Go-内存To Be

做一个快乐的互联网搬运工～

逃逸分析

逃逸分析的概念

在编译程序优化理论中，逃逸分析是一种确定指针动态范围的方法——分析在程序的哪些地方可以访问到指针。它涉及到指针分析和形状分析。当一个变量(或对象)在子程序中被分配时，一个指向变量的指针可能逃逸到其它执行线程中，或是返回到调用者子程序。

——维基百科

Go在一定程度消除了堆和栈的区别，因为go在编译的时候进行逃逸分析，来决定一个对象放栈上还是放堆上，不逃逸的对象放栈上，可能逃逸的放堆上。

逃逸分析的作用

堆与栈相比，栈具有速度快，易分配、易释放的特点，而堆适合不可预知大小的内存分配，但分配、回收成本大。通过逃逸分析，可以尽量把那些不需要分配到堆上的变量直接分配到栈上，堆上的变量少了，会减轻分配堆内存的开销，同时也会减少gc的压力，提高程序的运行速度。

另外，通过逃逸分析，还可以进行同步消除，如果你定义的对象的方法上有同步锁，但在运行时，却只有一个线程在访问，此时逃逸分析后的机器码，会去掉同步锁运行。

如何逃逸分析

根据维基百科的定义我们可以简化一下概念：如果一个函数返回对一个变量的引用，那么它就会发生逃逸。

看一个例子：

package main

import "fmt"

//func foo() *int {
//	t:=3
//	return &t
//}

func boo() int {
	t:=3
	return t
}

func main()  {
	//x:=foo()
	//fmt.Println(*x)
	//# command-line-arguments
	//./memory.go:7:9: &t escapes to heap
	//./memory.go:6:2: moved to heap: t
	//./memory.go:17:14: *x escapes to heap
	//./memory.go:17:13: main ... argument does not escape

	var x int
	x=boo()
	fmt.Println(x)
	//# command-line-arguments
	//./memory.go:20:13: x escapes to heap
	//./memory.go:20:13: main ... argument does not escape
}

View Code

其中有两个函数，一个传值，一个传地址，我们可以通过命令

go build -gcflags '-m -l' main.go

发现，传值变量没有发生逃逸，传址变量发生了逃逸。

但是代码中，main函数里的x也逃逸了，这是因为有些函数参数为interface类型，比如fmt.Println(a ...interface{})，编译期间很难确定其参数的具体类型，也会发生逃逸。

并且，如果一个变量需要申请的内存过大，超过了栈的存储能力，即便在函数返回后，它不会被引用，还是会被分配到堆上。

因此，当出现地址引用、申请内存过大、无法确定内存大小（比如切片）、无法确定引用类型（比如interface）这4种情况的时候，会发生逃逸。

所以啊，指针是把双刃剑，减少逃逸代码，有助于提升代码运行效率。

实例

type S struct {}

func main()  {
	var x S
	y:=&x
	_=*identity(y)
}

func identity(z *S) *S {
	return z
}
//# command-line-arguments
//./memory2.go:11:15: leaking param: z to result ~r1 level=0
//./memory2.go:7:5: main &x does not escape

//没有发生逃逸
//因为identity这个函数仅仅输入一个变量，
//又将这个变量作为返回输出，但identity并没有引用z，所以这个变量没有逃逸，
//而x没有被引用，且生命周期也在mian里，x没有逃逸，分配在栈上。
//但是参数z发生了泄漏（也有的说法是z是流式的），参数泄漏不是内存泄漏，
//仅仅是指而是指该传入参数的内容的生命期，超过函数调用期，也就是函数返回后，该参数的内容仍然存活

example1

package main

type M struct {}

func main()  {
	var x M
	_=*ref(x)
}

func ref(z M) *M {
	return &z
}

//# command-line-arguments
//./memory3.go:11:9: &z escapes to heap
//./memory3.go:10:10: moved to heap: z

//变量z发生了逃逸
//go都是值传递，ref函数copy了x的值，传给z，返回z的指针，
//然后在函数外被引用，说明z这个变量在函数內声明，可能会被函数外的其他程序访问。
//所以z逃逸了，分配在堆上。

example2

type Q struct {
	A *int
}

func main()  {
	var i int
	refStruct(i)
}

func refStruct(y int) (z Q) {
	z.A=&y
	return z
}

//# command-line-arguments
//./memory4.go:13:6: &y escapes to heap
//./memory4.go:12:16: moved to heap: y

