间的策略确实提高了运行效率。
计算出 sizeclass，那么就可以去 mcache.alloc[sizeclass] 分配了，注意这是一个 mspan 指针，真正的分配函数是 nextFreeFast() 函数，如下。
// nextFreeFast returns the next free object if one is quickly available.
// Otherwise it returns 0.
func nextFreeFast(s *mspan) gclinkptr {
theBit := sys.Ctz64(s.allocCache) // Is there a free object in the allocCache?
if theBit < 64 {
result := s.freeindex + uintptr(theBit)
if result < s.nelems {
freeidx := result + 1
if freeidx%64 == 0 && freeidx != s.nelems {
return 0
}
s.allocCache >>= uint(theBit + 1)
s.freeindex = freeidx
s.allocCount++
return gclinkptr(result*s.elemsize + s.base())
}
}
return 0
}
allocCache 这里是用位图表示内存是否可用，1 表示可用。然后通过 span 里面的 freeindex 和 elemsize 来计算地址即可。
如果 mcache.alloc[sizeclass] 已经不够用了，则从 mcentral 申请内存到 mcache。
// nextFree returns the next free object from the cached span if one is available.
// Otherwise it refills the cache with a span with an available object and
// returns that object along with a flag indicating that this was a heavy
// weight allocation. If it is a heavy weight allocation the caller must
// determine whether a new GC cycle needs to be started or if the GC is active
// whether this goroutine needs to assist the GC.
func (c *mcache) nextFree(sizeclass uint8) (v gclinkptr, s *mspan, shouldhelpgc bool) {
s = c.alloc[sizeclass]
shouldhelpgc = false
freeIndex := s.nextFreeIndex()
if freeIndex == s.nelems {
// The span is full.
if uintptr(s.allocCount) != s.nelems {
println("runtime: s.allocCount=", s.allocCount, "s.nelems=", s.nelems)
throw("s.allocCount != s.nelems && freeIndex == s.nelems")
}
systemstack(func() {
// 这个地方 mcache 向 mcentral 申请
c.refill(int32(sizeclass))
})
shouldhelpgc = true
s = c.alloc[sizeclass]
// mcache 向 mcentral 申请完之后，再次从 mcache 申请
freeIndex = s.nextFreeIndex()
}
...
}
// nextFreeIndex returns the index of the next free object in s at
// or after s.freeindex.
// There are hardware instructions that can be used to make this
// faster if profiling warrants it.
// 这个函数和 nextFreeFast 有点冗余了
func (s *mspan) nextFreeIndex() uintptr {
...
}
mcache 向 mcentral，如果 mcentral 不够，则向 mheap 申请。
//go version 1.10.8
func (c *mcache) refill(sizeclass int32) *mspan {
...
// 向 mcentral 申请
s = mheap_.central[sizeclass].mcentral.cacheSpan()
...
return s
}
// Allocate a span to use in an MCache.
func (c *mcentral) cacheSpan() *mspan {
...
// Replenish central list if empty.
s = c.grow()
}
func (c *mcentral) grow() *mspan {
npages := uintptr(class_to_allocnpages[c.sizeclass])
size := uintptr(class_to_size[c.sizeclass])
n := (npages << _PageShift) / size
//这里向 mheap 申请
s := mheap_.alloc(npages, c.sizeclass, false, true)
...
return s
}
如果 mheap 不足，则向 OS 申请。接上面的代码 mheap_.alloc()
func (h *mheap) alloc(npage uintptr, sizeclass int32, large bool, needzero bool) *mspan {
...
var s *mspan
systemstack(func() {
s = h.alloc_m(npage, sizeclass, large)
})
...
}
func (h *mheap) alloc_m(npage uintptr, sizeclass int32, large bool) *mspan {
...
s := h.allocSpanLocked(npage)
...
}
func (h *mheap) allocSpanLocked(npage uintptr) *mspan {
...
s = h.allocLarge(npage)
if s == nil {
if !h.grow(npage) {
return nil
}
s = h.allocLarge(npage)
if s == nil {
return nil
}
}
...
}
func (h *mheap) grow(npage uintptr) bool {
// Ask for a big chunk, to reduce the number of mappings
// the operating system needs to track; also amortizes
// the overhead of an operating system mapping.
// Allocate a multiple of 64kB.
npage = round(npage, (64<<10)/_PageSize)
ask := npage << _PageShift
if ask < _HeapAllocChunk {
ask = _HeapAllocChunk
}
v := h.sysAlloc(ask)
...
}
整个函数调用链如上所示，最后 sysAlloc 会调用系统调用（mmap 或者 VirtualAlloc，和初始化那部分有点类似）去向操作系统申请。
六、内存回收
这里只会介绍一些简单的内存回收。
6.1 mcache 回收
mcache 回收可以分两部分：第一部分是将 alloc 中未用完的内存归还给对应的 mcentral。
func freemcache(c *mcache) {
systemstack(func() {
c.releaseAll()
...
lock(&mheap_.lock)
purgecachedstats(c)
mheap_.cachealloc.free(unsafe.Pointer(c))
unlock(&mheap_.lock)
})
}
func (c *mcache) releaseAll() {
for i := 0; i < _NumSizeClasses; i++ {
s := c.alloc[i]
if s != &emptymspan {
mheap_.central[i].mcentral.uncacheSpan(s)
c.alloc[i] = &emptymspan
}
}
// Clear tinyalloc pool.
c.tiny = 0
c.tinyoffset = 0
}
6.2 mcentral 回收
当 mspan 没有 free object 的时候，将 mspan 归还给 mheap。
func (c *mcentral) freeSpan(s *mspan, preserve bool, wasempty bool) bool {
...
lock(&c.lock)
...
if s.allocCount != 0 {
unlock(&c.lock)
return false
}
c.nonempty.remove(s)
unlock(&c.lock)
mheap_.freeSpan(s, 0)
return true
}
6.3 mheap
mheap 并不会定时向操作系统归还，但是会对 span 做一些操作，比如合并相邻的 span。
七、总结
目前GoLang内存管理还处于学习阶段，上文多是整理学习了一些目前互联网的信息，参考资源都已列出，有些地方还没有理解，如有问题，请多交流。
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