Golang Map 源码阅读与分析
我们知道哈希表解决冲突一般要么是开放定址法，要么是再哈希法。Golang采用的是 第一种，但是实现上和教科书式的实现方式不同。这也是很有趣的一个东西，就像之前 看Python的 deque 一样，和教科书不一样，非常高效的实现方式。
通常教科书式的实现方式是，hash值重复的节点组成一个链表，首先我们根据hash值 定位到大概在哪里，然后遍历这个链表。这样有一个缺点，就是不知道这个链表到底有多长。
Golang的实现避免了这种问题，就是采用桶的方式，也就是每个坑后面接固定长度的键值对。
// A header for a Go map.
type hmap struct {
// Note: the format of the Hmap is encoded in ../../cmd/internal/gc/reflect.go and
// ../reflect/type.go. Don't change this structure without also changing that code!
count int // # live cells == size of map. Must be first (used by len() builtin)
flags uint8
B uint8 // log_2 of # of buckets (can hold up to loadFactor * 2^B items)
noverflow uint16 // approximate number of overflow buckets; see incrnoverflow for details
hash0 uint32 // hash seed
buckets unsafe.Pointer // array of 2^B Buckets. may be nil if count==0.
oldbuckets unsafe.Pointer // previous bucket array of half the size, non-nil only when growing
nevacuate uintptr // progress counter for evacuation (buckets less than this have been evacuated)
extra *mapextra // optional fields
}
// mapextra holds fields that are not present on all maps.
type mapextra struct {
// If both key and value do not contain pointers and are inline, then we mark bucket
// type as containing no pointers. This avoids scanning such maps.
// However, bmap.overflow is a pointer. In order to keep overflow buckets
// alive, we store pointers to all overflow buckets in hmap.overflow.
// Overflow is used only if key and value do not contain pointers.
// overflow[0] contains overflow buckets for hmap.buckets.
// overflow[1] contains overflow buckets for hmap.oldbuckets.
// The indirection allows to store a pointer to the slice in hiter.
overflow [2]*[]*bmap
// nextOverflow holds a pointer to a free overflow bucket.
nextOverflow *bmap
}
// A bucket for a Go map.
type bmap struct {
// tophash generally contains the top byte of the hash value
// for each key in this bucket. If tophash[0] < minTopHash,
// tophash[0] is a bucket evacuation state instead.
tophash [bucketCnt]uint8
// Followed by bucketCnt keys and then bucketCnt values.
// NOTE: packing all the keys together and then all the values together makes the
// code a bit more complicated than alternating key/value/key/value/... but it allows
// us to eliminate padding which would be needed for, e.g., map[int64]int8.
// Followed by an overflow pointer.
}
不过看到这里我就想吐槽了，Golang虽然标准库设计的很科学，但是很多命名实在是太简单了，
不方便记忆。估计只有写的人自己才能一眼反应过来吧。例如 B uint8 // log_2 of # of buckets (can hold up to loadFactor * 2^B items)
要是我的话，会选择稍微长一点，更容易记住的名字。
buckets 是一个指针，指向的就是 *bmap 这种键值对的数组。tophash[0] 是数组
里的第一个值，相当于链表里的第一个。不过如果冲突到了多于8个怎么办？所以
里面还有 overflow 这样的指针，指向两个 *[]*bmap slice。这样就相当于链表了，
假设冲突已经到了这种层次的话。
下面来看看Golang的map是怎么查找的，我们知道Go的一个坑点就是字典不管找没找到，
都有返回，默认返回value对应类型的 zero object。所以要用类似 a, ok := m["hello"]
的形式，然后判断 ok。麻烦。
// mapaccess1 returns a pointer to h[key]. Never returns nil, instead
// it will return a reference to the zero object for the value type if
// the key is not in the map.
// NOTE: The returned pointer may keep the whole map live, so don't
// hold onto it for very long.
func mapaccess1(t *maptype, h *hmap, key unsafe.Pointer) unsafe.Pointer {
if raceenabled && h != nil {
callerpc := getcallerpc(unsafe.Pointer(&t))
pc := funcPC(mapaccess1)
racereadpc(unsafe.Pointer(h), callerpc, pc)
raceReadObjectPC(t.key, key, callerpc, pc)
}
if msanenabled && h != nil {
msanread(key, t.key.size)
}
// 如果h里面啥也没有的话，直接返回
if h == nil || h.count == 0 {
return unsafe.Pointer(&zeroVal[0])
}
// 原来上面结构体里flags是用来做读写状态标识的
if h.flags&hashWriting != 0 {
throw("concurrent map read and map write")
}
// t是map的类型，Go是编译型语言，所以在编译的时候应该就确定好了
// 把key的类型确定好，hash算法固定好，直接用就好了
alg := t.key.alg
hash := alg.hash(key, uintptr(h.hash0))
m := uintptr(1)<<h.B - 1
b := (*bmap)(add(h.buckets, (hash&m)*uintptr(t.bucketsize)))
if c := h.oldbuckets; c != nil {
if !h.sameSizeGrow() {
// There used to be half as many buckets; mask down one more power of two.
