深入了解Golang的map增量扩容

	// Maximum average load of a bucket that triggers growth is 6.5.
	// Represent as loadFactorNum/loadFactorDen, to allow integer math.
	loadFactorNum = 13
	loadFactorDen = 2

// 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
//
// Keep in mind this data is for maximally loaded tables, i.e. just
// before the table grows. Typical tables will be somewhat less loaded.

// 只是分配新的buckets，并将老的buckets挂到oldbuckets字段上
// 真正搬迁的动作在growWork()中
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.
    // B+1 相当于之前的2倍空间
	bigger := uint8(1)
    // 对应条件2
	if !overLoadFactor(h.count+1, h.B) {
        // 进行等量扩容，B不变
		bigger = 0
		h.flags |= sameSizeGrow
	}
    // 将oldbuckets挂到buckets上
	oldbuckets := h.buckets
    // 申请新的buckets空间
	newbuckets, nextOverflow := makeBucketArray(t, h.B+bigger, nil)

    // 对标志位的处理
    // &^表示 按位置0
    // x=01010011
    // y=01010100
    // z=x&^y=00000011
    // 如果y的bit位为1，那么z相应的bit位就为0
    // 否则z对应的bit位就和x对应的bit位相同
    // 
    // 其实就是将h.flags的iterator和oldItertor位置为0
    // 如果发现iterator位为1，那就把它转接到 oldIterator 位
    // 使得 oldIterator 标志位变成 1
    // bucket挂到了oldbuckets下，那么标志位也一样转移过去
	flags := h.flags &^ (iterator | oldIterator)
	if h.flags&iterator != 0 {
		flags |= oldIterator
	}
    
    // // 可能有迭代器使用 buckets
    // iterator     = 1
    // 可能有迭代器使用 oldbuckets
    // oldIterator  = 2
    // 有协程正在向 map 中写入 key
    // hashWriting  = 4
    // 等量扩容（对应条件 2）
    // sameSizeGrow = 8
    // 提交grow的动作
	// commit the grow (atomic wrt GC)
	h.B += bigger
	h.flags = flags
	h.oldbuckets = oldbuckets
	h.buckets = newbuckets
    // 搬迁进度为0
	h.nevacuate = 0
    // 溢出bucket数量为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 真正执行搬迁工作的函数
// 调用其的动作在mapssign和mapdelete函数中，也就是插入、修改或删除的时候都会尝试进行搬迁
func growWork(t *maptype, h *hmap, bucket uintptr) {
	// make sure we evacuate the oldbucket corresponding
	// to the bucket we're about to use
    // 确保搬迁的老bucket对应的正在使用的新bucket
    // bucketmask 作用就是将key算出来的hash值与bucketmask相&，得到key应该落入的bucket
    // 只有hash值低B位决策key落入那个bucket
	evacuate(t, h, bucket&h.oldbucketmask())

	// evacuate one more oldbucket to make progress on growing
    // 再搬迁一个bucket，加快搬迁进度，这就是说为什么可能每次操作会搬迁1-2个bucket
	if h.growing() {
		evacuate(t, h, h.nevacuate)
	}
}

// 返回扩容前的bucketmask
// 
// 所谓的bucketmask作用就是将 key 计算出来的哈希值与 bucketmask 相与
// 得到的结果就是 key 应该落入的桶
// 比如 B = 5，那么 bucketmask 的低 5 位是 11111，其余位是 0
// hash 值与其相与的意思是，只有 hash 值的低 5 位决策 key 到底落入哪个 bucket。
// oldbucketmask provides a mask that can be applied to calculate n % noldbuckets().
func (h *hmap) oldbucketmask() uintptr {
	return h.noldbuckets() - 1
}

// 检查oldbuckets是否搬迁完
// 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
}

// evacDst is an evacuation destination.
type evacDst struct {
    // 标识bucket移动的目标地址
	b *bmap          // current destination bucket
    // k-v的索引
	i int            // key/elem index into b
    // 指向k
	k unsafe.Pointer // pointer to current key storage
    // 指向v
	e unsafe.Pointer // pointer to current elem storage
}
// evacuate 核心搬迁函数
func evacuate(t *maptype, h *hmap, oldbucket uintptr) {
    // 定位老的bucket的地址
	b := (*bmap)(add(h.oldbuckets, oldbucket*uintptr(t.bucketsize)))
	// 结果是2^B，进行计算 如 B = 5，结果为32
    newbit := h.noldbuckets()
    // 如果b没被搬迁过
	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.
        // xy包含了两个可能搬迁到的目的bucket地址
        // 默认是等量扩容的，用x来搬迁
		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))

        // 如果不是等量扩容，前后的bucket序号有变
        // 使用y来搬迁
		if !h.sameSizeGrow() {
			// Only calculate y pointers if we're growing bigger.
			// Otherwise GC can see bad pointers.
			y := &xy[1]
            // y代表的bucket序号增加了2^B
			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))
		}