//y发生逃逸
//在struct里好像并没有区别，有可能被函数外的程序访问就会逃逸

example3

type Q2 struct {
	A *int
}

func main()  {
	var i int
	refStruct2(&i)
}

func refStruct2(y *int) (z Q2) {
	z.A=y
	return z
}
//# command-line-arguments
//./memory5.go:12:17: leaking param: y to result z level=0
//./memory5.go:9:13: main &i does not escape

//y没有逃逸，只是发生了参数泄漏
//因为y只作为一个复制的载体，并不会被外部引用

example4

type D struct {
	J *int
}

func main()  {
	var x D
	var i int
	ref2(&i,&x)
}

func ref2(y *int,z *D)  {
	z.J=y
	//return z
}
//# command-line-arguments
//./memory6.go:13:11: leaking param: y
//./memory6.go:13:18: ref2 z does not escape
//./memory6.go:10:7: &i escapes to heap
//./memory6.go:9:6: moved to heap: i
//./memory6.go:10:10: main &x does not escape

//y没有逃逸，只是泄漏，因为显而易见的原因
//z也没有发生逃逸，因为它作为一个在函数内被定义的变量，只是被赋了个值而已，并不会再被外部引用
//而i发生了逃逸，因为i作为一个在main函数里被定义的变量，被传到函数ref2里，它这块内存锁标定的地址被外部引用，因此发生逃逸。
//x没有发生逃逸，因为它作为一个在main函数里被定义的变量，只是被赋值，并没有将自己的地址进行传递，因此没有发生逃逸

//加上return，是如下结果
//# command-line-arguments
//./memory6.go:13:11: leaking param: y
//./memory6.go:13:18: leaking param: z to result ~r2 level=0
//./memory6.go:10:7: &i escapes to heap
//./memory6.go:9:6: moved to heap: i
//./memory6.go:10:10: main &x does not escape
//大体结果一样，只不过z发生了泄漏

example5

看完这些例子，你应该对逃逸分析有了一个大体的了解，或者，更凌乱了。

内存管理

对于内置runtime system的编程语言，通常会抛弃传统的内存分配方式，改为自主管理内存。这样可以完成类似预分配、内存池、垃圾回收等操作，以避开频繁地向操作系统申请、释放内存，产生过多的系统调用而导致的性能问题。

