go1.18.1中的sync.Map

map记了一下，还有个sync.Map，其中对 atomic 包和锁的配合使用，很有代表性
go版本为1.18.1

源码 src/sync/map.go
源码加注释总共不超过 400 行源码，去掉注释只有 250 行了，可以花一会儿时间读一遍
原理很简单，空间换时间，即 read/dirty 两个 map 实现读写分离，降低锁竞争消耗
同时通过 atomic 包的应用完成原子性的操作
重点是 Load / Store / Delete 操作，加了部分注释

// Map is like a Go map[interface{}]interface{} but is safe for concurrent use
// by multiple goroutines without additional locking or coordination.
// Loads, stores, and deletes run in amortized constant time.
//
// The Map type is specialized. Most code should use a plain Go map instead,
// with separate locking or coordination, for better type safety and to make it
// easier to maintain other invariants along with the map content.
//
// The Map type is optimized for two common use cases: (1) when the entry for a given
// key is only ever written once but read many times, as in caches that only grow,
// or (2) when multiple goroutines read, write, and overwrite entries for disjoint
// sets of keys. In these two cases, use of a Map may significantly reduce lock
// contention compared to a Go map paired with a separate Mutex or RWMutex.
//
// The zero Map is empty and ready for use. A Map must not be copied after first use.
type Map struct {
	mu Mutex

	// 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[any]*entry

	// 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
}

// readOnly is an immutable struct stored atomically in the Map.read field.
type readOnly struct {
	m       map[any]*entry
	amended 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 points to the interface{} value stored for the entry.
	//
	// If p == nil, the entry has been deleted, and either m.dirty == nil or
	// m.dirty[key] is e.
	//
	// If p == expunged, the entry has been deleted, m.dirty != nil, and the entry
	// is missing from m.dirty.
	//
	// Otherwise, the entry is valid and recorded in m.read.m[key] and, if m.dirty
	// != nil, in m.dirty[key].
	//
	// An entry can be deleted by atomic replacement with nil: when m.dirty is
	// next created, it will atomically replace nil with expunged and leave
	// m.dirty[key] unset.
	//
	// An entry's associated value can be updated by atomic replacement, provided
	// p != expunged. If p == expunged, an entry's associated value can be updated
	// only after first setting m.dirty[key] = e so that lookups using the dirty
	// map find the entry.
	p unsafe.Pointer // *interface{}
}

func newEntry(i any) *entry {
	return &entry{p: unsafe.Pointer(&i)}
}

// 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) {
	read, _ := m.read.Load().(readOnly)
	e, ok := read.m[key]
	// 如果 read 中没有找到，且 amended 为 true（dirty中包含read中没有的key），则需要先加锁，然后尝试从dirty中寻找
	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]
		// 再次确认
		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.
			// 只要访问了 dirty 就记录一次未命中事件
			m.missLocked()
		}
		m.mu.Unlock()
	}
	if !ok {
		return nil, false
	}
	return e.load()
}

func (e *entry) load() (value any, ok bool) {
	// 同样是原子操作
	p := atomic.LoadPointer(&e.p)
	// 判断被删除的两种状态，返回不存在
	if p == nil || p == expunged {
		return nil, false
	}
	return *(*any)(p), true
}

// Store sets the value for a key.
func (m *Map) Store(key, value any) {
	read, _ := m.read.Load().(readOnly)
	// key存在于read中，且值不为 expunged，直接通过 CAS（atomic.CompareAndSwapPointer）进行指针修改实现无锁的 value 更新
	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 {
		// 值为 expunged，则通过 CAS 将 value 更新为 nil，并将 key 写入 dirty，以确保 dirty 提升为 read 时的数据完整性
		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
		}
		// 通过 atomic.StorePointer 直接修改 value 的指针值
		e.storeLocked(&value)
	} else if e, ok := m.dirty[key]; ok {
		// 不在read中，而存在于 dirty 中，直接修改
		e.storeLocked(&value)
	} else {
		// read 和 dirty 中都不存在的key，若 dirty 为空，则需要从 read.m 中取出未删除（非expunged）的key来创建一个 dirty
		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()
}

// tryStore stores a value if the entry has not been expunged.
//
// If the entry is expunged, tryStore returns false and leaves the entry
// unchanged.
func (e *entry) tryStore(i *any) bool {
	for {
		p := atomic.LoadPointer(&e.p)
		// 如果是 expunged 状态，需要继续加锁处理
		if p == expunged {
			return false
		}
		if atomic.CompareAndSwapPointer(&e.p, p, unsafe.Pointer(i)) {
			return true
		}
	}
}