m >>= 1
}
oldb := (*bmap)(add(c, (hash&m)*uintptr(t.bucketsize)))
if !evacuated(oldb) {
b = oldb
}
}
// 算出在哪个桶，哈希值取余？
top := uint8(hash >> (sys.PtrSize*8 - 8))
if top < minTopHash {
top += minTopHash
}
// 依次遍历，找出对象
for {
for i := uintptr(0); i < bucketCnt; i++ {
if b.tophash[i] != top {
continue
}
k := add(unsafe.Pointer(b), dataOffset+i*uintptr(t.keysize))
if t.indirectkey {
k = *((*unsafe.Pointer)(k))
}
if alg.equal(key, k) {
v := add(unsafe.Pointer(b), dataOffset+bucketCnt*uintptr(t.keysize)+i*uintptr(t.valuesize))
if t.indirectvalue {
v = *((*unsafe.Pointer)(v))
}
return v
}
}
b = b.overflow(t)
if b == nil {
return unsafe.Pointer(&zeroVal[0])
}
}
}
还有一个
> 2019.05.13 更新: 据网友 @rongfeixu 回复，mapaccess2 不知道是干啥的。mapaccess1 用于 v := map["bla"]
> mapaccess2 用于 v, ok := map["bla"] 这种形式。
mapaccessK 说是给iter用的。
接下来看看赋值操作，赋值操作其实就是先查找，找到了覆盖，没找到新建：
// Like mapaccess, but allocates a slot for the key if it is not present in the map.
func mapassign(t *maptype, h *hmap, key unsafe.Pointer) unsafe.Pointer {
if h == nil {
panic(plainError("assignment to entry in nil map"))
}
if raceenabled {
callerpc := getcallerpc(unsafe.Pointer(&t))
pc := funcPC(mapassign)
racewritepc(unsafe.Pointer(h), callerpc, pc)
raceReadObjectPC(t.key, key, callerpc, pc)
}
if msanenabled {
msanread(key, t.key.size)
}
if h.flags&hashWriting != 0 {
throw("concurrent map writes")
}
alg := t.key.alg
hash := alg.hash(key, uintptr(h.hash0))
// Set hashWriting after calling alg.hash, since alg.hash may panic,
// in which case we have not actually done a write.
h.flags |= hashWriting
if h.buckets == nil {
h.buckets = newarray(t.bucket, 1)
}
again:
bucket := hash & (uintptr(1)<<h.B - 1)
if h.growing() {
growWork(t, h, bucket)
}
b := (*bmap)(unsafe.Pointer(uintptr(h.buckets) + bucket*uintptr(t.bucketsize)))
top := uint8(hash >> (sys.PtrSize*8 - 8))
if top < minTopHash {
top += minTopHash
}
var inserti *uint8
var insertk unsafe.Pointer
var val unsafe.Pointer
for {
for i := uintptr(0); i < bucketCnt; i++ {
if b.tophash[i] != top {
if b.tophash[i] == empty && inserti == nil {
inserti = &b.tophash[i]
insertk = add(unsafe.Pointer(b), dataOffset+i*uintptr(t.keysize))
val = add(unsafe.Pointer(b), dataOffset+bucketCnt*uintptr(t.keysize)+i*uintptr(t.valuesize))
}
continue
}
k := add(unsafe.Pointer(b), dataOffset+i*uintptr(t.keysize))
if t.indirectkey {
k = *((*unsafe.Pointer)(k))
}
if !alg.equal(key, k) {
continue
}
// 如果找到了
// already have a mapping for key. Update it.
if t.needkeyupdate {
typedmemmove(t.key, k, key)
}
val = add(unsafe.Pointer(b), dataOffset+bucketCnt*uintptr(t.keysize)+i*uintptr(t.valuesize))
goto done
}
ovf := b.overflow(t)
if ovf == nil {
break
}
b = ovf
}
// 遍历完了，没找到，那就新建，前面记好了是否有空余处可以插入
// Did not find mapping for key. Allocate new cell & add entry.