golang的runtime system同样实现了一套内存池机制，接管了所有的内存申请和释放的动作。

基础概念

Golang程序启动伊始，会向系统申请一块内存（虚拟的地址空间），如下图：

盗了下面链接一个作者的图～

预申请的内存分为三部分：spans、bitmap、arena。

arena就是通俗意义上的堆区，它被分割成页，每一页8KB，所以一共有512GB/8KB页。

bitmap用于标识arena中哪些地址保存了对象，以及对象是否包含指针，如下图：

其中1个byte（8bit）标识arena中4页的信息，也就是说2bit标识1页，所以bitmap的大小为(（512GB/8KB）/4)*1B=16GB。

spans用于表示arena区中的某一页属于哪个mspan（就是由arena页组合起来的内存管理基本单元），如下图：

每个指针对应一页，所以spans的大小为（512GB/8K）*8B。

核心数据结构

内存管理单元

mspan

mspan是内存分配和管理的基本单位，用于管理预分配个数为sizeClass的连续地址内存。

如下源码：

package runtime

// class  bytes/obj  bytes/span  objects  tail waste  max waste
//     1          8        8192     1024           0     87.50%
//     2         16        8192      512           0     43.75%
//     3         32        8192      256           0     46.88%
//     4         48        8192      170          32     31.52%
//     5         64        8192      128           0     23.44%
//     6         80        8192      102          32     19.07%
//     7         96        8192       85          32     15.95%
//     8        112        8192       73          16     13.56%
//     9        128        8192       64           0     11.72%
//    10        144        8192       56         128     11.82%
//    11        160        8192       51          32      9.73%
//    12        176        8192       46          96      9.59%
//    13        192        8192       42         128      9.25%
//    14        208        8192       39          80      8.12%
//    15        224        8192       36         128      8.15%
//    16        240        8192       34          32      6.62%
//    17        256        8192       32           0      5.86%
//    18        288        8192       28         128     12.16%
//    19        320        8192       25         192     11.80%
//    20        352        8192       23          96      9.88%
//    21        384        8192       21         128      9.51%
//    22        416        8192       19         288     10.71%
//    23        448        8192       18         128      8.37%
//    24        480        8192       17          32      6.82%
//    25        512        8192       16           0      6.05%
//    26        576        8192       14         128     12.33%
//    27        640        8192       12         512     15.48%
//    28        704        8192       11         448     13.93%
//    29        768        8192       10         512     13.94%
//    30        896        8192        9         128     15.52%
//    31       1024        8192        8           0     12.40%
//    32       1152        8192        7         128     12.41%
//    33       1280        8192        6         512     15.55%
//    34       1408       16384       11         896     14.00%
//    35       1536        8192        5         512     14.00%
//    36       1792       16384        9         256     15.57%
//    37       2048        8192        4           0     12.45%
//    38       2304       16384        7         256     12.46%
//    39       2688        8192        3         128     15.59%
//    40       3072       24576        8           0     12.47%
//    41       3200       16384        5         384      6.22%
//    42       3456       24576        7         384      8.83%
//    43       4096        8192        2           0     15.60%
//    44       4864       24576        5         256     16.65%
//    45       5376       16384        3         256     10.92%
//    46       6144       24576        4           0     12.48%
//    47       6528       32768        5         128      6.23%
//    48       6784       40960        6         256      4.36%
//    49       6912       49152        7         768      3.37%
//    50       8192        8192        1           0     15.61%
//    51       9472       57344        6         512     14.28%
//    52       9728       49152        5         512      3.64%
//    53      10240       40960        4           0      4.99%
//    54      10880       32768        3         128      6.24%
//    55      12288       24576        2           0     11.45%
//    56      13568       40960        3         256      9.99%
//    57      14336       57344        4           0      5.35%
//    58      16384       16384        1           0     12.49%
//    59      18432       73728        4           0     11.11%
//    60      19072       57344        3         128      3.57%
//    61      20480       40960        2           0      6.87%
//    62      21760       65536        3         256      6.25%
//    63      24576       24576        1           0     11.45%
//    64      27264       81920        3         128     10.00%
//    65      28672       57344        2           0      4.91%
//    66      32768       32768        1           0     12.50%

const (
	_MaxSmallSize   = 32768
	smallSizeDiv    = 8
	smallSizeMax    = 1024
	largeSizeDiv    = 128
	_NumSizeClasses = 67
	_PageShift      = 13
)

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}
var class_to_allocnpages = [_NumSizeClasses]uint8{0, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 2, 1, 2, 1, 2, 1, 3, 2, 3, 1, 3, 2, 3, 4, 5, 6, 1, 7, 6, 5, 4, 3, 5, 7, 2, 9, 7, 5, 8, 3, 10, 7, 4}

type divMagic struct {
	shift    uint8
	shift2   uint8
	mul      uint16
	baseMask uint16
}

