听说map删除元素占用竟然不会减少？
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· 阅读数 6
引言
知道了map的底层实现，下面这些问题也就水落石出啦～
- map底层实现是什么？
- map什么情况下会扩容，扩容原理是什么？
- map为什么有时候会报并发读写错误？
- 未初始化的map读取会发生什么？
- map遍历顺序为什么是随机的？
- map删除元素 内存占用会减少吗？
构成
hmap
map的底层实现
- count
len(map)map中已经存放了多少元素 - flag 标志map所处于的阶段 下文有具体解释
- B 2^B 代表桶的个数 一个桶中可以保存8对键值对
- noverflow 溢出桶的数量
- hash0 hash计算的种子 map的初始化的时候会使用随机值
fastrand()作为hash种子 - buckets 存放桶的数组 桶的实现是
bmap - oldbuckets 扩容场景下 oldbuckets 存放的是扩容前的桶数组，buckets存放的是扩容完成的桶数组。
- nevacuate 已经完成迁移操作的桶的下标 因为扩容操作是渐进式的，所以需要保存迁移进度
- extra 一些额外的信息 mapextra 具体见下文
// A header for a Go map.
type hmap struct {
// Note: the format of the hmap is also encoded in cmd/compile/internal/reflectdata/reflect.go.
// Make sure this stays in sync with the compiler's definition.
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
}
bmap
map底层中一个桶的实现
其中tophash存放的是 key hash值的高八位，用于快速定位元素
紧随其后的八组键值对 key/key/... elem/elem/elem/...
最后还有一个指针指向一个溢出桶(如果有)
也就是说 map[key]的大概流程如下(省略了部分逻辑):
- key hash找到对应的桶下标
- 基于hash的高八位快速判断元素是否存在该桶中
- 如果桶中不存在，则看其是否有溢出桶，如果有则继续在溢出桶中寻找
- 继续递归寻找，直至元素找到或者不再有溢出桶了
// 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 elems.
// NOTE: packing all the keys together and then all the elems together makes the
// code a bit more complicated than alternating key/elem/key/elem/... but it allows
// us to eliminate padding which would be needed for, e.g., map[int64]int8.
// Followed by an overflow pointer.
}
mapextra
如果一个map中的key 和value都不包含指针 则可以标记这个map不包含指针，这样GC扫描的时候可以不去遍历整个map。但是bmap.overflow是指针(指向一个一个的溢出桶bmap)，为了保证这些溢出桶存活，需要有一个地方记录其对应的指针。这就是overflow和oldoverflow的作用
nextOverflow 指向 预先分配的溢出桶的数组，当需要溢出桶的时候可以优先从这里获取，这里的桶用完了才会使用new关键字生成一个
// mapextra holds fields that are not present on all maps.
type mapextra struct {
// If both key and elem 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.extra.overflow and hmap.extra.oldoverflow.
// overflow and oldoverflow are only used if key and elem do not contain pointers.
// overflow contains overflow buckets for hmap.buckets.
// oldoverflow contains overflow buckets for hmap.oldbuckets.
// The indirection allows to store a pointer to the slice in hiter.
overflow *[]*bmap
oldoverflow *[]*bmap
// nextOverflow holds a pointer to a free overflow bucket.
