// 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.
   // 元素个数，len(map)
   count     int // # live cells == size of map.  Must be first (used by len() builtin)
   // 标记是否有线程正在写
   flags     uint8
   // buckets 的对数 log_2
   B         uint8  // log_2 of # of buckets (can hold up to loadFactor * 2^B items)
   // overflow的bucket大致数量
   noverflow uint16 // approximate number of overflow buckets; see incrnoverflow for details
   //  哈希的种子，它能为哈希函数的结果引入随机性，这个值在创建哈希表时确定，并在调用哈希函数时作为参数传入
   hash0     uint32 // hash seed
   // buckets 数组，大小为 2^B
   buckets    unsafe.Pointer // array of 2^B Buckets. may be nil if count==0.
   // 旧map，不为nil时表示正在扩容
   oldbuckets unsafe.Pointer // previous bucket array of half the size, non-nil only when growing
   // 扩容进度，小于此地址的 buckets 是已经迁移完成的
   nevacuate  uintptr        // progress counter for evacuation (buckets less than this have been evacuated)
   extra *mapextra // optional fields
}

buckets 是一个指针，最终它指向的是一个结构体：

// 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.
}

运行时:

type bmap struct {
    topbits  [8]uint8
    keys     [8]keytype
    values   [8]valuetype
    pad      uintptr
    overflow uintptr
}

bmap 就是我们常说的“桶”，桶里面会最多装 8 个 key，这些 key 之所以会落入同一个桶，是因为它们经过哈希计算后，哈希结果是“一类”的。在桶内，又会根据 key 计算出来的 hash 值的高 8 位来决定 key 到底落入桶内的哪个位置（一个桶内最多有8个位置）。