概括
sync.Map的實現原理可概括為：
- 透過 read 和 dirty 兩個欄位將讀寫分離，讀取時會先查詢 read，不存在再查詢 dirty，寫入時則只寫入 dirty，所以read相當於dirty的快取。
- 讀取 read 並不需要加鎖，而讀或寫 dirty 都需要加鎖。
- misses 欄位統計 read 被穿透的次數，被穿透指需要讀 dirty 的情況，超過一定次數則交換read和dirty指標，使dirty成為新的read，而dirty將被設為nil
- 對於刪除資料則直接透過標記來延遲刪除
這些概括過於簡練，也不能完全準確的描述sunc.map的原理，我們直接來看原始碼
原始碼
// Copyright 2016 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package sync
import (
"sync/atomic"
)
// Map is like a Go map[any]any 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.
//
// In the terminology of [the Go memory model], Map arranges that a write operation
// “synchronizes before” any read operation that observes the effect of the write, where
// read and write operations are defined as follows.
// [Map.Load], [Map.LoadAndDelete], [Map.LoadOrStore], [Map.Swap], [Map.CompareAndSwap],
// and [Map.CompareAndDelete] are read operations;
// [Map.Delete], [Map.LoadAndDelete], [Map.Store], and [Map.Swap] are write operations;
// [Map.LoadOrStore] is a write operation when it returns loaded set to false;
// [Map.CompareAndSwap] is a write operation when it returns swapped set to true;
// and [Map.CompareAndDelete] is a write operation when it returns deleted set to true.
//
// [the Go memory model]: https://go.dev/ref/mem
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.Pointer[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 = 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 atomic.Pointer[any]
}
func newEntry(i any) *entry {
e := &entry{}
e.p.Store(&i)
return e
}
func (m *Map) loadReadOnly() readOnly {
if p := m.read.Load(); p != nil {
return *p
}
return readOnly{}
}
// 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.loadReadOnly()
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.loadReadOnly()
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 (e *entry) load() (value any, ok bool) {
p := e.p.Load()
if p == nil || p == expunged {
return nil, false
}
return *p, true
}
// Store sets the value for a key.
func (m *Map) Store(key, value any) {
_, _ = m.Swap(key, value)
}
// Clear deletes all the entries, resulting in an empty Map.
func (m *Map) Clear() {
read := m.loadReadOnly()
if len(read.m) == 0 && !read.amended {
// Avoid allocating a new readOnly when the map is already clear.
return
}
m.mu.Lock()
defer m.mu.Unlock()
read = m.loadReadOnly()
if len(read.m) > 0 || read.amended {
m.read.Store(&readOnly{})
}
clear(m.dirty)
// Don't immediately promote the newly-cleared dirty map on the next operation.
m.misses = 0
}
// tryCompareAndSwap compare the entry with the given old value and swaps
// it with a new value if the entry is equal to the old value, and the entry
// has not been expunged.
//
// If the entry is expunged, tryCompareAndSwap returns false and leaves
// the entry unchanged.
func (e *entry) tryCompareAndSwap(old, new any) bool {
p := e.p.Load()
if p == nil || p == expunged || *p != old {
return false
}
// Copy the interface after the first load to make this method more amenable
// to escape analysis: if the comparison fails from the start, we shouldn't
// bother heap-allocating an interface value to store.
nc := new
for {
if e.p.CompareAndSwap(p, &nc) {
return true
}
p = e.p.Load()
if p == nil || p == expunged || *p != old {
return false
}
}
}
// 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 e.p.CompareAndSwap(expunged, nil)
}
// swapLocked unconditionally swaps a value into the entry.
//
// The entry must be known not to be expunged.
func (e *entry) swapLocked(i *any) *any {
return e.p.Swap(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.loadReadOnly()
if e, ok := read.m[key]; ok {
actual, loaded, ok := e.tryLoadOrStore(value)
if ok {
return actual, loaded
}
}
m.mu.Lock()
read = m.loadReadOnly()
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 := e.p.Load()
if p == expunged {
return nil, false, false
}
if p != nil {
return *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 e.p.CompareAndSwap(nil, &ic) {
return i, false, true
}
p = e.p.Load()
if p == expunged {
return nil, false, false
}
if p != nil {
return *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) {
read := m.loadReadOnly()
e, ok := read.m[key]
if !ok && read.amended {
m.mu.Lock()
read = m.loadReadOnly()
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
}
// Delete deletes the value for a key.
func (m *Map) Delete(key any) {
m.LoadAndDelete(key)
}
func (e *entry) delete() (value any, ok bool) {
for {
p := e.p.Load()
if p == nil || p == expunged {
return nil, false
}
if e.p.CompareAndSwap(p, nil) {
return *p, true
}
}
}
// trySwap swaps a value if the entry has not been expunged.
//
// If the entry is expunged, trySwap returns false and leaves the entry
// unchanged.
func (e *entry) trySwap(i *any) (*any, bool) {
for {
p := e.p.Load()
if p == expunged {
return nil, false
}
if e.p.CompareAndSwap(p, i) {
return p, true
}
}
}
// Swap swaps the value for a key and returns the previous value if any.
