【go 源码】sync.Atomic 详解

xmge · · 444 次点击 · 开始浏览    置顶
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# sync.Atomic 源码阅读 ## 1.Demo ```go package main import ( "fmt" "time" "sync" "sync/atomic" ) func main() { test1() test2() } // count++ 并发不安全 func test1() { var wg sync.WaitGroup count := 0 t := time.Now() for i := 0 ; i < 100000 ; i++ { wg.Add(1) go func(i int) { count++ //count不是并发安全的 wg.Done() }(i) } wg.Wait() fmt.Printf("test1 花费时间:%d, count的值为:%d \n",time.Now().Sub(t),count) } // atomic.AddInt64(&count,1) //原子操作 func test2() { var wg sync.WaitGroup count := int64(0) t := time.Now() for i := 0 ; i < 100000 ; i++ { wg.Add(1) go func(i int) { atomic.AddInt64(&count,1) //原子操作 wg.Done() }(i) } wg.Wait() fmt.Printf("test2 花费时间:%d, count的值为:%d \n",time.Now().Sub(t),count) } ``` 结果: ``` 28.922ms count====> 93095 exit 27.9249ms count====> 100000 exit ``` ## 2.介绍 我们调用sync/atomic中的几个函数可以对几种简单的类型进行原子操作。这些类型包括int32,int64,uint32,uint64,uintptr,unsafe.Pointer,共6个。这些函数的原子操作共有5种:增减,存储,载入,交换,比较并交换。 sync/atomic 解决的典型问题就是 i++和CAS(Compare-and-Swap)的线程安全问题,它的实现原理大致是向CPU发送对某一个块内存的LOCK信号,然后就将此内存块加锁,从而保证了内存块操作的原子性。 与Mutex相比,它的优势主要有以下几点: 1. 更高效,因为atomic是直接作用与内存的锁,所以更底层,更高效。 2. 更简洁,atomic避免了加锁解锁的过程,一行代码就可以完成这个操作,使代码更简洁,更具有可读性。 ## 3.使用场景 sync/atomic 可以在并发场景下对变量进行非侵入式的操作。可以保证并发安全,虽然使用 `sync.Mutex` 可以实现,但是使用`sync/atomic`不仅是轻量级的,而且代码也更加简洁。 ## 4.源码 ```go // Copyright 2011 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 atomic provides low-level atomic memory primitives // useful for implementing synchronization algorithms. // // These functions require great care to be used correctly. // Except for special, low-level applications, synchronization is better // done with channels or the facilities of the sync package. // Share memory by communicating; // don't communicate by sharing memory. // // The swap operation, implemented by the SwapT functions, is the atomic // equivalent of: // // old = *addr // *addr = new // return old // // The compare-and-swap operation, implemented by the CompareAndSwapT // functions, is the atomic equivalent of: // // if *addr == old { // *addr = new // return true // } // return false // // The add operation, implemented by the AddT functions, is the atomic // equivalent of: // // *addr += delta // return *addr // // The load and store operations, implemented by the LoadT and StoreT // functions, are the atomic equivalents of "return *addr" and // "*addr = val". // package atomic import ( "unsafe" ) // BUG(rsc): On x86-32, the 64-bit functions use instructions unavailable before the Pentium MMX. // // On non-Linux ARM, the 64-bit functions use instructions unavailable before the ARMv6k core. // // On both ARM and x86-32, it is the caller's responsibility to arrange for 64-bit // alignment of 64-bit words accessed atomically. The first word in a // variable or in an allocated struct, array, or slice can be relied upon to be // 64-bit aligned. // SwapInt32 atomically stores new into *addr and returns the previous *addr value. func SwapInt32(addr *int32, new int32) (old int32) // SwapInt64 atomically stores new into *addr and returns the previous *addr value. func SwapInt64(addr *int64, new int64) (old int64) // SwapUint32 atomically stores new into *addr and returns the previous *addr value. func SwapUint32(addr *uint32, new uint32) (old uint32) // SwapUint64 atomically stores new into *addr and returns the previous *addr value. func SwapUint64(addr *uint64, new uint64) (old uint64) ... ``` ## 5.源码详解 源码中并没有go语言版本的实现,在此介绍一下 `sync/atomic` 中的方法(只介绍一种类型,其他类型都是一样的) 1.func AddInt64(addr *int64, delta int64) (new int64) 将addr增加delta(如何要减少,直接将delta为负数即可) ```go func main() { var addr int64 addr = atomic.AddInt64(&addr,1) fmt.Println(addr) } //1 ``` 2.func StoreUint64(addr *uint64, val uint64) 为addr赋值为val ```go func main() { var addr int64 atomic.StoreInt64(&addr,10) fmt.Println(addr) } ``` 3.func LoadInt64(addr *int64) (val int64) 加载addr的值 ```go func main() { var addr int64 = 10 addr = atomic.LoadInt64(&addr) fmt.Println(addr) } // 10 ``` 4.func SwapInt64(addr *int64, new int64) (old int64) 交换 将addr与new交换,并返回之前addr的值 ```go func main() { var i int64 = 10 old := atomic.SwapInt64(&i,20) fmt.Println(old) fmt.Println(i) } // 10 // 20 ``` 5.func CompareAndSwapInt64(addr *int64, old, new int64) (swapped bool) 将addr与old进行比较,想等则返回true,不相等返回false,并将addr赋值为new。 ```go func main() { var addr int64 = 2 compare := atomic.CompareAndSwapInt64(&addr,2,1) fmt.Println(compare) fmt.Println(addr) } // true // 1 ``` ---- 项目地址:github.com/xmge/gosc,更多go源码阅读文章将在公众号发布: ![img](https://gosc.oss-cn-beijing.aliyuncs.com/gosc.jpg)

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