Go 1.9 sync Map 源码阅读笔记

ゞ浴缸里的玫瑰 2022-06-03 02:20 194阅读 0赞

# 一、sync Map 包整体结构 #

![这里写图片描述][SouthEast]

本文主要阐述：Load、Store、Delete，更加详细的阐述可以参考源码描述（建议先大体浏览一下Map源码）。

## 导言： ##

1.  **空间换时间。 通过冗余的两个数据结构(read、dirty),实现加锁对性能的影响。**
2.  **使用只读数据(read)，避免读写冲突。**
3.  **动态调整，miss次数多了之后，将dirty数据提升为read。**
4.  **double-checking。**
5.  **延迟删除。 删除一个键值只是打标记（会将key对应value的pointer置为nil，但read中仍然有这个key:key;value:nil的键值对），只有在提升dirty的时候才清理删除的数据。**
6.  **优先从read读取、更新、删除，因为对read的读取不需要锁。**
7.  **虽然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，所以尽管Map的value会很大，但是冗余的空间占用还是有限的。**

# 二、基础数据结构 #

## 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 是线程安全的，读取，插入，删除也都保持着常数级的时间复杂度。
    //多个 goroutines 协程同时调用 Map 方法也是线程安全的。该 Map 的零值是有效的，
    //并且零值是一个空的 Map 。线程安全的 Map 在第一次使用之后，不允许被拷贝。
    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，所以不会有读写冲突。
        // 所以从这个数据中读取总是安全的。
        // 实际上，实际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已经被删除了，需要加锁，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数据包含当前的map包含的entries,它包含最新的entries(包括read中未删除的数据,虽有冗余，但是提升dirty字段为read的时候非常快，不用一个一个的复制，而是直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移动到read字段中。
        // 对于dirty的操作需要加锁，因为对它的操作可能会有读写竞争。
        // 当dirty为空的时候， 比如初始化或者刚提升完，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中不包含这个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提升为read,避免从dirty中miss太多次。因为操作dirty需要加锁。
    	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是一个正常的值
    	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][http_www.jianshu.com_p_43e66dab535b]

![这里写图片描述][SouthEast 1]

# 三、Load #

根据指定的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，需要从m.dirty查找，这个时候需要加锁
    	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,避免加锁的时候m.dirty提升为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
    }

![这里写图片描述][SouthEast 2]

# 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 成功并且取出的 entry 尝试存储 value 成功，直接返回
    	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，所以加入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 {
    		 // 将已经删除标记为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 没有被标记成已被清除。
    // 如果 entry 先前被清除过了，那么在 mutex 解锁之前，它一定要被加入到 dirty map 中
    
    //如果 entry 的 unexpungeLocked 返回为 true，那么就说明 entry 
    //之前被标记成了 expunged，并经过 CAS 操作成功把它置为 nil。
    func (e *entry) unexpungeLocked() (wasExpunged bool) {
    	return atomic.CompareAndSwapPointer(&e.p, expunged, nil)
    }

![这里写图片描述][SouthEast 3]

# 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
    		}
    	}
    }

![这里写图片描述][SouthEast 4]

# 六、疑问 #

## 1、已经删除的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方法中调用e.load()时，load方法会识别该值是否已被删除

本文map结构描述部分参考：[https://studygolang.com/articles/10511][https_studygolang.com_articles_10511]

Java 1.8 ConcurrentHashMap 源码注解部分：  
[https://github.com/jiankunking/backups/blob/master/jdk/ConcurrentHashMap.java][https_github.com_jiankunking_backups_blob_master_jdk_ConcurrentHashMap.java]

个人微信公众号：  
![这里写图片描述][SouthEast 5]

> 作者：jiankunking 出处：[http://blog.csdn.net/jiankunking][http_blog.csdn.net_jiankunking]

[SouthEast]: /images/20220603/8fdba4557adb4e8792b6d914c9fe0e89.png
[http_www.jianshu.com_p_43e66dab535b]: http://www.jianshu.com/p/43e66dab535b
[SouthEast 1]: /images/20220603/972dc79f80024243bc38ae9142b26a98.png
[SouthEast 2]: /images/20220603/a7015bdf707045428ac3ed12593f6537.png
[SouthEast 3]: /images/20220603/b7cdee9cb98f44a79989da5c28f0b231.png
[SouthEast 4]: /images/20220603/c088571c275442cca739e0f291d09fed.png
[https_studygolang.com_articles_10511]: https://studygolang.com/articles/10511
[https_github.com_jiankunking_backups_blob_master_jdk_ConcurrentHashMap.java]: https://github.com/jiankunking/backups/blob/master/jdk/ConcurrentHashMap.java
[SouthEast 5]: /images/20220603/352b6f02d3244dd690cf96c21e8c61b2.png
[http_blog.csdn.net_jiankunking]: http://blog.csdn.net/jiankunking