nextOverflow *bmap
}
关键代码解释
overLoadFactor
overLoadFactor 用于判断count/ 2^B 是否大于负载因子 6.5
map的实现中一个桶内可以存放8组 kv键值对，为什么负载因子设置为6.5呢？
该值设置的太大，会导致产生更多的溢出桶 ==> 特定key的时候需要检查更多的桶
设置的较小，虽然溢出桶数量减少了，但是会带来了较大的空间浪费
权衡之下，6.5是一个不错的选择
// Picking loadFactor: too large and we have lots of overflow
// buckets, too small and we waste a lot of space. I wrote
// a simple program to check some stats for different loads:
// (64-bit, 8 byte keys and elems)
// loadFactor %overflow bytes/entry hitprobe missprobe
// 4.00 2.13 20.77 3.00 4.00
// 4.50 4.05 17.30 3.25 4.50
// 5.00 6.85 14.77 3.50 5.00
// 5.50 10.55 12.94 3.75 5.50
// 6.00 15.27 11.67 4.00 6.00
// 6.50 20.90 10.79 4.25 6.50
// 7.00 27.14 10.15 4.50 7.00
// 7.50 34.03 9.73 4.75 7.50
// 8.00 41.10 9.40 5.00 8.00
//
// %overflow = percentage of buckets which have an overflow bucket
// bytes/entry = overhead bytes used per key/elem pair
// hitprobe = # of entries to check when looking up a present key
// missprobe = # of entries to check when looking up an absent key
// overLoadFactor reports whether count items placed in 1<<B buckets is over loadFactor. func overLoadFactor (count int , B uint8 ) bool { return count > bucketCnt && uintptr (count) > loadFactorNum*(bucketShift(B)/loadFactorDen) }
// bucketShift returns 1<<b, optimized for code generation.
func bucketShift(b uint8) uintptr {
// Masking the shift amount allows overflow checks to be elided.
// 1 << (b&(0xFF))
return uintptr(1) << (b & (goarch.PtrSize*8 - 1))
}
const (
// Maximum number of key/elem pairs a bucket can hold.
bucketCntBits = 3
bucketCnt = 1 << bucketCntBits. // 8
// Maximum average load of a bucket that triggers growth is 6.5.
// Represent as loadFactorNum/loadFactorDen, to allow integer math.
loadFactorNum = 13
loadFactorDen = 2
)
tooManyOverflowBuckets
判断溢出桶是否过多
一般来讲溢出桶内存使用是稀疏的(因为如果不是稀疏，早就会触发overLoadFactor扩容)，所以过多的溢出桶会导致有很多内存得不到使用。所以过多的溢出桶应该触发等容量迁移，将map中的元素重新整理
// tooManyOverflowBuckets reports whether noverflow buckets is too many for a map with 1<<B buckets.
// Note that most of these overflow buckets must be in sparse use;
// if use was dense, then we'd have already triggered regular map growth.
func tooManyOverflowBuckets(noverflow uint16, B uint8) bool {
// If the threshold is too low, we do extraneous work.
// If the threshold is too high, maps that grow and shrink can hold on to lots of unused memory.
// "too many" means (approximately) as many overflow buckets as regular buckets.
// See incrnoverflow for more details.
if B > 15 {
B = 15
}
// The compiler doesn't see here that B < 16; mask B to generate shorter shift code.
return noverflow >= uint16(1)<<(B&15)
}
tophash
截取hash值的高八位
如果计算出来的结果小于5 则人为地加上5。因为源码实现保留了一些值作为特定的标志位
emptyRest = 0该单元为空(特指一个桶中的一个单元)，并且该桶的更高索引上也没有元素，该桶的后面也没有溢出桶emptyOne = 1该单元为空evacuatedX = 2该单元有数据，且该数据已迁移到更大表的前半部分(扩容场景)evacuatedY = 3与上述相同，但迁移到更大表的后半部分(扩容场景)evacuatedEmpty = 4该单元为空，且该桶已经完成迁移minTopHash = 5正常计算出来的最小 tophash
// tophash calculates the tophash value for hash.
func tophash(hash uintptr) uint8 {
top := uint8(hash >> (goarch.PtrSize*8 - 8))
if top < minTopHash {
top += minTopHash
}
return top
}
// Possible tophash values. We reserve a few possibilities for special marks.
// Each bucket (including its overflow buckets, if any) will have either all or none of its
// entries in the evacuated* states (except during the evacuate() method, which only happens
// during map writes and thus no one else can observe the map during that time).
emptyRest = 0 // this cell is empty, and there are no more non-empty cells at higher indexes or overflows.
emptyOne = 1 // this cell is empty
evacuatedX = 2 // key/elem is valid. Entry has been evacuated to first half of larger table.
evacuatedY = 3 // same as above, but evacuated to second half of larger table.
evacuatedEmpty = 4 // cell is empty, bucket is evacuated.
minTopHash = 5 // minimum tophash for a normal filled cell.
