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2023-06-26 17:30:52 +00:00
< ol class = "chapter" > < li class = "chapter-item expanded affix " > < a href = "../index.html" > Go语言圣经< / a > < / li > < li class = "chapter-item expanded affix " > < a href = "../preface-zh.html" > 译者序< / a > < / li > < li class = "chapter-item expanded affix " > < a href = "../preface.html" > 前言< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1.html" > < strong aria-hidden = "true" > 1.< / strong > 入门< / a > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-01.html" > < strong aria-hidden = "true" > 1.1.< / strong > Hello, World< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-02.html" > < strong aria-hidden = "true" > 1.2.< / strong > 命令行参数< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-03.html" > < strong aria-hidden = "true" > 1.3.< / strong > 查找重复的行< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-04.html" > < strong aria-hidden = "true" > 1.4.< / strong > GIF动画< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-05.html" > < strong aria-hidden = "true" > 1.5.< / strong > 获取URL< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-06.html" > < strong aria-hidden = "true" > 1.6.< / strong > 并发获取多个URL< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-07.html" > < strong aria-hidden = "true" > 1.7.< / strong > Web服务< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-08.html" > < strong aria-hidden = "true" > 1.8.< / strong > 本章要点< / a > < / li > < / ol > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2.html" > < strong aria-hidden = "true" > 2.< / strong > 程序结构< / a > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-01.html" > < strong aria-hidden = "true" > 2.1.< / strong > 命名< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-02.html" > < strong aria-hidden = "true" > 2.2.< / strong > 声明< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-03.html" > < strong aria-hidden = "true" > 2.3.< / strong > 变量< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-04.html" > < strong aria-hidden = "true" > 2.4.< / strong > 赋值< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-05.html" > < strong aria-hidden = "true" > 2.5.< / strong > 类型< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-06.html" > < strong aria-hidden = "true" > 2.6.< / strong > 包和文件< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-07.html" > < strong aria-hidden = "true" > 2.7.< / strong > 作用域< / a > < / li > < / ol > < / li > < li class = "chapter-item expanded " > < a href = "../ch3/ch3.html" > < strong aria-hidden = "true" > 3.< / strong > 基础数据类型< / a > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../ch3/ch3-01.html" > < strong aria-hidden = "true" > 3.1.< / strong > 整型< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch3/ch3-02.html" > < strong aria-hidden = "true" > 3.2.< / strong > 浮点数< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch3/ch3-03.html" > < strong aria-hidden = "true" > 3.3.< / strong > 复数< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch3/ch3-04.html" > < strong aria-hidden = "true" > 3.4.< / strong > 布尔型< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch3/ch3-05.html" > < strong aria-hidden = "true" > 3.5.< / strong > 字符串< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch3/ch3-06.html" > < strong aria-hidden = "true" > 3.6.< / strong > 常量< / a > < / li > < / ol > < / li > < li class = "chapter-item expanded " > < a href = "../ch4/ch4.html" > < strong aria-hidden = "true" > 4.< / strong > 复合数据类型< / a > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../ch4/ch4-01.html" > < strong aria-hidden = "true" > 4.1.< / strong > 数组< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch4/ch4-02.html" > < strong aria-hidden = "true" > 4.2.< / strong > Slice< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch4/ch4-03.html" > < strong aria-hidden = "true" > 4.3.< / strong > Map< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch4/ch4-04.html" > < strong aria-hidden = "true" > 4.4.< / strong > 结构体< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch4/ch4-05.html" > < strong aria-hidden = "true" > 4.5.< / strong > JSON< /
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2023-06-26 17:46:27 +00:00
< li > < em > 凹语言(专为 WebAssembly 设计): < a href = "https://github.com/wa-lang/wa" > https://github.com/wa-lang/wa< / a > < / em > < / li >
< li > < em > KCL 配置语言(Rust): < a href = "https://github.com/kcl-lang/kcl" > https://github.com/kcl-lang/kcl< / a > < / em > < / li >
< / ul >
2022-08-04 06:59:33 +00:00
< hr >
< 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(" y:" , y, " " ) // A2
}()
go func() {
y = 1 // B1
fmt.Print(" x:" , x, " " ) // 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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