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<li><em>Go语言圣经读书笔记(挖坑中...): <a href="https://github.com/chai2010/gopl-notes-zh">https://github.com/chai2010/gopl-notes-zh</a></em></li>
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<h2 id="94-内存同步"><a class="header" href="#94-内存同步">9.4. 内存同步</a></h2>
<p>你可能比较纠结为什么Balance方法需要用到互斥条件无论是基于channel还是基于互斥量。毕竟和存款不一样它只由一个简单的操作组成所以不会碰到其它goroutine在其执行“期间”执行其它逻辑的风险。这里使用mutex有两方面考虑。第一Balance不会在其它操作比如Withdraw“中间”执行。第二更重要的是“同步”不仅仅是一堆goroutine执行顺序的问题同样也会涉及到内存的问题。</p>
<p>在现代计算机中可能会有一堆处理器每一个都会有其本地缓存local cache。为了效率对内存的写入一般会在每一个处理器中缓冲并在必要时一起flush到主存。这种情况下这些数据可能会以与当初goroutine写入顺序不同的顺序被提交到主存。像channel通信或者互斥量操作这样的原语会使处理器将其聚集的写入flush并commit这样goroutine在某个时间点上的执行结果才能被其它处理器上运行的goroutine得到。</p>
<p>考虑一下下面代码片段的可能输出:</p>
<pre><code class="language-go">var x, y int
go func() {
x = 1 // A1
fmt.Print(&quot;y:&quot;, y, &quot; &quot;) // A2
}()
go func() {
y = 1 // B1
fmt.Print(&quot;x:&quot;, x, &quot; &quot;) // B2
}()
</code></pre>
<p>因为两个goroutine是并发执行并且访问共享变量时也没有互斥会有数据竞争所以程序的运行结果没法预测的话也请不要惊讶。我们可能希望它能够打印出下面这四种结果中的一种相当于几种不同的交错执行时的情况</p>
<pre><code>y:0 x:1
x:0 y:1
x:1 y:1
y:1 x:1
</code></pre>
<p>第四行可以被解释为执行顺序A1,B1,A2,B2或者B1,A1,A2,B2的执行结果。然而实际运行时还是有些情况让我们有点惊讶</p>
<pre><code>x:0 y:0
y:0 x:0
</code></pre>
<p>根据所使用的编译器CPU或者其它很多影响因子这两种情况也是有可能发生的。那么这两种情况要怎么解释呢</p>
<p>在一个独立的goroutine中每一个语句的执行顺序是可以被保证的也就是说goroutine内顺序是连贯的。但是在不使用channel且不使用mutex这样的显式同步操作时我们就没法保证事件在不同的goroutine中看到的执行顺序是一致的了。尽管goroutine A中一定需要观察到x=1执行成功之后才会去读取y但它没法确保自己观察得到goroutine B中对y的写入所以A还可能会打印出y的一个旧版的值。</p>
<p>尽管去理解并发的一种尝试是去将其运行理解为不同goroutine语句的交错执行但看看上面的例子这已经不是现代的编译器和cpu的工作方式了。因为赋值和打印指向不同的变量编译器可能会断定两条语句的顺序不会影响执行结果并且会交换两个语句的执行顺序。如果两个goroutine在不同的CPU上执行每一个核心有自己的缓存这样一个goroutine的写入对于其它goroutine的Print在主存同步之前就是不可见的了。</p>
<p>所有并发的问题都可以用一致的、简单的既定的模式来规避。所以可能的话将变量限定在goroutine内部如果是多个goroutine都需要访问的变量使用互斥条件来访问。</p>
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