isEmpty
通过判断tophash 中的值 是否小于emptyOne 从而判断bmap中对应槽位是否为空
为什么可以这么判断 详见 tophash
// isEmpty reports whether the given tophash array entry represents an empty bucket entry.
func isEmpty(x uint8) bool {
return x <= emptyOne
}
flags
底层实现中通过flag标志来代表map处于的阶段
-
iterator(1)map正在被迭代(for ... range map) -
oldIterator(2)正在oldbuckets迭代(for ... range map)，说明扩容阶段也正在进行 -
hashWriting(4)map正在被写入元素，其他并发读请求如果观察到这个flag会报并发读写错误，程序异常退出。因为该map不是并发安全的，存在并发读写说明该map被滥用 -
sameSizeGrow(8)说明map正在等容量迁移。具体来讲，触发map的扩容有两种情况:- 溢出桶过多(map找key需要遍历更多的桶)，会触发等容量迁移(容量不变，元素重新分配)
count/(1^B)超出负载因子6.5，说明容量不足，桶的数量会变为原来的两倍
// flags
iterator = 1 // there may be an iterator using buckets
oldIterator = 2 // there may be an iterator using oldbuckets
hashWriting = 4 // a goroutine is writing to the map
sameSizeGrow = 8 // the current map growth is to a new map of the same size
evacuated
判断一个桶是否已经完成迁移
具体见 tophash 中的解释
func evacuated(b *bmap) bool {
h := b.tophash[0]
return h > emptyOne && h < minTopHash
}
makeBucketArray
给定B, 初始化一个bmap数组
关键点
B<4时(也就是说桶的数量小于16时)，不额外预先分配溢出桶- 当
B>4时，预分配溢出桶的数量为1<<(B-4) - 正常的
bmap数组和预分配的溢出桶，是一块连续的内存地址
// makeBucketArray initializes a backing array for map buckets.
// 1<<b is the minimum number of buckets to allocate.
// dirtyalloc should either be nil or a bucket array previously
// allocated by makeBucketArray with the same t and b parameters.
// If dirtyalloc is nil a new backing array will be alloced and
// otherwise dirtyalloc will be cleared and reused as backing array.
func makeBucketArray(t *maptype, b uint8, dirtyalloc unsafe.Pointer) (buckets unsafe.Pointer, nextOverflow *bmap) {
base := bucketShift(b)
nbuckets := base
// For small b, overflow buckets are unlikely.
// Avoid the overhead of the calculation.
if b >= 4 {
// Add on the estimated number of overflow buckets
// required to insert the median number of elements
// used with this value of b.
nbuckets += bucketShift(b - 4)
sz := t.bucket.size * nbuckets
up := roundupsize(sz)
if up != sz {
nbuckets = up / t.bucket.size
}
}
if dirtyalloc == nil {
buckets = newarray(t.bucket, int(nbuckets))
} else {
// dirtyalloc was previously generated by
// the above newarray(t.bucket, int(nbuckets))
// but may not be empty.
buckets = dirtyalloc
size := t.bucket.size * nbuckets
if t.bucket.ptrdata != 0 {
memclrHasPointers(buckets, size)
} else {
memclrNoHeapPointers(buckets, size)
}
}
if base != nbuckets {
// We preallocated some overflow buckets.
// To keep the overhead of tracking these overflow buckets to a minimum,
// we use the convention that if a preallocated overflow bucket's overflow
// pointer is nil, then there are more available by bumping the pointer.
// We need a safe non-nil pointer for the last overflow bucket; just use buckets.
nextOverflow = (*bmap)(add(buckets, base*uintptr(t.bucketsize)))
last := (*bmap)(add(buckets, (nbuckets-1)*uintptr(t.bucketsize)))
last.setoverflow(t, (*bmap)(buckets))
}
return buckets, nextOverflow
}
func (b *bmap) setoverflow(t *maptype, ovf *bmap) {
*(**bmap)(add(unsafe.Pointer(b), uintptr(t.bucketsize)-goarch.PtrSize)) = ovf
}
sameSizeGrow
用于判断 map是不是处于 等容量迁移阶段(溢出桶过多导致的)
// sameSizeGrow reports whether the current growth is to a map of the same size.
