剖析golang map的实现

[TOC]

本文参考的是golang 1.10源码实现。

golang中map是一个kv对集合。底层使用hash table，用链表来解决冲突，通过编译器配合runtime，所有的map对象都是共用一份代码。

对比其他语言
c++使用红黑树组织，性能稍低但是稳定性很好。使用模版在编译期生成代码，好处是效率高，但是缺点是代码膨胀、编译时间也会变长。

java使用的是hash table+链表/红黑树，当bucket内元素超过某个阈值时，该bucket的链表会转换成红黑树。java为了所有map共用一份代码，规定了只有Object的子类才能使用作为map的key，缺点是基础数据类型必须使用object包装一下才能使用map。

1. 函数选择

hash函数，有加密型和非加密型。加密型的一般用于加密数据、数字摘要等，典型代表就是md5、sha1、sha256、aes256这种；非加密型的一般就是查找。在map的应用场景中，用的是查找。选择hash函数主要考察的是两点：性能、碰撞概率。

具体hash函数的性能比较可以看：http://aras-p.info/blog/2016/08/09/More-Hash-Function-Tests/

golang使用的hash算法根据硬件选择，如果cpu支持aes，那么使用aes hash，否则使用memhash，memhash是参考xxhash、cityhash实现的，性能炸裂。

把hash值映射到buckte时，golang会把bucket的数量规整为2的次幂，而有m=2^b，则n%m=n&(m-1)，用位运算规避mod的昂贵代价。

2. 结构组成

首先我们看下map的结构：

// A header for a Go map.
type hmap struct {
    // Note: the format of the hmap is also encoded in cmd/compile/internal/gc/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
}

// 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.extra.overflow and hmap.extra.oldoverflow.
    // overflow and oldoverflow are only used if key and value 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
}

// 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.
}

一个map主要是由三个结构构成:

1. hmap --- map的最外层的数据结构，包括了map的各种基础信息、如大小、bucket。
2. mapextra --- 记录map的额外信息，例如overflow bucket。
3. bmap --- 代表bucket，每一个bucket最多放8个kv，最后由一个overflow字段指向下一个bmap，注意key、value、overflow字段都不显示定义，而是通过maptype计算偏移获取的。

hmap.001.png

其中hmap.extra.nextOverflow指向的是预分配的overflow bucket，预分配的用完了那么值就变成nil。
hmap.noverflow是overflow bucket的数量，当B小于16时是准确值，大于等于16时是大概的值。
hmap.count是当前map的元素个数，也就是len()返回的值。

2.1 设计原理

介绍完结构，我们就细说一下这么设计的原因。

2.1.1 bmap细节

在golang map中出现冲突时，不是每一个key都申请一个结构通过链表串起来，而是以bmap为最小粒度挂载，一个bmap可以放8个kv。这样减少对象数量，减轻管理内存的负担，利于gc。
如果插入时，bmap中key超过8，那么就会申请一个新的bmap（overflow bucket）挂在这个bmap的后面形成链表，优先用预分配的overflow bucket，如果预分配的用完了，那么就malloc一个挂上去。注意golang的map不会shrink，内存只会越用越多，overflow bucket中的key全删了也不会释放

hash值的高8位存储在bucket中的tophash字段。每个桶最多放8个kv对，所以tophash类型是数组[8]uint8。把高八位存储起来，这样不用完整比较key就能过滤掉不符合的key，加快查询速度。实际上当hash值的高八位小于常量minTopHash时，会加上minTopHash，区间[0, minTophash)的值用于特殊标记。查找key时，计算hash值，用hash值的高八位在tophash中查找，有tophash相等的，再去比较key值是否相同。

????? 这里我不太清楚，1.为啥小于minTopHash才加 2.为什么不是位运算而用加。 刚好top在[0,minHash)，或着加上minHash之后溢出到这个区间，岂不是可能误判？

// tophash calculates the tophash value for hash.
func tophash(hash uintptr) uint8 {
    top := uint8(hash >> (sys.PtrSize*8 - 8))
    if top < minTopHash {
        top += minTopHash
    }
    return top
}

bmap中所有key存在一块，所有value存在一块，这样做方便内存对齐。
当key大于128字节时，bucket的key字段存储的会是指针，指向key的实际内容；value也是一样。

我们还知道golang中没有范型，为了支持map的范型，golang定义了一个maptype类型，定义了这类key用什么hash函数、bucket的大小、怎么比较之类的，通过这个变量来实现范型。

