Go实现map并发安全的三种方式

实现map并发读写线程安全

1. 加锁

对整个map加上读写锁sync.RWMutex

//keyType为key的类型，valueType为value的类型
type RWMap struct {
	Map map[keyType]valueType
	sync.RWMutex
}

func NewRWMap(capacity int) *RWMap {
	if capacity < 0 {
		capacity = 0
	}
	return &RWMap{
		Map: make(map[keyType]valueType, capacity),
	}
}

//add or update
func (m *RWMap) Set(key keyType, value valueType) {
	m.Lock()
	defer m.Unlock()
	m.Map[key] = value
}

//delete
func (m *RWMap) Delete(key int)  {
	m.Lock()
	defer m.Unlock()
	delete(m.Map, key)
}
//get
func (m *RWMap) Get(key int) valueType {
	m.RLock()
	defer m.RUnlock()
	return m.Map[key]
}

优点：解决了问题。
缺点：锁粒度大。

2. 分片加锁

一个操作会导致整个map被锁住，导致性能降低。所以提出了分片思想，将一个map分成几个片，按片加锁。
第三方包实现：github.com/orcaman/concurrent-map
github上用 map language:go 搜索：

3.4kstar

插曲：注意，如果你的goland ide 版本太老的话，github.com/orcaman/concurrent-map/v2 版本是用不了的:

所以我最后换成VSCode，发现就没这个问题了。（因为新版本的GoLand还得继续想法子破解）

源码New方法返回的map，看到key只支持string

// Creates a new concurrent map.
func New[V any]() ConcurrentMap[string, V] {
	return create[string, V](fnv32)
}

Example and usage

package main

import (
	"fmt"
	"time"

	cmap "github.com/orcaman/concurrent-map/v2"
)

func main() {
	m := cmap.New[int]()
	for i := 0; i < 300; i   {
		go func(i int) {
			m.Set(fmt.Sprintf("%v", i), i*2)  //并发写
		}(i)
	}
	time.Sleep(4 * time.Second)
	fmt.Println(len(m.Keys()))
}

execute and output:

PS C:\GoWork\src\asset-manager\mytest> go run main.go
300

并发写没问题。
更多使用示例包里的concurrent_map_test.go里面提供了。

3. sync.Map

标准库中的 sync.Map是专为 append-only 场景设计的。

sync.Map在读多写少性能比较好，否则并发性能很差。

Go源码：

// 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. 

//=====自注释======
sync.Map 很像Go map[interface{}]interface{}。但sync.Map是线程安全的，能被多个协程在没有额外的锁或者协调的情况下并发使用。
Loads,stores，deletes操作都运行在分摊常数时间内。
amortized 平摊的（adj.)英  /əˈmɔːtaɪzd/
//=====自注释======


//
// 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.

//=====自注释======
invariants (n.) 不变量（invariant的复数）/ɪnˈveriənts/
sync.Map 类型是为特殊情况专门设计的。
大多数代码都应该使用普通的Go map   单独的锁或者协调 ，这种形式，来获得更好的类型安全
以及使得在维护映射内容的同时维护其他不变量更容易。
//=====自注释======


//
// 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.

//=====自注释======
disjoint (adj.) 不连贯的，（两个集合）不相交的 /dɪsˈdʒɔɪnt/
sync.Map类型针对两个常见用例进行了优化:
(1)对于一个给定的key，只会写一次，但是读很多次，就像在只增长的缓存中一样。
(2)当多个协程读，写，重写不相交的keys。
以上两种情况，相比于使用Go map   Mutex（或者RWMutex），使用sync.Map能显著减少锁竞争。
//=====自注释======
//

// The zero Map is empty and ready for use. A Map must not be copied after first use.
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.

//=====自注释======
expunged (adj.)/ɪkˈspʌndʒ/ 被擦去的，被删掉的
dirty map 包含map内容的部分，该部分要求持有mu锁。为了确保dirty map能快速提升到read map，
它还包括read map 中所有未删除的项。
//=====自注释======

	//
	// 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
}

read atomic.Value

sync/stomic包里都是go提供的原子操作。

sync.Map思想：就是用两个数据结构（只读的 read 和可写的 dirty）尽量将读写操作分开，并最小粒度加锁，来减少锁对性能的影响。

总结

较常使用的是前两种：加读写锁和分片加锁。特定场景下sync.Map性能会有更优的表现（要满足那两个场景条件比较苛刻，实际很少用）。

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