func (h *hmap) sameSizeGrow() bool {
return h.flags&sameSizeGrow != 0
}
growing
用于判断 map是不是处于迁移阶段(等容量迁移/扩容迁移)
// growing reports whether h is growing. The growth may be to the same size or bigger.
func (h *hmap) growing() bool {
return h.oldbuckets != nil
}
hashGrow
- 如果是因为overLoadFactor 触发的扩容 ，则申请的内存为原来两倍
- 如果是溢出桶过多触发的迁移，申请的内存与原来一致
该方法只是完成内存的申请与一些变量的赋值，但是并没有真正地触发桶的迁移
- 如果是因为overLoadFactor触发的扩容，则设置flag |= sameSizeGrow
- iterator -> oldIterator 如此迭代器便能够感知到map触发了迁移
- 设置期望的 B
- 清空统计的溢出桶数量
noverflow- 设置当前桶迁移进度
nevacuate为0- ...
func hashGrow(t *maptype, h *hmap) {
// If we've hit the load factor, get bigger.
// Otherwise, there are too many overflow buckets,
// so keep the same number of buckets and "grow" laterally.
bigger := uint8(1)
if !overLoadFactor(h.count+1, h.B) {
bigger = 0
h.flags |= sameSizeGrow
}
oldbuckets := h.buckets
newbuckets, nextOverflow := makeBucketArray(t, h.B+bigger, nil)
flags := h.flags &^ (iterator | oldIterator)
if h.flags&iterator != 0 {
flags |= oldIterator
}
// commit the grow (atomic wrt gc)
h.B += bigger
h.flags = flags
h.oldbuckets = oldbuckets
h.buckets = newbuckets
h.nevacuate = 0
h.noverflow = 0
if h.extra != nil && h.extra.overflow != nil {
// Promote current overflow buckets to the old generation.
if h.extra.oldoverflow != nil {
throw("oldoverflow is not nil")
}
h.extra.oldoverflow = h.extra.overflow
h.extra.overflow = nil
}
if nextOverflow != nil {
if h.extra == nil {
h.extra = new(mapextra)
}
h.extra.nextOverflow = nextOverflow
}
// the actual copying of the hash table data is done incrementally
// by growWork() and evacuate().
}
growWork
真正开始执行桶的迁移
先迁移传入的bucket下标的桶
然后在尝试迁移下一个桶，推动整个map的迁移
func growWork(t *maptype, h *hmap, bucket uintptr) {
// make sure we evacuate the oldbucket corresponding
// to the bucket we're about to use
evacuate(t, h, bucket&h.oldbucketmask())
// evacuate one more oldbucket to make progress on growing
if h.growing() {
evacuate(t, h, h.nevacuate)
}
}
迁移的逻辑位于 evacuate
- 如果是等量迁移，则直接计算对应的hash值，往对应的桶中放置元素
- 如果是扩容迁移，计算完hash值后，决定其最终放置在哪个桶
比如原来容量为4 hash值 0b111 和0b011 都在0b11号桶(hash & 0b11)
扩容后容量为8 hash值 0b111 和0b011 分别存放在 0b111号桶和0b11号桶(hash & 0b111)
- 当该桶以及其溢出桶都迁移完毕，
Unlink the overflow buckets & clear key/elem to help GC.(这里只是清空了 桶中的键值对以及指向溢出桶的指针，使得这些变量可以被GC回收，但是桶本身占据的内存空间并没有被回收) - oldbuckets中的所有桶都完成迁移后，才会设置
h.oldbuckets = nil。这时h.oldbuckets指向的内存空间才可以真正被释放
func evacuate(t *maptype, h *hmap, oldbucket uintptr) {
b := (*bmap)(add(h.oldbuckets, oldbucket*uintptr(t.bucketsize)))
newbit := h.noldbuckets()
if !evacuated(b) {
// TODO: reuse overflow buckets instead of using new ones, if there
// is no iterator using the old buckets. (If !oldIterator.)
