介绍
因为golang的原生map是非并发安全的，所以为了保证map的并发安全，最简单的方式就是给map加锁。直接对一个map加锁，当访问map的请求越来越多，都竞争这一把锁使得并发访问变慢。
分段锁是一种锁的设计，并不是具体的一种锁，分段锁顾名思义就是将锁分段，将锁的粒度变小，将存储的对象分散到各个分片（shard）中，每个分片由一把锁控制，我们将 key 分散到固定数量的 shard 中避免 rehash 操作。shard 是有锁保护的 map, 当 shard 进行 rehash 时会阻塞shard内的读写，但不会对其他 shard 造成影响。这样使得当需要对在A分片上的数据进行读写时不会影响B分片的读写。
虽然有sync.Map存在，但是通过压力测试对比，分段锁的性能更好，下面给出分段锁代码
代码
package main import ( "math" "sync" "sync/atomic" ) type ConcurrentMap struct { shard []*MapShard count int32 shardCount uint32 } type MapShard struct { m map[string]interface{} mutex sync.RWMutex } //该参数转成二进制，每个位都赋为1 func computeCapacity(param int) int { if param <= 16 { return 16 } n := param - 1 n |= n >> 1 n |= n >> 2 n |= n >> 4 n |= n >> 8 n |= n >> 16 if n < 0 { return math.MaxInt32 } return n + 1 } // MakeConcurrentMap 返回一个分段锁的实例 func MakeConcurrentMap(shardCount int) *ConcurrentMap { shardCount = computeCapacity(shardCount) shard := make([]*MapShard, shardCount) for idx := range shard { shard[idx] = &MapShard{ m: make(map[string]interface{}), mutex: sync.RWMutex{}, } } return &ConcurrentMap{ shard: shard, count: 0, shardCount: uint32(shardCount), } } const prime32 = uint32(16777619) func fnv32(key string) uint32 { hash := uint32(2166136261) for i := 0; i < len(key); i++ { hash *= prime32 hash ^= uint32(key[i]) } return hash } func (dict *ConcurrentMap) getShardMap(key string) *MapShard { hashCode := fnv32(key) idx := (dict.shardCount - 1) & hashCode return dict.shard[idx] } func (dict *ConcurrentMap) Get(key string) (val interface{}, exists bool) { shard := dict.getShardMap(key) shard.mutex.RLock() defer shard.mutex.RUnlock() val, exists = shard.m[key] return } func (dict *ConcurrentMap) Len() int { return int(atomic.LoadInt32(&dict.count)) } // Set 插入 func (dict *ConcurrentMap) Set(key string, val interface{}) int { shard := dict.getShardMap(key) shard.mutex.Lock() defer shard.mutex.Unlock() if _, ok := shard.m[key]; ok { shard.m[key] = val return 0 } shard.m[key] = val atomic.AddInt32(&dict.count, 1) return 1 } // Remove 删除一个key func (dict *ConcurrentMap) Remove(key string) int { shard := dict.getShardMap(key) shard.mutex.Lock() defer shard.mutex.Unlock() if _, ok := shard.m[key]; ok { delete(shard.m, key) atomic.AddInt32(&dict.count, -1) return 1 } return 0 }
