golang map 并发读写 sync.Map 实现原理

一. map并发读写不安全

Go  map 是不支持并发写操作的，当 Goroutine 操作同一个 map，会产生报错：fatal error: concurrent map writes。

1. map 并发读写 

//map  并发读写 产生错误
//fatal error: concurrent map read and map write
func MapConcurrentErr() {
	c := make(map[string]int)
	go func() { //开一个goroutine写map
		for j := 0; j < 1000000; j++ {
			c[fmt.Sprintf("%d", j)] = j
		}
	}()
	go func() { //开一个goroutine读map
		for j := 0; j < 1000000; j++ {
			fmt.Println(c[fmt.Sprintf("%d", j)])
		}
	}()
	time.Sleep(time.Second * 20)
 
}

2.使用sync map 解决并发安全问题

//sync map 并发读写安全
func SyncMapConcurrent() {
	c := sync.Map{}
	go func() { //开一个goroutine写map
		for j := 0; j < 1000000; j++ {
			c.Store(j, j)
			fmt.Println("store", j)
		}
	}()
	go func() { //开一个goroutine读map
		for j := 0; j < 1000000; j++ {
			load, ok := c.Load(j)
			fmt.Println("load", load, ok)
		}
	}()
	time.Sleep(time.Second * 20)
 
}

二.sync map实现

2.1 结构体

1.Map结构体

type Map struct {
    
    //锁，用于保护 dirty 和 read。读写dirty、将dirty copy到 read时需要持有该锁。
	mu Mutex
 
    //实际结构为 readOnly
	// read contains the portion of the map's contents that are safe for
	// concurrent access (with or without mu held).
	//
	// The read field itself is always safe to load, but must only be stored with
	// mu held.
	//
	// Entries stored in read may be updated concurrently without mu, but updating
	// a previously-expunged entry requires that the entry be copied to the dirty
	// map and unexpunged with mu held.
	read atomic.Value // readOnly
    
    
	// dirty contains the portion of the map's contents that require mu to be
	// held. To ensure that the dirty map can be promoted to the read map quickly,
	// it also includes all of the non-expunged entries in the read map.
	//
	// Expunged entries are not stored in the dirty map. An expunged entry in the
	// clean map must be unexpunged and added to the dirty map before a new value
	// can be stored to it.
	//
	// If the dirty map is nil, the next write to the map will initialize it by
	// making a shallow copy of the clean map, omitting stale entries.
	dirty map[interface{}]*entry
    
    //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 数据同步到 read 上
	// misses counts the number of loads since the read map was last updated that
	// needed to lock mu to determine whether the key was present.
	//
	// Once enough misses have occurred to cover the cost of copying the dirty
	// map, the dirty map will be promoted to the read map (in the unamended
	// state) and the next store to the map will make a new dirty copy.
	misses int
}

2.readOnly

// readOnly is an immutable struct stored atomically in the Map.read field.
type readOnly struct {
	m       map[interface{}]*entry
    //标记ditry中是否包含元素
	amended bool // true if the dirty map contains some key not in m.
}

 3.entry
entry 用来存储值的地址属性 p 有三种状

• p == nil: 键值已经被删除，且 m.dirty == nil
• p == expunged: 键值已经被删除，但 m.dirty!=nil 且 m.dirty 不存在该键值（expunged 实际是空接口指针）
• 否则键值对存在，存在于 m.read.m 中，如果 m.dirty!=nil 则也存在于 m.dirty

// expunged is an arbitrary pointer that marks entries which have been deleted
// from the dirty map.
var expunged = unsafe.Pointer(new(interface{}))

 2.2 map结构体的方法

1.Load

Load方法用于获取map中的值

// 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 interface{}) (value interface{}, ok bool) {
	read, _ := m.read.Load().(readOnly)
	e, ok := read.m[key]
	if !ok && read.amended {
		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]
            //触发miss逻辑， 符合条件时会将dirty数据copy 到readOnly
			// 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 {
		return nil, false
	}
    // 读取entry中值
	return e.load()
}

2.Store

store用户设置key的值

 
// Store sets the value for a key.
func (m *Map) Store(key, value interface{}) {
	read, _ := m.read.Load().(readOnly)
	if e, ok := read.m[key]; ok && e.tryStore(&value) {
		return
	}
 
	m.mu.Lock()
	read, _ = m.read.Load().(readOnly)
	if e, ok := read.m[key]; ok {
		if 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.
			m.dirtyLocked()
			m.read.Store(readOnly{m: read.m, amended: true})
		}
		m.dirty[key] = newEntry(value)
	}
	m.mu.Unlock()
}

3.Delete
 

// Delete deletes the value for a key.
func (m *Map) Delete(key interface{}) {
	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 interface{}) (value interface{}, loaded bool) {
	read, _ := m.read.Load().(readOnly)
	e, ok := read.m[key]
	if !ok && read.amended {
		m.mu.Lock()
		read, _ = m.read.Load().(readOnly)
		e, ok = read.m[key]
		if !ok && read.amended {
			e, 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 {
		return e.delete()
	}
	return nil, false
}

三 总结 

• 通过 read 和 dirty 两个字段将读写分离，读的数据存在只读字段 read 上，将最新写入的数据则存在 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 需要加锁，cory dirty到read时需要加锁
• 另外有 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 数据同步到 read 上
• 对于删除数据则直接通过标记来延迟删除