// unexpungeLocked ensures that the entry is not marked as expunged.
//
// If the entry was previously expunged, it must be added to the dirty map
// before m.mu is unlocked.
func (e *entry) unexpungeLocked() (wasExpunged bool) {
	return atomic.CompareAndSwapPointer(&e.p, expunged, nil)
}

// storeLocked unconditionally stores a value to the entry.
//
// The entry must be known not to be expunged.
func (e *entry) storeLocked(i *any) {
	atomic.StorePointer(&e.p, unsafe.Pointer(i))
}

// LoadOrStore returns the existing value for the key if present.
// Otherwise, it stores and returns the given value.
// The loaded result is true if the value was loaded, false if stored.
func (m *Map) LoadOrStore(key, value any) (actual any, loaded bool) {
	// Avoid locking if it's a clean hit.
	read, _ := m.read.Load().(readOnly)
	if e, ok := read.m[key]; ok {
		actual, loaded, ok := e.tryLoadOrStore(value)
		if ok {
			return actual, loaded
		}
	}

	m.mu.Lock()
	read, _ = m.read.Load().(readOnly)
	if e, ok := read.m[key]; ok {
		if e.unexpungeLocked() {
			m.dirty[key] = e
		}
		actual, loaded, _ = e.tryLoadOrStore(value)
	} else if e, ok := m.dirty[key]; ok {
		actual, loaded, _ = e.tryLoadOrStore(value)
		m.missLocked()
	} 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)
		actual, loaded = value, false
	}
	m.mu.Unlock()

	return actual, loaded
}

// tryLoadOrStore atomically loads or stores a value if the entry is not
// expunged.
//
// If the entry is expunged, tryLoadOrStore leaves the entry unchanged and
// returns with ok==false.
func (e *entry) tryLoadOrStore(i any) (actual any, loaded, ok bool) {
	p := atomic.LoadPointer(&e.p)
	if p == expunged {
		return nil, false, false
	}
	if p != nil {
		return *(*any)(p), true, true
	}

	// Copy the interface after the first load to make this method more amenable
	// to escape analysis: if we hit the "load" path or the entry is expunged, we
	// shouldn't bother heap-allocating.
	ic := i
	for {
		if atomic.CompareAndSwapPointer(&e.p, nil, unsafe.Pointer(&ic)) {
			return i, false, true
		}
		p = atomic.LoadPointer(&e.p)
		if p == expunged {
			return nil, false, false
		}
		if p != nil {
			return *(*any)(p), true, true
		}
	}
}

// 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) {
	// 和 Load 代码逻辑一致，只是多了删除操作
	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]
			// 无论是否存在，直接删除 dirty 这个 map 中的 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
}

// Delete deletes the value for a key.
func (m *Map) Delete(key any) {
	// 直接复用了，只是忽略返回值
	m.LoadAndDelete(key)
}

func (e *entry) delete() (value any, ok bool) {
	for {
		p := atomic.LoadPointer(&e.p)
		if p == nil || p == expunged {
			return nil, false
		}
		// 将 key 对应的 value 值改为 nil，而不是真正的删除
		if atomic.CompareAndSwapPointer(&e.p, p, nil) {
			return *(*any)(p), true
		}
	}
}

// Range calls f sequentially for each key and value present in the map.
// If f returns false, range stops the iteration.
//
// Range does not necessarily correspond to any consistent snapshot of the Map's
// contents: no key will be visited more than once, but if the value for any key
// is stored or deleted concurrently (including by f), Range may reflect any
// mapping for that key from any point during the Range call. Range does not
// block other methods on the receiver; even f itself may call any method on m.
//
// Range may be O(N) with the number of elements in the map even if f returns
// false after a constant number of calls.
func (m *Map) Range(f func(key, value any) bool) {
	// We need to be able to iterate over all of the keys that were already
	// present at the start of the call to Range.
	// If read.amended is false, then read.m satisfies that property without
	// requiring us to hold m.mu for a long time.
	read, _ := m.read.Load().(readOnly)
	if read.amended {
		// m.dirty contains keys not in read.m. Fortunately, Range is already O(N)
		// (assuming the caller does not break out early), so a call to Range
		// amortizes an entire copy of the map: we can promote the dirty copy
		// immediately!
		m.mu.Lock()
		read, _ = m.read.Load().(readOnly)
		if read.amended {
			read = readOnly{m: m.dirty}
			m.read.Store(read)
			m.dirty = nil
			m.misses = 0
		}
		m.mu.Unlock()
	}

	for k, e := range read.m {
		v, ok := e.load()
		if !ok {
			continue
		}
		if !f(k, v) {
			break
		}
	}
}