var class_to_divmagic = [_NumSizeClasses]divMagic{{0, 0, 0, 0}, {3, 0, 1, 65528}, {4, 0, 1, 65520}, {5, 0, 1, 65504}, {4, 9, 171, 0}, {6, 0, 1, 65472}, {4, 10, 205, 0}, {5, 9, 171, 0}, {4, 11, 293, 0}, {7, 0, 1, 65408}, {4, 9, 57, 0}, {5, 10, 205, 0}, {4, 12, 373, 0}, {6, 7, 43, 0}, {4, 13, 631, 0}, {5, 11, 293, 0}, {4, 13, 547, 0}, {8, 0, 1, 65280}, {5, 9, 57, 0}, {6, 9, 103, 0}, {5, 12, 373, 0}, {7, 7, 43, 0}, {5, 10, 79, 0}, {6, 10, 147, 0}, {5, 11, 137, 0}, {9, 0, 1, 65024}, {6, 9, 57, 0}, {7, 6, 13, 0}, {6, 11, 187, 0}, {8, 5, 11, 0}, {7, 8, 37, 0}, {10, 0, 1, 64512}, {7, 9, 57, 0}, {8, 6, 13, 0}, {7, 11, 187, 0}, {9, 5, 11, 0}, {8, 8, 37, 0}, {11, 0, 1, 63488}, {8, 9, 57, 0}, {7, 10, 49, 0}, {10, 5, 11, 0}, {7, 10, 41, 0}, {7, 9, 19, 0}, {12, 0, 1, 61440}, {8, 9, 27, 0}, {8, 10, 49, 0}, {11, 5, 11, 0}, {7, 13, 161, 0}, {7, 13, 155, 0}, {8, 9, 19, 0}, {13, 0, 1, 57344}, {8, 12, 111, 0}, {9, 9, 27, 0}, {11, 6, 13, 0}, {7, 14, 193, 0}, {12, 3, 3, 0}, {8, 13, 155, 0}, {11, 8, 37, 0}, {14, 0, 1, 49152}, {11, 8, 29, 0}, {7, 13, 55, 0}, {12, 5, 7, 0}, {8, 14, 193, 0}, {13, 3, 3, 0}, {7, 14, 77, 0}, {12, 7, 19, 0}, {15, 0, 1, 32768}}
var size_to_class8 = [smallSizeMax/smallSizeDiv + 1]uint8{0, 1, 2, 3, 3, 4, 4, 5, 5, 6, 6, 7, 7, 8, 8, 9, 9, 10, 10, 11, 11, 12, 12, 13, 13, 14, 14, 15, 15, 16, 16, 17, 17, 18, 18, 18, 18, 19, 19, 19, 19, 20, 20, 20, 20, 21, 21, 21, 21, 22, 22, 22, 22, 23, 23, 23, 23, 24, 24, 24, 24, 25, 25, 25, 25, 26, 26, 26, 26, 26, 26, 26, 26, 27, 27, 27, 27, 27, 27, 27, 27, 28, 28, 28, 28, 28, 28, 28, 28, 29, 29, 29, 29, 29, 29, 29, 29, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31}
var size_to_class128 = [(_MaxSmallSize-smallSizeMax)/largeSizeDiv + 1]uint8{31, 32, 33, 34, 35, 36, 36, 37, 37, 38, 38, 39, 39, 39, 40, 40, 40, 41, 42, 42, 43, 43, 43, 43, 43, 44, 44, 44, 44, 44, 44, 45, 45, 45, 45, 46, 46, 46, 46, 46, 46, 47, 47, 47, 48, 48, 49, 50, 50, 50, 50, 50, 50, 50, 50, 50, 50, 51, 51, 51, 51, 51, 51, 51, 51, 51, 51, 52, 52, 53, 53, 53, 53, 54, 54, 54, 54, 54, 55, 55, 55, 55, 55, 55, 55, 55, 55, 55, 55, 56, 56, 56, 56, 56, 56, 56, 56, 56, 56, 57, 57, 57, 57, 57, 57, 58, 58, 58, 58, 58, 58, 58, 58, 58, 58, 58, 58, 58, 58, 58, 58, 59, 59, 59, 59, 59, 59, 59, 59, 59, 59, 59, 59, 59, 59, 59, 59, 60, 60, 60, 60, 60, 61, 61, 61, 61, 61, 61, 61, 61, 61, 61, 61, 62, 62, 62, 62, 62, 62, 62, 62, 62, 62, 63, 63, 63, 63, 63, 63, 63, 63, 63, 63, 63, 63, 63, 63, 63, 63, 63, 63, 63, 63, 63, 63, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 66, 66, 66, 66, 66, 66, 66, 66, 66, 66, 66, 66, 66, 66, 66, 66, 66, 66, 66, 66, 66, 66, 66, 66, 66, 66, 66, 66, 66, 66, 66, 66}

/usr/local/go/src/runtime/sizeclasses.go

可以看到，

mspan共有67种sizeClass，每个mspan按照自身的sizeClass分割成若干个object，每个object可存储一个对象。

比如，某mspan的sizeClass为3，那么其划分的object大小就是32B，可以存储(16B，32B]大小的的对象。同时，sizeClass还决定了mspan能够分到的页数，比如，sizeClass对应能分到的页数是1。

另外，数组中最大的数是32768，也就是32KB，超过32KB即是大对象，sizeClass=0就表示大对象，不通过mspan，直接由mheap分配内存。对于，小于16B的是微小对象，分配器会将其合并，将几个对象分配到同一个object中。

