一、sync Map 包整体构造
本文次要论述：Load、Store、Delete，更加具体的论述能够参考源码形容（倡议先大体浏览一下Map源码）。
导言：
- 空间换工夫。 通过冗余的两个数据结构(read、dirty),实现加锁对性能的影响。
- 应用只读数据(read)，防止读写抵触。
- 动静调整，miss次数多了之后，将dirty数据晋升为read。
- double-checking。
- 提早删除。 删除一个键值只是打标记（会将key对应value的pointer置为nil，但read中依然有这个key:key;value:nil的键值对），只有在晋升dirty的时候才清理删除的数据。
- 优先从read读取、更新、删除，因为对read的读取不须要锁。
- 尽管read和dirty有冗余数据，但这些数据是通过指针指向同一个数据，所以只管Map的value会很大，然而冗余的空间占用还是无限的。
二、根底数据结构
1、Map
// Map is a concurrent map with amortized-constant-time loads, stores, and deletes.// It is safe for multiple goroutines to call a Map's methods concurrently.//// It is optimized for use in concurrent loops with keys that are// stable over time, and either few steady-state stores, or stores// localized to one goroutine per key.//// For use cases that do not share these attributes, it will likely have// comparable or worse performance and worse type safety than an ordinary// map paired with a read-write mutex.//// The zero Map is valid and empty.//// A Map must not be copied after first use.//该 Map 是线程平安的，读取，插入，删除也都放弃着常数级的工夫复杂度。//多个 goroutines 协程同时调用 Map 办法也是线程平安的。该 Map 的零值是无效的，//并且零值是一个空的 Map 。线程平安的 Map 在第一次应用之后，不容许被拷贝。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. // 一个只读的数据结构，因为只读，所以不会有读写抵触。 // 所以从这个数据中读取总是平安的。 // 实际上，理论也会更新这个数据的entries,如果entry是未删除的(unexpunged), 并不需要加锁。如果entry曾经被删除了，须要加锁，以便更新dirty数据。 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数据蕴含以后的map蕴含的entries,它蕴含最新的entries(包含read中未删除的数据,虽有冗余，然而晋升dirty字段为read的时候十分快，不必一个一个的复制，而是间接将这个数据结构作为read字段的一部分),有些数据还可能没有挪动到read字段中。 // 对于dirty的操作须要加锁，因为对它的操作可能会有读写竞争。 // 当dirty为空的时候， 比方初始化或者刚晋升完，下一次的写操作会复制read字段中未删除的数据到这个数据中。 dirty map[interface{}]*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. // 当从Map中读取entry的时候，如果read中不蕴含这个entry,会尝试从dirty中读取，这个时候会将misses加一， // 当misses累积到 dirty的长度的时候， 就会将dirty晋升为read,防止从dirty中miss太屡次。因为操作dirty须要加锁。 misses int}2、readOnly
// readOnly is an immutable struct stored atomically in the Map.read field.type readOnly struct { m map[interface{}]*entry // true if the dirty map contains some key not in m. // 如果Map.dirty有些数据不在中的时候，这个值为true amended bool }3、entry
// 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 m.dirty == nil. // // 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有三种值： //nil: entry已被删除了，并且m.dirty为nil //expunged: entry已被删除了，并且m.dirty不为nil，而且这个entry不存在于m.dirty中 //其它： entry是一个失常的值 p unsafe.Pointer // *interface{}}4、Value
// A Value provides an atomic load and store of a consistently typed value.// Values can be created as part of other data structures.// The zero value for a Value returns nil from Load.// Once Store has been called, a Value must not be copied.//// A Value must not be copied after first use.type Value struct { noCopy noCopy v interface{}}下图来自：http://www.jianshu.com/p/43e66dab535b
三、Load
依据指定的key,查找对应的值value,如果不存在，通过ok反映
func (m *Map) Load(key interface{}) (value interface{}, ok bool) { read, _ := m.read.Load().(readOnly) e, ok := read.m[key] // 如果没找到，并且m.dirty中有新数据，须要从m.dirty查找，这个时候须要加锁 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.) //double check,防止加锁的时候m.dirty晋升为m.read,这个时候m.read可能被替换了。 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. 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 } m.read.Store(readOnly{m: m.dirty}) m.dirty = nil m.misses = 0}四、Store
更新或者新增一个entry
// Store sets the value for a key.func (m *Map) Store(key, value interface{}) { read, _ := m.read.Load().(readOnly) // 从 read map 中读取 key 胜利并且取出的 entry 尝试存储 value 胜利，间接返回 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() {//确保未被标记成删除，即e 指向的是非 nil 的 // The entry was previously expunged, which implies that there is a // non-nil dirty map and this entry is not in it. //m.dirty中不存在这个键，所以退出m.dirty 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()}// 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 *interface{}) bool { p := atomic.LoadPointer(&e.p) if p == expunged { return false } for { if atomic.CompareAndSwapPointer(&e.p, p, unsafe.Pointer(i)) { return true } p = atomic.LoadPointer(&e.p) if p == expunged { return false } }}func (m *Map) dirtyLocked() { if m.dirty != nil { return } read, _ := m.read.Load().(readOnly) m.dirty = make(map[interface{}]*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 { // 将曾经删除标记为nil的数据标记为expunged if atomic.CompareAndSwapPointer(&e.p, nil, expunged) { return true } p = atomic.LoadPointer(&e.p) } return p == expunged}// 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.// unexpungeLocked 函数确保了 entry 没有被标记成已被革除。// 如果 entry 先前被革除过了，那么在 mutex 解锁之前，它肯定要被退出到 dirty map 中//如果 entry 的 unexpungeLocked 返回为 true，那么就阐明 entry //之前被标记成了 expunged，并通过 CAS 操作胜利把它置为 nil。func (e *entry) unexpungeLocked() (wasExpunged bool) { return atomic.CompareAndSwapPointer(&e.p, expunged, nil)}五、Delete
删除一个键值
// Delete deletes the value for a key.func (m *Map) Delete(key interface{}) { 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 { delete(m.dirty, key) } m.mu.Unlock() } if ok { e.delete() }}func (e *entry) delete() (hadValue bool) { for { p := atomic.LoadPointer(&e.p) // 已标记为删除 if p == nil || p == expunged { return false } // 原子操作，e.p标记为nil if atomic.CompareAndSwapPointer(&e.p, p, nil) { return true } }}六、疑难
1、曾经删除的key,再次Load的时候，会怎么样？
func (e *entry) load() (value interface{}, ok bool) { p := atomic.LoadPointer(&e.p) if p == nil || p == expunged { return nil, false } return *(*interface{})(p), true}在Map Load办法中调用e.load()时，load办法会辨认该值是否已被删除
本文map构造形容局部参考：https://studygolang.com/articles/10511