        // 遍历所有的bucket，包括溢出bucket
        // b是老bucket的地址
		for ; b != nil; b = b.overflow(t) {
			k := add(unsafe.Pointer(b), dataOffset)
			e := add(k, bucketCnt*uintptr(t.keysize))
            // 遍历bucket里所有的cell
			for i := 0; i < bucketCnt; i, k, e = i+1, add(k, uintptr(t.keysize)), add(e, uintptr(t.elemsize)) {
				// 当前cell的tophash值
                top := b.tophash[i]
                // 如果cell为空，即没有key
                // 说明其被搬迁过，作标记然后继续下一个cell
				if isEmpty(top) {
					b.tophash[i] = evacuatedEmpty
					continue
				}

                // 一般不会出现这种情况
                // 未搬迁的cell只可能是empty或者正常的tophash
                // 不会小于minTopHash
				if top < minTopHash {
					throw("bad map state")
				}
                // 进行一次拷贝避免相同内存地址问题
				k2 := k
                // key如果是指针就进行解引用
				if t.indirecTKEy() {
					k2 = *((*unsafe.Pointer)(k2))
				}
                // 默认值为0标识默认是使用x，进行等量扩容
				var useY uint8
                // 增量扩容
				if !h.sameSizeGrow() {
					// Compute hash to make our evacuation decision (whether we need
					// to send this key/elem to bucket x or bucket y).
                    // 计算hash值，与第一次写入一样
					hash := t.hasher(k2, uintptr(h.hash0))

                    // 有协程在遍历map 且 出现相同的key，计算出的hash值不同
                    // 这里只会有一种情况，也就是float64的时候
                    // 每次hash出来都会是不同的hash值，这就意味着无法通过get去获取其key确切位置
                    // 因此采用取最低位位置来分辨
                    // 为下一个level重新计算一个随机的tophash
                    // 这些key将会在多次增长后均匀的分布在所有的存储桶中
					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.
                        // 第B位 置1
                        // 如果tophash最低位是0就分配到x part 否则分配到y part
						useY = top & 1
						top = tophash(hash)
					} else {
                        // 对于正常的key
                        // 第B位 置0
						if hash&newbit != 0 {
                            // 使用y部分
							useY = 1
						}
					}
				}

				if evacuatedX+1 != evacuatedY || evacuatedX^1 != evacuatedY {
					throw("bad evacuatedN")
				}
                // 这里其实就是重新设置tophash值
                // 标记老的cell的tophash值，表示搬到useT部分（可能是x也可能是y，看具体取值）
				b.tophash[i] = evacuatedX + useY // evacuatedX + 1 == evacuatedY
				// 选择目标bucket的内存起始部分
                dst := &xy[useY]                 // evacuation destination
                
                // 如果i=8说明要溢出了
				if dst.i == bucketCnt {
                    // 新建一个溢出bucket
					dst.b = h.newoverflow(t, dst.b)
                    // 从0开始计数
					dst.i = 0
                    // 标识key要移动到的位置
					dst.k = add(unsafe.Pointer(dst.b), dataOffset)
                    // 标识value要移动到的位置
					dst.e = add(dst.k, bucketCnt*uintptr(t.keysize))
				}
                // 重新设置tophash
				dst.b.tophash[dst.i&(bucketCnt-1)] = top // mask dst.i as an optimization, to avoid a bounds check
				if t.indirectkey() {
                    // 将原key（指针类型）复制到新的位置
					*(*unsafe.Pointer)(dst.k) = k2 // copy pointer
				} else {
                    // 将原key（值类型）复制到新位置
					typedmemmove(t.key, dst.k, k) // copy elem
				}
                // 如果v是指针，操作同key
				if t.indirectelem() {
					*(*unsafe.Pointer)(dst.e) = *(*unsafe.Pointer)(e)
				} else {
					typedmemmove(t.elem, dst.e, e)
				}
                // 定位到下一个cell
				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.
                // 两个溢出指针在bucket末尾用于保证 遍历到bucket末尾的指针
				dst.k = add(dst.k, uintptr(t.keysize))
				dst.e = add(dst.e, uintptr(t.elemsize))
			}
		}
        // 如果没有协程在用老的bucket，就将老的bucket清除，帮助gc
		// 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
            // 只清除k-v部分，tophash用于标识搬迁状态
			memclrHasPointers(ptr, n)
		}
	}

    // 如果此次搬迁的bucket等于当前搬迁进度，更新搬迁进度
	if oldbucket == h.nevacuate {
		advanceEvacuationMark(h, t, newbit)
	}
}

// 更新搬迁进度
func advanceEvacuationMark(h *hmap, t *maptype, newbit uintptr) {
    // 进度+1
	h.nevacuate++
    // 尝试往后看1024个bucket，确保行为是O(1)的
	// 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
	}
    // 寻找没有搬迁过的bucket
	for h.nevacuate != stop && bucketEvacuated(t, h, h.nevacuate) {
		h.nevacuate++
	}

    // 现在h.nevacuate之前的bucket都被搬迁完毕了

    // 如果所有bucket搬迁完毕
	if h.nevacuate == newbit { // newbit == # of oldbuckets
        // 清除oldbuckets，释放bucket array
		// Growing is all done. Free old main bucket array.
		h.oldbuckets = nil
        // 清除老的溢出bucket
        // [0]表示当前溢出bucket
        // [1]表示老的溢出bucket
		// 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
		}
        // 清除正在扩容的标志位
		h.flags &^= sameSizeGrow
	}
}