// xy contains the x and y (low and high) evacuation destinations.
var xy [2]evacDst
x := &xy[0]
x.b = (*bmap)(add(h.buckets, oldbucket*uintptr(t.bucketsize)))
x.k = add(unsafe.Pointer(x.b), dataOffset)
x.e = add(x.k, bucketCnt*uintptr(t.keysize))
if !h.sameSizeGrow() {
// Only calculate y pointers if we're growing bigger.
// Otherwise GC can see bad pointers.
y := &xy[1]
y.b = (*bmap)(add(h.buckets, (oldbucket+newbit)*uintptr(t.bucketsize)))
y.k = add(unsafe.Pointer(y.b), dataOffset)
y.e = add(y.k, bucketCnt*uintptr(t.keysize))
}
for ; b != nil; b = b.overflow(t) {
k := add(unsafe.Pointer(b), dataOffset)
e := add(k, bucketCnt*uintptr(t.keysize))
for i := 0; i < bucketCnt; i, k, e = i+1, add(k, uintptr(t.keysize)), add(e, uintptr(t.elemsize)) {
top := b.tophash[i]
if isEmpty(top) {
b.tophash[i] = evacuatedEmpty
continue
}
if top < minTopHash {
throw("bad map state")
}
k2 := k
if t.indirectkey() {
k2 = *((*unsafe.Pointer)(k2))
}
var useY uint8
if !h.sameSizeGrow() {
// Compute hash to make our evacuation decision (whether we need
// to send this key/elem to bucket x or bucket y).
hash := t.hasher(k2, uintptr(h.hash0))
if h.flags&iterator != 0 && !t.reflexivekey() && !t.key.equal(k2, k2) {
// If key != key (NaNs), then the hash could be (and probably
// will be) entirely different from the old hash. Moreover,
// it isn't reproducible. Reproducibility is required in the
// presence of iterators, as our evacuation decision must
// match whatever decision the iterator made.
// Fortunately, we have the freedom to send these keys either
// way. Also, tophash is meaningless for these kinds of keys.
// We let the low bit of tophash drive the evacuation decision.
// We recompute a new random tophash for the next level so
// these keys will get evenly distributed across all buckets
// after multiple grows.
useY = top & 1
top = tophash(hash)
} else {
if hash&newbit != 0 {
useY = 1
}
}
}
if evacuatedX+1 != evacuatedY || evacuatedX^1 != evacuatedY {
throw("bad evacuatedN")
}
b.tophash[i] = evacuatedX + useY // evacuatedX + 1 == evacuatedY
dst := &xy[useY] // evacuation destination
if dst.i == bucketCnt {
dst.b = h.newoverflow(t, dst.b)
dst.i = 0
dst.k = add(unsafe.Pointer(dst.b), dataOffset)
dst.e = add(dst.k, bucketCnt*uintptr(t.keysize))
}
dst.b.tophash[dst.i&(bucketCnt-1)] = top // mask dst.i as an optimization, to avoid a bounds check
if t.indirectkey() {
*(*unsafe.Pointer)(dst.k) = k2 // copy pointer
} else {
typedmemmove(t.key, dst.k, k) // copy elem
}
if t.indirectelem() {
*(*unsafe.Pointer)(dst.e) = *(*unsafe.Pointer)(e)
} else {
typedmemmove(t.elem, dst.e, e)
}
dst.i++
// These updates might push these pointers past the end of the
// key or elem arrays. That's ok, as we have the overflow pointer
// at the end of the bucket to protect against pointing past the
// end of the bucket.
dst.k = add(dst.k, uintptr(t.keysize))
dst.e = add(dst.e, uintptr(t.elemsize))
}
}
// Unlink the overflow buckets & clear key/elem to help GC.
if h.flags&oldIterator == 0 && t.bucket.ptrdata != 0 {
b := add(h.oldbuckets, oldbucket*uintptr(t.bucketsize))
// Preserve b.tophash because the evacuation
// state is maintained there.
ptr := add(b, dataOffset)
n := uintptr(t.bucketsize) - dataOffset
// memclrHasPointers clears n bytes of typed memory starting at ptr.
memclrHasPointers(ptr, n)
}
}
if oldbucket == h.nevacuate {
// 当 oldbuckets中的所有桶都完成迁移后，才会设置 h.oldbuckets = nil
// 这时h.oldbuckets指向的内存空间才可以真正被释放
advanceEvacuationMark(h, t, newbit)
}
}
func advanceEvacuationMark(h *hmap, t *maptype, newbit uintptr) {
h.nevacuate++
// Experiments suggest that 1024 is overkill by at least an order of magnitude.
