go sync.map源码解析
go中的map是并发不安全的，同时多个协程读取不会出现问题，但是多个协程 同时读写就会出现 fatal error:concurrent map read and map write的错误。通用的解决办法如下：
1. 加锁
1.1 通用锁
import "sync"
type SafeMap struct {
data map[string]string
lock sync.Mutex
}
func (this *SafeMap) get(key string) string{
this.lock.Lock()
defer this.lock.Unlock()
return this.data[key]
}
func (this *SafeMap) set(key, value string) {
this.lock.Lock()
defer this.lock.Unlock()
this.data[key] = value
}
1.2 读写锁
import "sync"
type SafeMap struct {
data map[string]string
lock sync.RWMutex
}
func (this *SafeMap) get(key string) string{
this.lock.RLock()
defer this.lock.RUnlock()
return this.data[key]
}
func (this *SafeMap) set(key, value string) {
this.lock.Lock()
defer this.lock.Unlock()
this.data[key] = value
}
1.3 在go1.9之后，go引入了并发安全的map: sync.map
sync.map的原理可以概括为：
1. 通过read和dirty两个字段将读写分离，读的数据存在于read字段的，最新写的数据位于dirty字段上。
2. 读取时先查询read，不存在时查询dirty，写入时只写入dirty
3. 读取read不需要加锁，而读或写dirty需要加锁
4. 使用misses字段来统计read被穿透的次数，超过一定次数将数据从dirty同步到read上
5. 删除数据通过标记来延迟删除
sync.Map结构如下所示：
type Map struct {
mu Mutex //加锁，宝座dirty字段
read atomic.Value // 只读数据，实例类型为 readOnly
dirty map[interface{}]*entry //最新写入的数据
misses int //read被穿透的次数
}
readOnly结构
type readOnly struct {
m map[interface{}]*entry
amended bool // true if the dirty map contains some key not in m.
}
entery结构
type entry struct {
// p == nil entry已经被删除且 dirty == nil
// p == expunged entry已经被删除，但是dirty != nil且dirty中不存在该元素，这种情况出现于重建dirty时，将read复制到dirty中，复制的过程中将nil标记为expunged,不将其复制到dirty
// 除此之外，entry存在于read中，如果dirty != nil则也存在于dirty中
p unsafe.Pointer // *interface{} }
Load()方法
func (m *Map) Load(key interface{}) (value interface{}, ok bool) {
//首先尝试从read中读取 readOnly对象
read, _ := m.read.Load().(readOnly)
e, ok := read.m[key]
//如果不存在则尝试从dirty中读取
if !ok && read.amended {
m.mu.Lock()
//再读取一次read中内容，主要是用于防止上一步加锁过程中dirty map转换为read map导致dirty中读取不到数据
read, _ = m.read.Load().(readOnly)
e, ok = read.m[key]
//如果确实不存在，则从dirty中读取
if !ok && read.amended {
e, ok = m.dirty[key]
// 不管dirty中存不存在，都将miss + 1, 如果misses值等于dirty中元素个数，就会把dirty中元素迁移到read中
m.missLocked()
}
m.mu.Unlock()
}
if !ok {
return nil, false
}
return e.load()
}
Store()方法
// Store sets the value for a key.
func (m *Map) Store(key, value interface{}) {
//直接再read中查找
read, _ := m.read.Load().(readOnly)
//如果找到了，直接更新read中值，返回
if e, ok := read.m[key]; ok && e.tryStore(&value) {
return
}
//如不存在，去dirty中读
m.mu.Lock()
//二次检测
read, _ = m.read.Load().(readOnly)
//如果此时读到，read中不允许直接的添加删除值，此种情况说明加锁之前存在dirty升级为read的操作
if e, ok := read.m[key]; ok {
//如果读到的值为expunged, 说明生成dirty时，复制read中的元素，对于nil的元素，搞成了expunged，所以意味着dirty不为nil，且dirty中没有该元素
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.
