内容简介:[TOC]本文参考的是golang 1.10源码实现。golang中map是一个kv对集合。
[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:
- 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。
- mapextra --- 记录map的额外信息,例如overflow bucket。
- 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]) }
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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。
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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 }
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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)。
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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;
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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。
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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 }
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如果正在扩容,并且操作的bucket还没搬迁完,那么搬迁bucket。
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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。
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[美] Scott Meyers / 侯捷 / 华中科技大学出版社 / 2001-9 / 49.80元
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