// Put it in there as a safeguard anyway, to ensure O(1) behavior.
stop := h.nevacuate + 1024
if stop > newbit {
stop = newbit
}
for h.nevacuate != stop && bucketEvacuated(t, h, h.nevacuate) {
h.nevacuate++
}
if h.nevacuate == newbit { // newbit == # of oldbuckets
// Growing is all done. Free old main bucket array.
h.oldbuckets = nil
// Can discard old overflow buckets as well.
// If they are still referenced by an iterator,
// then the iterator holds a pointers to the slice.
if h.extra != nil {
h.extra.oldoverflow = nil
}
h.flags &^= sameSizeGrow // 清除 扩容标识
}
}
newoverflow
初始化一个可用的
bmap溢出桶优先尝试从预先分配的溢出桶数组中拿(参看makeBucketArray)
如果上述用尽，才调用newobject 初始化一个
func (h *hmap) newoverflow(t *maptype, b *bmap) *bmap {
var ovf *bmap
if h.extra != nil && h.extra.nextOverflow != nil {
// We have preallocated overflow buckets available.
// See makeBucketArray for more details.
ovf = h.extra.nextOverflow
if ovf.overflow(t) == nil {
// We're not at the end of the preallocated overflow buckets. Bump the pointer.
h.extra.nextOverflow = (*bmap)(add(unsafe.Pointer(ovf), uintptr(t.bucketsize)))
} else {
// This is the last preallocated overflow bucket.
// Reset the overflow pointer on this bucket,
// which was set to a non-nil sentinel value.
ovf.setoverflow(t, nil)
h.extra.nextOverflow = nil
}
} else {
ovf = (*bmap)(newobject(t.bucket))
}
h.incrnoverflow()
if t.bucket.ptrdata == 0 {
h.createOverflow()
*h.extra.overflow = append(*h.extra.overflow, ovf)
}
b.setoverflow(t, ovf)
return ovf
}
核心实现
创建map
make(map[k]v, hint)
make(map[k]v)
主要流程
- 根据传入的
hint计算出合适的B(参看overLoadFactor) - 使用随机数初始化一个hash种子，用于后续hash计算
- 如果计算出来的B==0，则先不初始化对应的
bmap数组。如果B>0则申请一块连续的内存空间(参看makeBucketArray)，初始化buckets(用于存放bmap数组)和extra.nextOverflow(预先分配的溢出桶)
// makemap implements Go map creation for make(map[k]v, hint).
// If the compiler has determined that the map or the first bucket
// can be created on the stack, h and/or bucket may be non-nil.
// If h != nil, the map can be created directly in h.
// If h.buckets != nil, bucket pointed to can be used as the first bucket.
func makemap(t *maptype, hint int, h *hmap) *hmap {
mem, overflow := math.MulUintptr(uintptr(hint), t.bucket.size)
if overflow || mem > maxAlloc {
hint = 0
}
// initialize Hmap
if h == nil {
h = new(hmap)
}
h.hash0 = fastrand()
// Find the size parameter B which will hold the requested # of elements.
// For hint < 0 overLoadFactor returns false since hint < bucketCnt.