//更新dirty中的值
m.dirty[key] = e
}
//更新read中的值
e.storeLocked(&value)
//此时，read中没有该元素，需要更新dirty中的值
} else if e, ok := m.dirty[key]; ok {
e.storeLocked(&value)
} else {
// 如果 !read.amended, 说明dirty为nil, 需要将read map复制一份到dirty map
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()
//设置read.amended == true
m.read.Store(readOnly{m: read.m, amended: true})
}
m.dirty[key] = newEntry(value)
}
m.mu.Unlock()
}
LoadOrStoce()
// 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 interface{}) (actual interface{}, loaded bool) {
// Avoid locking if it's a clean hit.
//读取read中是否存在该key
read, _ := m.read.Load().(readOnly)
if e, ok := read.m[key]; ok {
//如果存在(是否标识为删除由tryLoadOrStore处理)，尝试获取该元素的值，或者将值写入
actual, loaded, ok := e.tryLoadOrStore(value)
if ok {
return actual, loaded
}
}
m.mu.Lock()
//二次检测
read, _ = m.read.Load().(readOnly)
//如果此时读到，read中不允许直接的添加删除值，此种情况说明加锁之前存在dirty升级为read的操作
if e, ok := read.m[key]; ok {
//如果读到的值为expunged, 说明生成dirty时，复制read中的元素，对于nil的元素，搞成了expunged，所以意味着dirty不为nil，且dirty中没有该元素
if e.unexpungeLocked() {
m.dirty[key] = e
}
//如果存在(是否标识为删除由tryLoadOrStore处理)，尝试获取该元素的值，或者将值写入
actual, loaded, _ = e.tryLoadOrStore(value)
// 此时，read中没有元素，需要 tryLoadOrStore dirty中值
} else if e, ok := m.dirty[key]; ok {
actual, loaded, _ = e.tryLoadOrStore(value)
m.missLocked()
} else {
// 如果 !read.amended, 说明dirty为nil, 需要将read map复制一份到dirty map
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})
}
// 将值写入dirty中
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.
// 如果元素是 expunged, tryLoadOrStore 保持entry不变并直接返回false
func (e *entry) tryLoadOrStore(i interface{}) (actual interface{}, loaded, ok bool) {
p := atomic.LoadPointer(&e.p)
// 标识删除，直接返回
if p == expunged {
return nil, false, false
}
// 如果元素存在真实值，则直接返回该真实值
if p != nil {
return *(*interface{})(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.
// 如果 p == nil, 则更新该元素值
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 *(*interface{})(p), true, true
}
}
}
Delete()方法
// Delete deletes the value for a key.
func (m *Map) Delete(key interface{}) {
// 检查read中是否存在
read, _ := m.read.Load().(readOnly)
e, ok := read.m[key]
// 如果不存在，并且dirty中存在元素
if !ok && read.amended {
m.mu.Lock()
// 二次检测
read, _ = m.read.Load().(readOnly)
e, ok = read.m[key]
if !ok && read.amended {
// dirty中删除
delete(m.dirty, key)
}
m.mu.Unlock()
}
if ok {
// 如果存在，直接删除
e.delete()
}
}
func (e *entry) delete() (hadValue bool) {
for {
p := atomic.LoadPointer(&e.p)
if p == nil || p == expunged {
return false
}
if atomic.CompareAndSwapPointer(&e.p, p, nil) {
return true
}
}
}
Range()方法
// 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, Range may reflect any mapping for that key
// from any point during the Range call.
//
// 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 interface{}) 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)
// 如果 amended == true, 说明dirty中存在元素，且包含所有有效元素，此时，使用dirty map
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 {
//使用dirty map并将其升级为 read map
read = readOnly{m: m.dirty}
m.read.Store(read)
m.dirty = nil
m.misses = 0
}
m.mu.Unlock()
}
// 使用read map读
for k, e := range read.m {
v, ok := e.load()
// 被删除的不计入
if !ok {
continue
}
if !f(k, v) {
break
}
}
}
当sync.Map中存在大量写操作的情况下，会导致read中读不到数据，依然会频繁加锁，同时dirty升级为read，整体性能就会很低，所以sync.Map更加适合大量读、少量写的场景。
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