// The loaded result reports whether the key was present.
func (m *Map) Swap(key, value any) (previous any, loaded bool) {
read := m.loadReadOnly()
if e, ok := read.m[key]; ok {
if v, ok := e.trySwap(&value); ok {
if v == nil {
return nil, false
}
return *v, true
}
}
m.mu.Lock()
read = m.loadReadOnly()
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
}
if v := e.swapLocked(&value); v != nil {
loaded = true
previous = *v
}
} else if e, ok := m.dirty[key]; ok {
if v := e.swapLocked(&value); v != nil {
loaded = true
previous = *v
}
} 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()
return previous, loaded
}
// CompareAndSwap swaps the old and new values for key
// if the value stored in the map is equal to old.
// The old value must be of a comparable type.
func (m *Map) CompareAndSwap(key, old, new any) (swapped bool) {
read := m.loadReadOnly()
if e, ok := read.m[key]; ok {
return e.tryCompareAndSwap(old, new)
} else if !read.amended {
return false // No existing value for key.
}
m.mu.Lock()
defer m.mu.Unlock()
read = m.loadReadOnly()
swapped = false
if e, ok := read.m[key]; ok {
swapped = e.tryCompareAndSwap(old, new)
} else if e, ok := m.dirty[key]; ok {
swapped = e.tryCompareAndSwap(old, new)
// We needed to lock mu in order to load the entry for key,
// and the operation didn't change the set of keys in the map
// (so it would be made more efficient by promoting the dirty
// map to read-only).
// Count it as a miss so that we will eventually switch to the
// more efficient steady state.
m.missLocked()
}
return swapped
}
// CompareAndDelete deletes the entry for key if its value is equal to old.
// The old value must be of a comparable type.
//
// If there is no current value for key in the map, CompareAndDelete
// returns false (even if the old value is the nil interface value).
func (m *Map) CompareAndDelete(key, old any) (deleted bool) {
read := m.loadReadOnly()
e, ok := read.m[key]
if !ok && read.amended {
m.mu.Lock()
read = m.loadReadOnly()
e, ok = read.m[key]
if !ok && read.amended {
e, ok = m.dirty[key]
// Don't delete key from m.dirty: we still need to do the “compare” part
// of the operation. The entry will eventually be expunged when the
// dirty map is promoted to the read map.
//
// 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()
}
for ok {
p := e.p.Load()
if p == nil || p == expunged || *p != old {
return false
}
if e.p.CompareAndSwap(p, nil) {
return true
}
}
return false
}
// 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.loadReadOnly()
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.loadReadOnly()
if read.amended {
read = readOnly{m: m.dirty}
copyRead := read
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() {
m.misses++
if m.misses < len(m.dirty) {
return
}
m.read.Store(&readOnly{m: m.dirty})
m.dirty = nil
m.misses = 0
}
func (m *Map) dirtyLocked() {
if m.dirty != nil {
return
}
read := m.loadReadOnly()
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 := e.p.Load()
for p == nil {
if e.p.CompareAndSwap(nil, expunged) {
return true
}
p = e.p.Load()
}
return p == expunged
}
結構
type Map struct {
mu Mutex // 讀寫dirty上的鎖
read atomic.Value // readOnly
dirty map[interface{}]*entry
misses int // 穿透次數
}
type readOnly struct {
m map[interface{}]*entry
amended bool // 這個值標誌著read和dirty是否完全相同，不完全相同則為true
}
type entry struct {
p unsafe.Pointer // *interface{}
}
請注意，read和dirty都是指標型別，並且經過實驗，我們可以確定read和dirty中相同的鍵值對實際上是指向了同一塊地址，所以map中並不是儲存了兩份資料。
讀取鍵值
func (m *Map) Load(key interface{}) (value interface{}, ok bool) {
// 首先嚐試從 read 中讀取
read, _ := m.read.Load().(readOnly)
e, ok := read.m[key]
// read中如果不存在且read和dirty不完全相同，則嘗試從 dirty 中獲取
if !ok && read.amended {
m.mu.Lock()
// 由於上面 read 獲取沒有加鎖，為了安全再檢查一次
