go sync包（七）Sync.Map

Sync.Map

原理

通过 read 和 dirty 两个字段实现数据的读写分离，读的数据存在只读字段 read 上，将最新写入的数据存在 dirty 字段上。
读取时会先查询 read，不存在再查询 dirty，写入时则只写入 dirty。
读取 read 并不需要加锁，而读或写 dirty 则需要加锁。
misses 字段来统计 read 被穿透的次数（被穿透指需要读 dirty 的情况），超过一定次数则将 dirty 数据更新到 read 中（触发条件：misses=len(dirty)）。

优缺点

优点：通过读写分离，降低锁时间来提高效率。
缺点：不适用于大量写的场景，这样会导致 read map 读不到数据而进一步加锁读取，同时dirty map也会一直晋升为read map，整体性能较差，甚至没有单纯的 map+metux 高。
适用场景：读多写少的场景。

Sync.Map 数据结构

type Map struct {mu Mutexread atomic.Value // readOnly// 包含需要加锁才能访问的元素// 包括所有在read字段中但未被expunged（删除）的元素以及新加的元素dirty map[any]*entry// 记录从read中读取miss的次数，一旦miss数和dirty长度一样了，就会把dirty提升为read    misses int
}// readOnly is an immutable struct stored atomically in the Map.read field.
type readOnly struct {m       map[any]*entryamended bool // true if the dirty map contains some key not in m.
}// expunged is an arbitrary pointer that marks entries which have been deleted
// from the dirty map.
var expunged = unsafe.Pointer(new(any))// An entry is a slot in the map corresponding to a particular key.
type entry struct {p unsafe.Pointer // *interface{}
}

Sync.Map 操作

Store

// Store sets the value for a key.
func (m *Map) Store(key, value any) {// 判断 read 有没有包含这个 key，有则 cas 更新read, _ := m.read.Load().(readOnly)if e, ok := read.m[key]; ok && e.tryStore(&value) {return}// read 没有，需要加锁访问 dirtym.mu.Lock()read, _ = m.read.Load().(readOnly)// 双重检查，在查一下 read 里面有没有（包含标记删除的）if e, ok := read.m[key]; ok {// 之前被标记为删除，重新加入 dirtyif e.unexpungeLocked() {// The entry was previously expunged, which implies that there is a// non-nil dirty map and this entry is not in it.m.dirty[key] = e}e.storeLocked(&value)} else if e, ok := m.dirty[key]; ok {e.storeLocked(&value)} else {if !read.amended {// We're adding the first new key to the dirty map.// Make sure it is allocated and mark the read-only map as incomplete.// 创建一个新的 dirty，遍历 read，将没有标记删除的加入m.dirtyLocked()m.read.Store(readOnly{m: read.m, amended: true})}m.dirty[key] = newEntry(value)}m.mu.Unlock()
}func (m *Map) dirtyLocked() {if m.dirty != nil {return}read, _ := m.read.Load().(readOnly)m.dirty = make(map[any]*entry, len(read.m))for k, e := range read.m {if !e.tryExpungeLocked() {m.dirty[k] = e}}
}

Store 是 新增/修改 操作：

read 里面有，直接 cas 更新。
read 没有，加锁。
再次检查 read，有已经标记删除的，重用，并更新 value。
dirty 有，直接更新。
dirty 没有，新增一个。如果 dirty 为 nil，创建一个 dirty，遍历 read 将元素加入 dirty。

Load

// Load returns the value stored in the map for a key, or nil if no
// value is present.
// The ok result indicates whether value was found in the map.
func (m *Map) Load(key any) (value any, ok bool) {// 先从 read 获取read, _ := m.read.Load().(readOnly)e, ok := read.m[key]// dirty 中有包含 read 中没有的元素if !ok && read.amended {// 加锁，双检查 read，read 中没有，从 dirty 中获取m.mu.Lock()// Avoid reporting a spurious miss if m.dirty got promoted while we were// blocked on m.mu. (If further loads of the same key will not miss, it's// not worth copying the dirty map for this key.)read, _ = m.read.Load().(readOnly)e, ok = read.m[key]if !ok && read.amended {e, ok = m.dirty[key]// Regardless of whether the entry was present, record a miss: this key// will take the slow path until the dirty map is promoted to the read// map.// miss++m.missLocked()}m.mu.Unlock()}if !ok {return nil, false}return e.load()
}func (m *Map) missLocked() {m.misses++if m.misses < len(m.dirty) {return}// dirty 提升为 readm.read.Store(readOnly{m: m.dirty})// dirty 置为 nilm.dirty = nilm.misses = 0
}

Load 是读取操作：

从 read 中获取，有则直接返回。
read 中没有，加锁
- 双检查 read， read 有，赋值。
- read 没有，从 dirty 中获取，不管 dirty 中有没有，miss++
  当 miss == len(dirty)，dirty 提升为 read 字段，清空 dirty。

Delete

// Delete deletes the value for a key.
func (m *Map) Delete(key any) {m.LoadAndDelete(key)
}// LoadAndDelete deletes the value for a key, returning the previous value if any.
// The loaded result reports whether the key was present.
func (m *Map) LoadAndDelete(key any) (value any, loaded bool) {// 从 read 中获取read, _ := m.read.Load().(readOnly)e, ok := read.m[key]if !ok && read.amended {// 加锁，双重检查 readm.mu.Lock()read, _ = m.read.Load().(readOnly)e, ok = read.m[key]if !ok && read.amended {// read 没有，dirty 有，则直接删除 dirty 的keye, ok = m.dirty[key]delete(m.dirty, key)// Regardless of whether the entry was present, record a miss: this key// will take the slow path until the dirty map is promoted to the read// map.m.missLocked()}m.mu.Unlock()}if ok {// 延迟删除：read 的 entry 标记为删除，设置为 nilreturn e.delete()}return nil, false
}func (e *entry) delete() (value any, ok bool) {for {p := atomic.LoadPointer(&e.p)if p == nil || p == expunged {return nil, false}if atomic.CompareAndSwapPointer(&e.p, p, nil) {return *(*any)(p), true}}
}