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golang sync map

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Symbols count in article: 13k Reading time ≈ 12 mins.

struct

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// 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.
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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[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.
	misses int
}

map

Load()

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// 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 interface{}) (value interface{}, ok bool) {
	read, _ := m.read.Load().(readOnly)
	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.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()
}

note

  • 先从read查，查到就直接返回。查不到并且read.amended=true(dirty!=read)再去dirty中查，查dirty要加互斥锁。
  • 这里用到了两次查询，参考单例的设计。
  • read是atomic.value类型的，atomic.value的Load/Store是线程安全的，所以不用加锁，读性能比较好。但是其实也用到了cas,所以也用到了lock指令，不是完全无锁定的。

demo

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package main

import (
	"fmt"
	"sync"
	"sync/atomic"
)

type Map struct {
	mu     sync.Mutex
	read   atomic.Value // readOnly
	dirty  map[interface{}]*int
	misses int
}

type readOnly struct {
	m       map[interface{}]*int
	amended bool // true if the dirty map contains some key not in m.
}

func main() {
	m := Map{}
	//fmt.Println(m.read.Load().(readOnly))		//panic
	fmt.Println(m.read.Load()) //<nil>

	var v interface{}
	read, _ := (v).(readOnly)
	fmt.Println(read)	//{map[] false}
}

Store()

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// Store sets the value for a key.
func (m *Map) Store(key, value interface{}) {
	read, _ := m.read.Load().(readOnly)
	//先从read读，读成功再尝试store更新
	//tryStore先load,如果标记为expunged则ret false, ow store
	if e, ok := read.m[key]; ok && e.tryStore(&value) {
		return
	}

	//read读不到则store到dirty，要加锁
	m.mu.Lock()
	read, _ = m.read.Load().(readOnly)
	//两次读，第2次读到了
	if e, ok := read.m[key]; ok {
		//unexpungeLocked：如果e.p=expunged，则set e.p=nil
		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
		}
		//atomic.StorePointer 更新entry
		e.storeLocked(&value)
	//dirty中有，则更新
	} else if e, ok := m.dirty[key]; ok {
		e.storeLocked(&value)
	//read/dirty都没有
	//新增数据
	} else {
		//并且read.amended=0
		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.
			//如果dirty=nil,则把read(未删除的)拷贝给dirty
			//初始化后第一次store会进入这里，或者misscount>=len(dirty)也会进来重新刷新dirty.
			m.dirtyLocked()
			//更新了amended
			m.read.Store(readOnly{m: read.m, amended: true})
		}
		//新增entry并加到dirty
		m.dirty[key] = newEntry(value)
	}
	m.mu.Unlock()
}

note

  • 任何值可以赋值给空接口的值，空接口的值也可以赋值给任何值
  • 第一次新增和misscount达到限制都会刷新dirty(吧read赋值给dirty,如果read为空，这一步就提前ret)
  • store包含了更新和新增
  • 这里加锁是给m.dirty加锁，加锁是内核的调度器执行的主要用于大的复合对象/临界区，原子操作是硬件指令主要针对单个值
  • (新增元素或者刷新dirty的时候)tryExpungeLocked将nil的标记为已删除expunged，新增entry的时候read如果能读到，unexpungeLocked对于expunged又改为nil并更新dirty

demo

missLocked

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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
}

LoadOrStore

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// 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.
	read, _ := m.read.Load().(readOnly)
	//如果read里面有
	if e, ok := read.m[key]; ok {
		//tryLoadOrStore逻辑：
		//tryLoadOrStore:先load
		//if e.p=expunged, ret nil, false, false
		//if e.p=nil, 返回原来的值，true, true
		//else store，return value, false, true
		actual, loaded, ok := e.tryLoadOrStore(value)
		if ok {
			return actual, loaded
		}
		//如果read读到了，并且e.p=expunged,则往下执行，走dirty
	}

	m.mu.Lock()
	read, _ = m.read.Load().(readOnly)
	//如果从read读到
	if e, ok := read.m[key]; ok {
		//unexpungeLocked: return atomic.CompareAndSwapPointer(&e.p, expunged, nil)
		if e.unexpungeLocked() {
		//If the entry was previously expunged, it must be added to the dirty map
		//before m.mu is unlocked.
			m.dirty[key] = e
		}
		actual, loaded, _ = e.tryLoadOrStore(value)
	//如果从dirty读到
	} 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
}

note

load和store的结合体，不赘述

Delete

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// Delete deletes the value for a key.
func (m *Map) Delete(key interface{}) {
	m.LoadAndDelete(key)
}

LoadAndDelete

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// 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 interface{}) (value interface{}, loaded bool) {
	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]
			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 {
		//if e.p=nil/expunged,ret
		//else set e.p = nil
		return e.delete()
	}
	return nil, false
}

note

  • 如果read中找到了，直接在read标记删除，否则才去找dirty
  • read的删除，对于nil/expunged直接ret,不然标记为nil
  • dirty的删除，才是真的删除
  • 删除dirty也会使misscount+1

Range

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// 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)
	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 {
			//如果dirty有新数据，则先同步到read在range
			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
		}
	}
}

dirtyLocked

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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
		}
	}
}

entry

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// 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 unsafe.Pointer // *interface{}
}

tryStore

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// 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 {
	for {
		p := atomic.LoadPointer(&e.p)
		if p == expunged {
			return false
		}
		if atomic.CompareAndSwapPointer(&e.p, p, unsafe.Pointer(i)) {
			return true
		}
	}
}

unexpungeLocked

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// 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

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

tryLoadOrStore

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// 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 interface{}) (actual interface{}, loaded, ok bool) {
	p := atomic.LoadPointer(&e.p)
	if p == expunged {
		return nil, false, false
	}
	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.
	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

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func (e *entry) delete() (value interface{}, ok bool) {
	for {
		p := atomic.LoadPointer(&e.p)
		if p == nil || p == expunged {
			return nil, false
		}
		if atomic.CompareAndSwapPointer(&e.p, p, nil) {
			return *(*interface{})(p), true
		}
	}
}

tryExpungeLocked

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func (e *entry) tryExpungeLocked() (isExpunged bool) {
	p := atomic.LoadPointer(&e.p)
	for p == nil {
		if atomic.CompareAndSwapPointer(&e.p, nil, expunged) {
			return true
		}
		p = atomic.LoadPointer(&e.p)
	}
	return p == expunged
}

总结

  • misscount达到阈值，将dirty刷回read,set dirty=nil
  • 新增元素的时候(区别与修改)，如果amend=false,则将read刷回dirty

reference

[1]https://colobu.com/2017/07/11/dive-into-sync-Map/

[2]https://draveness.me/golang/docs/part2-foundation/ch03-datastructure/golang-hashmap/

[3]https://juejin.im/post/6844903895227957262

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