有一点我有些恍惚，最大的容量是32KB，而arena中每一页的大小是8KB，也就是说，定然存在跨页存储，虽然没有找到可靠的资料，但是，观察一下源码可以发现，页数为1的最大object容量是8192B，也就正好是8KB。当object为32KB时，页数为4，当object为24KB时，页数为3，也就是说，在golang写死的sizeClass里，针对页面容量、可存放对象大小和页数已经预置了对应关系，或许，这也是写死sizeClass的原因之一。

mspan的结构体如下：

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
// if sweepgen == h->sweepgen + 1, the span was cached before sweep began and is still cached, and needs sweeping
// if sweepgen == h->sweepgen + 3, the span was swept and then cached and is still cached
// 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)
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
scavenged bool // whether this span has had its pages released to the OS
elemsize uintptr // computed from sizeclass or from npages
unusedsince int64 // first time spotted by gc in mspanfree state
limit uintptr // end of data in span
speciallock mutex // guards specials list
specials *special // linked list of special records sorted by offset.
}

/usr/local/go/src/runtime/mheap.go

next *mspan

指向链表里的下一个mspan

prev *mspan

指向链表里的上一个mspan

list *mSpanList

mspan链表

startAddr uintptr

管理页的起始地址，直接指向arena区域的的某个位置

npages uintptr

管理的页数

manualFreeList gclinkptr

空闲对象列表

freeindex uintptr

freeindex是0到nelems之间的位置索引，在该索引处开始扫描此mspan的下一个空闲对象。每次分配从freeindex开始扫描allocBits，直到遇到空闲对象。然后调整freeindex，以便于后续扫描发现下一个空闲对象。如果freeindex等于nelems，那么这片mspan没有空闲对象。

nelems uintptr

管理的对象数

allocBits *gcBits

allocBits是此mspan中对象的位图。当n>=freeindex并且其位图标记为0，那么这个对象就是空闲的，否则，这个对象就是被占用的。

allocCount uint16

已分配的对象的个数

elemsize uintptr

对象的大小

内存管理组件

内存分配由内存分配器完成。分配器由3种组件构成：mcache、mcentral、mheap。

mcache

首先我们需要先简单了解一个概念，Go调度器的基本结构，也就是G-P-M模型：

G: 表示Goroutine，每个Goroutine对应一个G结构体，G存储Goroutine的运行堆栈、状态以及任务函数，可重用。G并非执行体，每个G需要绑定到P才能被调度执行。
P: Processor，表示逻辑处理器，对G来说，P相当于CPU核，G只有绑定到P(在P的local runq中)才能被调度。对M来说，P提供了相关的执行环境(Context)，如内存分配状态(mcache)，任务队列(G)等，P的数量决定了系统内最大可并行的G的数量（前提：物理CPU核数 >= P的数量），P的数量由用户设置的GOMAXPROCS决定，但是不论GOMAXPROCS设置为多大，P的数量最大为256。
M: Machine，OS线程抽象，代表着真正执行计算的资源，在绑定有效的P后，进入schedule循环；而schedule循环的机制大致是从Global队列、P的Local队列以及wait队列中获取G，切换到G的执行栈上并执行G的函数，调用goexit做清理工作并回到M，如此反复。M并不保留G状态，这是G可以跨M调度的基础，M的数量是不定的，由Go Runtime调整，为了防止创建过多OS线程导致系统调度不过来，目前默认最大限制为10000个。

其中，P是一个虚拟资源，同时只能被一个协程访问，所以P中的数据不需要锁。为了分配对象时有更好的性能，各个P中都有各种sizeClass的mspan的缓存，这样就可以直接给G分配。其结构如下：

其中，对于67中sizeClass各有两个，一个用于分配包含指针的对象，一个用于分配不包含指针的对象，因此一个P中共有134个mspan的缓存。对于无指针对象的mspan在进行垃圾回收时，不需要进一步扫描它是否引用了其他活跃的对象。

mcache在初始化时没有任何mspan资源，在使用过程中会动态地从mcentral申请，之后缓存下来。当对象<=32KB时，使用mcache相应规格的mspan分配。

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_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)

	// 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
}

/usr/local/go/src/runtime/mcache.go

其中numSpanClasses的就是134，而且mcache中也没有任何加锁信息。

mcentral

为所有mcache提供切分好的mspan资源，包含已分配出去的和未分配出去的。每个mcentral对应一种sizeClass的mspan，因此共有134个mcentral。其结构如下：

当P中没有特定大小的mspan时，就会向mcentral申请。mcentral由于是共享资源，因此需要有锁。

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
}

/usr/local/go/src/runtime/mcentral.go

mcentral的结构体源码很短，清晰地体现出了它的特点。其中的nmalloc用于累计已分配的mspan数量。

mcache向mcentral获取、归还mspan的流程：

获取：加锁；从nonempty中取出一个可用mspan；将其从nonempty链表删除；将其加入到empty链表；解锁。
归还：加锁；将mspan从empty链表删除；加入到nonempty链表；解锁。

mheap

代表Go程序持有的所有堆空间。当mcentral没有空闲的mspan时，会向mheap申请。若mheap也没有，会向操作系统申请。mheap主要用于大对象的内存分配，以及管理未切割的mspan，用于给mcentral切割成小对象。其结构如下：

mheap的源码如下：

type mheap struct {
lock mutex
free mTreap // free and non-scavenged spans
scav mTreap // free and scavenged spans
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

// 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.