B := uint8(0)
for overLoadFactor(hint, B) {
B++
}
h.B = B
// allocate initial hash table
// if B == 0, the buckets field is allocated lazily later (in mapassign)
// If hint is large zeroing this memory could take a while.
if h.B != 0 {
var nextOverflow *bmap
h.buckets, nextOverflow = makeBucketArray(t, h.B, nil)
if nextOverflow != nil {
h.extra = new(mapextra)
h.extra.nextOverflow = nextOverflow
}
}
return h
}
从map中Get元素
val, ok := h[key] 对应的源码实现为 mapaccess2_fat
val := h[key] 源码实现为mapaccess1_fat，原理类似不再赘述
- 如果map为空或者map内元素数量为0，直接返回元素的默认值
- 如果发现
h.flags&hashWriting != 0说明发生了并发读写 直接panic - 计算hash，找到对应的桶
- 如果发现map正在扩容，且对应的桶没有迁移完毕，则从旧桶寻找，如果迁移完毕则从新桶寻找
- 寻找路径为：先看第一个桶中元素是否存在，再看溢出桶是否存在，再看溢出桶的溢出桶...
func mapaccess2_fat(t *maptype, h *hmap, key, zero unsafe.Pointer) (unsafe.Pointer, bool) {
e := mapaccess1(t, h, key)
if e == unsafe.Pointer(&zeroVal[0]) {
return zero, false
}
return e, true
}
// mapaccess1 returns a pointer to h[key]. Never returns nil, instead
// it will return a reference to the zero object for the elem 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 {
// 省略一些非核心代码
if h == nil || h.count == 0 {
if t.hashMightPanic() {
t.hasher(key, 0) // see issue 23734
}
return unsafe.Pointer(&zeroVal[0])
}
if h.flags&hashWriting != 0 {
fatal("concurrent map read and map write")
}
hash := t.hasher(key, uintptr(h.hash0))
m := bucketMask(h.B)
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
}
}
top := tophash(hash)
bucketloop:
for ; b != nil; b = b.overflow(t) {
for i := uintptr(0); i < bucketCnt; i++ {
if b.tophash[i] != top {
if b.tophash[i] == emptyRest {
break bucketloop
}
continue
}
k := add(unsafe.Pointer(b), dataOffset+i*uintptr(t.keysize))
if t.indirectkey() {
k = *((*unsafe.Pointer)(k))
}
if t.key.equal(key, k) {
e := add(unsafe.Pointer(b), dataOffset+bucketCnt*uintptr(t.keysize)+i*uintptr(t.elemsize))
if t.indirectelem() {
e = *((*unsafe.Pointer)(e))
}
return e
}
}
}
return unsafe.Pointer(&zeroVal[0])
}
向map中Set元素
h[key] = val 对应的源码实现为 mapassign
- 如果map未初始化则直接panic
- 如果发现
h.flags&hashWriting != 0说明发生了并发写 直接panic - 如果发现对应的桶未初始化，则先初始化之(对应于初始化map的时候未指定长度或者指定长度为0，在此处延迟创建)
- 计算hash，找到对应的桶，如果发现map正在迁移，则调用
growWork触发当前桶和隔壁桶的迁移工作 - 如果发现map需要迁移(溢出桶过多tooManyOverflowBuckets或者overLoadFactor)，会先触发map的迁移(具体参看hashGrow)
- 遍历桶以及溢出桶，找到可插入的位置(已存在或者找到一个空闲可用的槽)，如果没找到说明槽位都满了，初始化一个新的
bmap(具体见newoverflow)使用之
// 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"))
}
// 省略一些非核心代码
if h.flags&hashWriting != 0 {
fatal("concurrent map writes")
}
hash := t.hasher(key, uintptr(h.hash0))
// Set hashWriting after calling t.hasher, since t.hasher may panic,
// in which case we have not actually done a write.
h.flags ^= hashWriting
if h.buckets == nil {
h.buckets = newobject(t.bucket) // newarray(t.bucket, 1)
}
again:
bucket := hash & bucketMask(h.B)
if h.growing() {
growWork(t, h, bucket)
}
b := (*bmap)(add(h.buckets, bucket*uintptr(t.bucketsize)))
top := tophash(hash)
var inserti *uint8
var insertk unsafe.Pointer
var elem unsafe.Pointer
bucketloop:
for {
for i := uintptr(0); i < bucketCnt; i++ {
if b.tophash[i] != top {
if isEmpty(b.tophash[i]) && inserti == nil {
inserti = &b.tophash[i]
insertk = add(unsafe.Pointer(b), dataOffset+i*uintptr(t.keysize))
elem = add(unsafe.Pointer(b), dataOffset+bucketCnt*uintptr(t.keysize)+i*uintptr(t.elemsize))
}
if b.tophash[i] == emptyRest {
break bucketloop
}
continue
}
k := add(unsafe.Pointer(b), dataOffset+i*uintptr(t.keysize))
if t.indirectkey() {
k = *((*unsafe.Pointer)(k))
}
if !t.key.equal(key, k) {
continue
}
// already have a mapping for key. Update it.