func (m *Map) missLocked() {
	// 未命中计数加一
	m.misses++
	if m.misses < len(m.dirty) {
		return
	}
	// 如果达到了阈值，则将 dirty 提升为 read，并重置 dirty 和 misses
	m.read.Store(readOnly{m: m.dirty})
	m.dirty = nil
	m.misses = 0
}

func (m *Map) dirtyLocked() {
	if m.dirty != nil {
		return
	}

	read, _ := m.read.Load().(readOnly)
	// 预分配空间，并通过此步骤真正删除掉了之前被标记删除的 key
	m.dirty = make(map[any]*entry, len(read.m))
	for k, e := range read.m {
		if !e.tryExpungeLocked() {
			m.dirty[k] = e
		}
	}
}

func (e *entry) tryExpungeLocked() (isExpunged bool) {
	p := atomic.LoadPointer(&e.p)
	for p == nil {
		// 将已被删除(nil状态)的 p 改成 expunged，如果这个 entry 成功变成了 expunged 状态，也就不需要写入 dirty 了
		if atomic.CompareAndSwapPointer(&e.p, nil, expunged) {
			return true
		}
		p = atomic.LoadPointer(&e.p)
	}
	// expunged 状态
	return p == expunged
}

• expunged

  • 一个特殊指针unsafe.Pointer(new(any))，用于标记相应的 key 已被删除，且已经不在 dirty 中了

• Map.mu

  • 对 dirty 读写时需要加锁
  • 更新 read 时，要加锁

• Map.read

  • atomic.Value 类型，支持并发的读，存储不需要加锁直接访问的部分
  • 对其中存储的 entry，可以被并发的更新（CAS）
    • 如果更新已被标记删除的 entry（expunged状态），则需要将此 entry 值修改为 nil，并将 key 存入 dirty 中

• Map.dirty

  • 访问其中的内容需要加锁，包含新写入的 key，以及 read 中所有未被删除的 key
  • 可以被提升为 read，然后置为 nil，然后下次写入时新建一个包含 read 中所有未被删除 key 的 dirty

• Map.misses

  • 自 read 被更新以来访问 key 失败的次数，如果达到阈值（len(dirty)）则将 dirty 提升为 read（readOnly.m）,并重置0

• Map

  • 真正存储 k/v 的是 read/dirty 两个字段
  • read 存储的是 readOnly 类型，其中包含一个 map（m） 和 布尔值（amended）
    • m 定义为 map[any]*entry，其key不会发生变化；value是 entry 指针
    • amended 用于标记 dirty 中是否包含 m 中不存在的 key
  • read 和 dirty 各自维护了一个map，其中 key 会有所不同，但同一个 key 指向的是同一个 value
    • 由于 value 是 *entry，所以双方对 value 的修改都是互相可见的

• entry

  • 只包含一个指针 p （unsafe.Pointer）
  • 只有三种状态
    1. nil
       • 说明其 key 已被删除，dirty == nil 或者该 key 仍存在于 dirty 中
         • Delete 时，如果 key 在 read 中，则直接将其 value 更新为 nil
           • 如果此时 dirty != nil，则 key 仍在 dirty 中，其 value 也变为 nil
           • 还有一种情况就是此时 dirty == nil
    2. expunged
       • 说明其 key 已被删除，dirty != nil 且 dirty 中已经没有这个 key 了
       • 根据 read 重建 dirty 时，会将 read 中状态为 nil 的 key，更新为 expunged，且不会放入新的 dirty
         • 见 tryExpungeLocked 方法
    3. 正常值
       • 指向 value 的实际地址，且如果 dirty != nil，则该 key 存在于 dirty 中

• 一些细节

  • go 1.18.1 中已经使用 any 替换 interface了，毕竟支持泛型了
  • Load / Store 等操作中，都会在加锁后二次确认 double check
  • 随处可见的原子操作，atomic
  • dirty 提升为 read，有以下几个场景
    • 访问 key 时，read 中不存在，且 misses 未命中计数达到阈值
      • 包括 Load/LoadOrStore/LoadOrDelete（Delete）操作
    • 遍历 时，dirty 不为空
      • Range 操作
  • dirty 中一定是包含 read 中所有未被删除的 key
  • 如果 key 在 dirty 中，提升 dirty 能提升 read 的命中率
    • 但如果频繁访问一个不存在的 key，将导致加锁及无意义的 dirty 提升操作
  • 众所周知，sync.Map 适合读多写少的场景，从 Load / Store 的源码也能看出原因
    • 更新已存在 key 的操作效率很高，只需要 CAS 操作即可完成
    • 新 key 写入，将导致加锁，同时在 dirty 提升之前将导致所有访问不存在于read中的key都触发加锁
      • 无论是否存在于 dirty 中，只要 read.amended 为 true，或者说 dirty 不为 nil

        参考链接

        原文链接