// Page reclaimer state

// reclaimIndex is the page index in allArenas of next page to
// reclaim. Specifically, it refers to page (i %
// pagesPerArena) of arena allArenas[i / pagesPerArena].
//
// If this is >= 1<<63, the page reclaimer is done scanning
// the page marks.
//
// This is accessed atomically.
reclaimIndex uint64
// reclaimCredit is spare credit for extra pages swept. Since
// the page reclaimer works in large chunks, it may reclaim
// more than requested. Any spare pages released go to this
// credit pool.
//
// This is accessed atomically.
reclaimCredit uintptr

// scavengeCredit is spare credit for extra bytes scavenged.
// Since the scavenging mechanisms operate on spans, it may
// scavenge more than requested. Any spare pages released
// go to this credit pool.
//
// This is protected by the mheap lock.
scavengeCredit uintptr

// 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)

// arenas is the heap arena map. It points to the metadata for
// the heap for every arena frame of the entire usable virtual
// address space.
//
// Use arenaIndex to compute indexes into this array.
//
// For regions of the address space that are not backed by the
// Go heap, the arena map contains nil.
//
// Modifications are protected by mheap_.lock. Reads can be
// performed without locking; however, a given entry can
// transition from nil to non-nil at any time when the lock
// isn't held. (Entries never transitions back to nil.)
//
// In general, this is a two-level mapping consisting of an L1
// map and possibly many L2 maps. This saves space when there
// are a huge number of arena frames. However, on many
// platforms (even 64-bit), arenaL1Bits is 0, making this
// effectively a single-level map. In this case, arenas[0]
// will never be nil.
arenas [1 << arenaL1Bits]*[1 << arenaL2Bits]*heapArena

// heapArenaAlloc is pre-reserved space for allocating heapArena
// objects. This is only used on 32-bit, where we pre-reserve
// this space to avoid interleaving it with the heap itself.
heapArenaAlloc linearAlloc

// arenaHints is a list of addresses at which to attempt to
// add more heap arenas. This is initially populated with a
// set of general hint addresses, and grown with the bounds of
// actual heap arena ranges.
arenaHints *arenaHint

// arena is a pre-reserved space for allocating heap arenas
// (the actual arenas). This is only used on 32-bit.
arena linearAlloc

// allArenas is the arenaIndex of every mapped arena. This can
// be used to iterate through the address space.
//
// Access is protected by mheap_.lock. However, since this is
// append-only and old backing arrays are never freed, it is
// safe to acquire mheap_.lock, copy the slice header, and
// then release mheap_.lock.
allArenas []arenaIdx

// sweepArenas is a snapshot of allArenas taken at the
// beginning of the sweep cycle. This can be read safely by
// simply blocking GC (by disabling preemption).
sweepArenas []arenaIdx

// _ uint32 // ensure 64-bit alignment of central

// central free lists for small size classes.
// the padding makes sure that the mcentrals are
// spaced CacheLinePadSize bytes apart, so that each mcentral.lock
// gets its own cache line.
// central is indexed by spanClass.
central [numSpanClasses]struct {
mcentral mcentral
pad [cpu.CacheLinePadSize - unsafe.Sizeof(mcentral{})%cpu.CacheLinePadSize]byte
}

spanalloc fixalloc // allocator for span*
cachealloc fixalloc // allocator for mcache*
treapalloc fixalloc // allocator for treapNodes*
specialfinalizeralloc fixalloc // allocator for specialfinalizer*
specialprofilealloc fixalloc // allocator for specialprofile*
speciallock mutex // lock for special record allocators.
arenaHintAlloc fixalloc // allocator for arenaHints

unused *specialfinalizer // never set, just here to force the specialfinalizer type into DWARF
}

/usr/local/go/src/runtime/mheap.go

其中具备了，锁信息、spans信息、arena区信息、central信息。