if t.needkeyupdate() {
typedmemmove(t.key, k, key)
}
elem = add(unsafe.Pointer(b), dataOffset+bucketCnt*uintptr(t.keysize)+i*uintptr(t.elemsize))
goto done
}
ovf := b.overflow(t)
if ovf == nil {
break
}
b = ovf
}
// 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(h.count+1, h.B) || tooManyOverflowBuckets(h.noverflow, h.B)) {
hashGrow(t, h)
goto again // Growing the table invalidates everything, so try again
}
if inserti == nil {
// The current bucket and all the overflow buckets connected to it are full, allocate a new one.
newb := h.newoverflow(t, b)
inserti = &newb.tophash[0]
insertk = add(unsafe.Pointer(newb), dataOffset)
elem = add(insertk, bucketCnt*uintptr(t.keysize))
}
// store new key/elem at insert position
if t.indirectkey() {
kmem := newobject(t.key)
*(*unsafe.Pointer)(insertk) = kmem
insertk = kmem
}
if t.indirectelem() {
vmem := newobject(t.elem)
*(*unsafe.Pointer)(elem) = vmem
}
typedmemmove(t.key, insertk, key)
*inserti = top
h.count++
done:
if h.flags&hashWriting == 0 {
fatal("concurrent map writes")
}
h.flags &^= hashWriting
if t.indirectelem() {
elem = *((*unsafe.Pointer)(elem))
}
return elem
}
map迭代
具体细节暂按下不表
- 每次迭代 返回元素的顺序并不是固定的
// mapiterinit initializes the hiter struct used for ranging over maps.
// The hiter struct pointed to by 'it' is allocated on the stack
// by the compilers order pass or on the heap by reflect_mapiterinit.
// Both need to have zeroed hiter since the struct contains pointers.
func mapiterinit(t *maptype, h *hmap, it *hiter) {
// ...
it.t = t
if h == nil || h.count == 0 {
return
}
// grab snapshot of bucket state
it.B = h.B
it.buckets = h.buckets
if t.bucket.ptrdata == 0 {
// Allocate the current slice and remember pointers to both current and old.
// This preserves all relevant overflow buckets alive even if
// the table grows and/or overflow buckets are added to the table
// while we are iterating.
h.createOverflow()
it.overflow = h.extra.overflow
it.oldoverflow = h.extra.oldoverflow
}
// decide where to start
var r uintptr
if h.B > 31-bucketCntBits {
r = uintptr(fastrand64())
} else {
r = uintptr(fastrand())
}
it.startBucket = r & bucketMask(h.B)
it.offset = uint8(r >> h.B & (bucketCnt - 1))
// iterator state
it.bucket = it.startBucket
// Remember we have an iterator.
// Can run concurrently with another mapiterinit().
if old := h.flags; old&(iterator|oldIterator) != iterator|oldIterator {
atomic.Or8(&h.flags, iterator|oldIterator)
}
mapiternext(it)
}
map删除
具体细节暂按下不表
- map删除元素并不会导致map占用的内存减小
// Only clear key if there are pointers in it.
if t.indirectkey() {
*(*unsafe.Pointer)(k) = nil
} else if t.key.ptrdata != 0 {
memclrHasPointers(k, t.key.size)
}
e := add(unsafe.Pointer(b), dataOffset+bucketCnt*uintptr(t.keysize)+i*uintptr(t.elemsize))
if t.indirectelem() {
*(*unsafe.Pointer)(e) = nil
} else if t.elem.ptrdata != 0 {
memclrHasPointers(e, t.elem.size)
} else {
memclrNoHeapPointers(e, t.elem.size)
}
转载自:https://juejin.cn/post/7349124917378727948