2.1.2 扩容设计

bcuket挂接的链表越来越长，性能会退化，那么就要进行扩容，扩大bucket的数量。

当元素个数/bucket个数大于等于6.5时，就会进行扩容，把bucket数量扩成原本的两倍，当hash表扩容之后，需要将那些老数据迁移到新table上(源代码中称之为evacuate)， 数据搬迁不是一次性完成，而是逐步的完成（在insert和remove时进行搬移），这样就分摊了扩容的耗时。同时为了避免有个bucket一直访问不到导致扩容无法完成，还会进行一个顺序扩容，每次因为写操作搬迁对应bucket后，还会按顺序搬迁未搬迁的bucket，所以最差情况下n次写操作，就保证搬迁完大小为n的map。

扩容会建立一个大小是原来2倍的新的表，将旧的bucket搬到新的表中之后，并不会将旧的bucket从oldbucket中删除，而是加上一个已删除的标记。

只有当所有的bucket都从旧表移到新表之后，才会将oldbucket释放掉。 如果扩容过程中，阈值又超了呢？如果正在扩容，那么不会再进行扩容。

总体思路描述完，就看源码创建、查询、赋值、删除的具体实现。

3. 源码实现

3.1 创建

// 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 {
    if hint < 0 || hint > int(maxSliceCap(t.bucket.size)) {
        hint = 0
    }

    // initialize Hmap
    if h == nil {
        h = new(hmap)
    }
    h.hash0 = fastrand()

    // find size parameter which will hold the requested # of elements
    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
}

hint是一个启发值，启发初建map时创建多少个bucket，如果hint是0那么就先不分配bucket，lazy分配。大概流程就是设置一下hash seed、bucket数量、实际申请bucket之类的，流程很简单。

然后我们在看下申请bucket实际干了啥：

// 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.kind&kindNoPointers == 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
}

默认创建2^b个bucket，如果b大于等于4，那么就预先额外创建一些overflow bucket。除了最后一个overflow bucket，其余overflow bucket的overflow指针都是nil，最后一个overflow bucket的overflow指针指向bucket数组第一个元素，作为哨兵，说明到了到结尾了.

创建简单流程

3.2 查询

// 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()
        pc := funcPC(mapaccess1)
        racereadpc(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 unsafe.Pointer(&zeroVal[0])
    }
    if h.flags&hashWriting != 0 {
        throw("concurrent map read and map write")
    }
    alg := t.key.alg
    hash := alg.hash(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)
    for ; b != nil; b = b.overflow(t) {
        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
            }
        }
    }
    return unsafe.Pointer(&zeroVal[0])
}

1. 先定位出bucket，如果正在扩容，并且这个bucket还没搬到新的hash表中，那么就从老的hash表中查找。

2. 在bucket中进行顺序查找，使用高八位进行快速过滤，高八位相等，再比较key是否相等，找到就返回value。如果当前bucket找不到，就往下找overflow bucket，都没有就返回零值。

这里我们可以看到，访问的时候，并不进行扩容的数据搬迁。并且并发有写操作时抛异常。

这里要注意的是，t.bucketsize并不是bmap的size，而是bmap加上存储key、value、overflow指针，所以查找bucket的时候时候用的不是bmap的szie。

查询简单流程

3.3 赋值

// 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()
        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 = newobject(t.bucket) // newarray(t.bucket, 1)
    }

again:
    bucket := hash & bucketMask(h.B)
    if h.growing() {
        growWork(t, h, bucket)
    }
    b := (*bmap)(unsafe.Pointer(uintptr(h.buckets) + bucket*uintptr(t.bucketsize)))
    top := tophash(hash)

    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
            }
            // 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
    }

    // 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 {
        // 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
}