// If we hit the max load factor or we have too many overflow buckets,
// and we're not already in the middle of growing, start growing.
if !h.growing() && (overLoadFactor(int64(h.count), h.B) || tooManyOverflowBuckets(h.noverflow, h.B)) {
hashGrow(t, h)
goto again // Growing the table invalidates everything, so try again
}
if inserti == nil {
// all current buckets are full, allocate a new one.
newb := h.newoverflow(t, b)
inserti = &newb.tophash[0]
insertk = add(unsafe.Pointer(newb), dataOffset)
val = add(insertk, bucketCnt*uintptr(t.keysize))
}
// store new key/value at insert position
if t.indirectkey {
kmem := newobject(t.key)
*(*unsafe.Pointer)(insertk) = kmem
insertk = kmem
}
if t.indirectvalue {
vmem := newobject(t.elem)
*(*unsafe.Pointer)(val) = vmem
}
typedmemmove(t.key, insertk, key)
*inserti = top
h.count++
done:
if h.flags&hashWriting == 0 {
throw("concurrent map writes")
}
h.flags &^= hashWriting
if t.indirectvalue {
val = *((*unsafe.Pointer)(val))
}
return val
}
删除操作操作也是类似，先找，找到了删除，没找到就没动作：
func mapdelete(t *maptype, h *hmap, key unsafe.Pointer) {
if raceenabled && h != nil {
callerpc := getcallerpc(unsafe.Pointer(&t))
pc := funcPC(mapdelete)
racewritepc(unsafe.Pointer(h), callerpc, pc)
raceReadObjectPC(t.key, key, callerpc, pc)
}
if msanenabled && h != nil {
msanread(key, t.key.size)
}
if h == nil || h.count == 0 {
return
}
if h.flags&hashWriting != 0 {
throw("concurrent map writes")
}
alg := t.key.alg
hash := alg.hash(key, uintptr(h.hash0))
// Set hashWriting after calling alg.hash, since alg.hash may panic,
// in which case we have not actually done a write (delete).
h.flags |= hashWriting
bucket := hash & (uintptr(1)<<h.B - 1)
if h.growing() {
growWork(t, h, bucket)
}
b := (*bmap)(unsafe.Pointer(uintptr(h.buckets) + bucket*uintptr(t.bucketsize)))
top := uint8(hash >> (sys.PtrSize*8 - 8))
if top < minTopHash {
top += minTopHash
}
for {
for i := uintptr(0); i < bucketCnt; i++ {
if b.tophash[i] != top {
continue
}
k := add(unsafe.Pointer(b), dataOffset+i*uintptr(t.keysize))
k2 := k
if t.indirectkey {
k2 = *((*unsafe.Pointer)(k2))
}
if !alg.equal(key, k2) {
continue
}
if t.indirectkey {
*(*unsafe.Pointer)(k) = nil
} else {
typedmemclr(t.key, k)
}
v := unsafe.Pointer(uintptr(unsafe.Pointer(b)) + dataOffset + bucketCnt*uintptr(t.keysize) + i*uintptr(t.valuesize))
if t.indirectvalue {
*(*unsafe.Pointer)(v) = nil
} else {
typedmemclr(t.elem, v)
}
// 标记为空
b.tophash[i] = empty
h.count--
goto done
}
b = b.overflow(t)
if b == nil {
goto done
}
}
done:
if h.flags&hashWriting == 0 {
throw("concurrent map writes")
}
h.flags &^= hashWriting
}
综上，我们发现，Golang的map是不能并发读写的，只是简单的依靠设置flag，
然后检查并且 throw 来完成的，太不安全了，所以有了 sync.Map
可以愉快的并发读写了。
更多文章
邮件 订阅
RSS 订阅
Web开发简介系列
数据结构的实际使用
Golang 简明教程
Python 教程
本站热门
- socks5 协议详解
- zerotier简明教程
- 搞定面试中的系统设计题
- frp 源码阅读与分析(一)：流程和概念
- 用peewee代替SQLAlchemy
- Golang(Go语言)中实现典型的fork调用
- DNSCrypt简明教程
- 一个Gunicorn worker数量引发的血案
- Golang validator使用教程
- Docker组件介绍（二）：shim, docker-init和docker-proxy
- Docker组件介绍（一）：runc和containerd
- 使用Go语言实现一个异步任务框架
- 协程(coroutine)简介 - 什么是协程？
- SQLAlchemy简明教程
- Go Module 简明教程