read, _ = m.read.Load().(readOnly)
e, ok = read.m[key]
// 確實不存在則從 dirty 獲取
if !ok && read.amended {
// 注意這裡的=不是:=說明這個e,ok不是新的，而是複用舊的，可以傳出去的
e, ok = m.dirty[key]
// read穿透，統計穿透次數
m.missLocked()
}
m.mu.Unlock()
}
// dity中也沒有
if !ok {
return nil, false
}
// 從 entry.p 讀取值
return e.load()
}
func (m *Map) missLocked() {
m.misses++ // 穿透次數增加
if m.misses < len(m.dirty) { // 穿透次數還可以接受
return
}
// 當 miss 積累過多，會將 dirty 存入 read，然後 將 amended = false，且 m.dirty = nil
m.read.Store(readOnly{m: m.dirty}) // dirty成為新的read
m.dirty = nil
m.misses = 0
}
新增或更改鍵值
func (m *Map) Store(key, value interface{}) {
read, _ := m.read.Load().(readOnly)
// 如果 read 裡存在，則嘗試直接修改read
// 如果read中的這個值其實被刪除，那就不要改read了
if e, ok := read.m[key]; ok && e.tryStore(&value) {
return
}
// 如果上一步沒執行成功，則要分情況處理
m.mu.Lock()
read, _ = m.read.Load().(readOnly)
// 和 Load 一樣，重新從 read 獲取一次
if e, ok := read.m[key]; ok {
// 情況 1：read 裡存在
if e.unexpungeLocked() {
// 如果 p == expunged，則需要先將 entry 賦值給 dirty（因為 expunged 資料不會留在 dirty）
m.dirty[key] = e
}
// 用值更新 entry
e.storeLocked(&value)
} else if e, ok := m.dirty[key]; ok {
// 情況 2：read 裡不存在，但 dirty 裡存在，則用值更新 entry
e.storeLocked(&value)
} else {
// 情況 3：read 和 dirty 裡都不存在
if !read.amended {
// 如果 amended == false，則呼叫 dirtyLocked 將 read 複製到 dirty（除了被標記刪除的資料）
m.dirtyLocked()
// 當我們新增一個在read和dirty中都不存在的鍵值時，會先把read中的nil資料先設定成哨兵，然後將read中的不是nil也不是哨兵的資料新增到dirty中
// 然後將 amended 改為 true
m.read.Store(readOnly{m: read.m, amended: true})
}
// 將新的鍵值存入 dirty
m.dirty[key] = newEntry(value)
}
m.mu.Unlock()
}
func (e *entry) tryStore(i *interface{}) bool {
for {
p := atomic.LoadPointer(&e.p)
if p == expunged { // 這個值其實已經被刪除
return false
}
// 這個值仍未被刪除
if atomic.CompareAndSwapPointer(&e.p, p, unsafe.Pointer(i)) {
return true
}
}
}
func (e *entry) unexpungeLocked() (wasExpunged bool) {
return atomic.CompareAndSwapPointer(&e.p, expunged, nil)
}
func (e *entry) storeLocked(i *interface{}) {
atomic.StorePointer(&e.p, unsafe.Pointer(i))
}
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 {
// 判斷 entry 是否被刪除，否則就存到 dirty 中
if !e.tryExpungeLocked() {
m.dirty[k] = e
}
}
}
func (e *entry) tryExpungeLocked() (isExpunged bool) {
p := atomic.LoadPointer(&e.p)
for p == nil {
// 如果有 p == nil（即鍵值對被 delete），則會在這個時機被置為 expunged
if atomic.CompareAndSwapPointer(&e.p, nil, expunged) {
return true
}
p = atomic.LoadPointer(&e.p)
}
return p == expunged
}
刪除鍵值
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()
}
}
// Delete deletes the value for a key.
func (m *Map) Delete(key any) {
m.LoadAndDelete(key)
}
func (e *entry) delete() (hadValue bool) {
for {
p := atomic.LoadPointer(&e.p)
// 已標記為刪除
if p == nil || p == expunged {
return false
}
// 原子操作，e.p標記為nil
if atomic.CompareAndSwapPointer(&e.p, p, nil) {
return true
}
}
}
流程
問題
問題1：為什麼sync.map要有nil和expunged兩個刪除標記，全用nil不可以嗎？
我猜想有可能是因為想要去分兩種情況，一種是有這read中有這個鍵值對，但是被刪除了，另一種情況是，根本就沒有過這個鍵值對，這兩種情況做不同的處理。
問題2：如果store只修改了read中的資料，而沒有修改dirty中的資料，當我們下一此promote的時候dirty中的舊資料不會覆蓋read中的新資料嗎？
例如，我先存幾個資料，然後連續miss讓資料全部上到read，清空dirty，現在存一個新的值到dirty，會把read也複製下來，這樣，dirty和read會有重合的資料，現在只更新read，然後連續miss，讓dirty替換read，read中不就出現髒資料了嗎？然而我做了實驗，並不會這樣。
package main
import (
"fmt"
"sync"
)
func main() {
// 建立一個 sync.Map
var myMap sync.Map
// 向 sync.Map 中新增鍵值對
myMap.Store("key1", "value1")
myMap.Store("key2", "value2")
myMap.Store("key3", "value3")
// 從 sync.Map 中獲取值
value, ok := myMap.Load("key5")
value, ok = myMap.Load("key5")
value, ok = myMap.Load("key5")
myMap.Store("key4", "value4")
myMap.Store("key1", "modify")
value, ok = myMap.Load("key5")
value, ok = myMap.Load("key5")
value, ok = myMap.Load("key5")
value, ok = myMap.Load("key1")
if ok {
fmt.Println("Found value for key1:", value)
} else {
fmt.Println("Value not found for key1")
}
}
初步判斷和amended有關