1. hash表如果正在扩容，并且这次要操作的bucket还没搬到新hash表中，那么先进行搬迁（扩容细节下面细说）。

2. 在buck中寻找key，同时记录下第一个空位置，如果找不到，那么就在空位置中插入数据；如果找到了，那么就更新对应的value；

3. 找不到key就看下需不需要扩容，需要扩容并且没有正在扩容，那么就进行扩容，然后回到第一步。

4. 找不到key，不需要扩容，但是没有空slot，那么就分配一个overflow bucket挂在链表结尾，用新bucket的第一个slot放存放数据。

3.4 删除

func mapdelete(t *maptype, h *hmap, key unsafe.Pointer) {
    if raceenabled && h != nil {
        callerpc := getcallerpc()
        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 & bucketMask(h.B)
    if h.growing() {
        growWork(t, h, bucket)
    }
    b := (*bmap)(add(h.buckets, bucket*uintptr(t.bucketsize)))
    top := tophash(hash)
search:
    for ; b != nil; b = b.overflow(t) {
        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
            }
            // Only clear key if there are pointers in it.
            if t.indirectkey {
                *(*unsafe.Pointer)(k) = nil
            } else if t.key.kind&kindNoPointers == 0 {
                memclrHasPointers(k, t.key.size)
            }
            v := add(unsafe.Pointer(b), dataOffset+bucketCnt*uintptr(t.keysize)+i*uintptr(t.valuesize))
            if t.indirectvalue {
                *(*unsafe.Pointer)(v) = nil
            } else if t.elem.kind&kindNoPointers == 0 {
                memclrHasPointers(v, t.elem.size)
            } else {
                memclrNoHeapPointers(v, t.elem.size)
            }
            b.tophash[i] = empty
            h.count--
            break search
        }
    }

    if h.flags&hashWriting == 0 {
        throw("concurrent map writes")
    }
    h.flags &^= hashWriting
}

1. 如果正在扩容，并且操作的bucket还没搬迁完，那么搬迁bucket。

2. 找出对应的key，如果key、value是包含指针的那么会清理指针指向的内存，否则不会回收内存。

3.5 扩容

首先通过赋值、删除流程，我们可以知道，触发扩容的是赋值、删除操作，具体判断要不要扩容的代码片段如下：

// 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)
}

// 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)
}

{
    ....
    // 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
    }
    ....
}

翻译一下代码，意思就是：

func overLoadFactor(countint, Buint8) bool {
  // return count>bucketCnt&&uintptr(count) >loadFactorNum*(bucketShift(B)/loadFactorDen)
   return 元素个数>8 && count>bucket数量*6.5
   其中loadFactorNum是常量13，loadFactorDen是常量2,所以是6.5
   bucket数量不算overflow bucket
}
​
func tooManyOverflowBuckets(noverflowuint16, Buint8) bool{
   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)
}


if (不是正在扩容 && (元素个数/bucket数超过某个值 || 太多overflow bucket)) {
    进行扩容
}

判断完扩容后，如果需要扩容，那么第一步需要做的，就是对hash表进行扩容：

//仅对hash表进行扩容，这里不进行搬迁
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().
}

扩容的函数hashGrow其实仅仅是进行一些空间分配，字段的初始化，实际的搬迁操作是在growWork函数中

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)
    }
}

evacuate是进行具体搬迁某个bucket的函数，可以看出growWork会搬迁两个bucket，一个是入参bucket；另一个是h.nevacuate。这个nevacuate是一个顺序累加的值。可以想想如果每次仅仅搬迁进行写操作（赋值/删除）的bucket，那么有可能某些bucket就是一直没有机会访问到，那么扩容就一直没法完成，总是在扩容中的状态，因此会额外进行一次顺序迁移，理论上，有N个old bucket，最多N次写操作，那么必定会搬迁完。

然后我们再看下evacuate具体的实现

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.v = 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.v = add(y.k, bucketCnt*uintptr(t.keysize))
        }

        for ; b != nil; b = b.overflow(t) {
            k := add(unsafe.Pointer(b), dataOffset)
            v := add(k, bucketCnt*uintptr(t.keysize))
            for i := 0; i < bucketCnt; i, k, v = i+1, add(k, uintptr(t.keysize)), add(v, uintptr(t.valuesize)) {
                top := b.tophash[I]
                if top == empty {
                    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/value to bucket x or bucket y).
                    hash := t.key.alg.hash(k2, uintptr(h.hash0))
                    if h.flags&iterator != 0 && !t.reflexivekey && !t.key.alg.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 {
                    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.v = 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 value
                }
                if t.indirectvalue {
                    *(*unsafe.Pointer)(dst.v) = *(*unsafe.Pointer)(v)
                } else {
                    typedmemmove(t.elem, dst.v, v)
                }
                dst.i++
                // These updates might push these pointers past the end of the
                // key or value 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.v = add(dst.v, uintptr(t.valuesize))
            }
        }
        // Unlink the overflow buckets & clear key/value to help GC.
        if h.flags&oldIterator == 0 && t.bucket.kind&kindNoPointers == 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(ptr, n)
        }
    }

    if oldbucket == h.nevacuate {
        advanceEvacuationMark(h, t, newbit)
    }
}

在advanceEvacuationMark中进行nevacuate的累加，遇到已经迁移的bucket会继续累加，一次最多加1024。