“在单核多线程环境中需要使用 VOLATILE 吗?”

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英文:

VOLATILE required in single core multi Threaded environment?

问题

我需要使用VOLATILE变量吗,如果我在单核机器上,而且是多线程环境?

根据我的理解,线程没有自己的缓存空间。

它们使用所在核心的缓存。

如果是单核机器,它们将共享相同的缓存。

请纠正/补充我的理解。

英文:

Do I need to use VOLATILE variable, if I am on a single core machine & multi Threaded environment

From my understanding, Threads do not have a cache space of their own.

They use the cache of the core they are running in.

Now if it is a single core machine, they will all be sharing the same cache.

Please correct/addMore over my understanding

答案1

得分: 3

编译器也可以重新排列指令。

例如:

class SomeRunnable implements Runnable {
  public boolean stop;

  public void run() {
    while (!stop)
      println("hello");
  }
}

由于stop变量没有标记为volatile(或者没有在同步块中使用),编译器可以自由地将上述代码转换为:

class SomeRunnable implements Runnable {
  public boolean stop;

  public void run() {
    if (stop) return;
    for (;;) {
      println("hello");
    }
  }
}

这种优化称为“循环不变代码移动”。之所以进行这种优化是为了减少读取stop的开销;如果它在循环中不会改变,那么读取它有什么意义。

最好不要考虑缓存,因为:

  • 除非你知道你在谈论什么,否则你可能没有正确理解
  • 它取决于CPU架构。

如果你想理解Java并发,那么你需要了解Java内存模型。

英文:

The compiler can also reorder instructions.

For example:

class SomeRunnable implements Runnable{
  public boolean stop;

  public void run(){
     while(!stop)
         println("hello");
  }
}

Since the stop variable isn't marked as volatile (or used in a synchronized block), the compiler is free to transform the above code to:

class SomeRunnable implements Runnable{
  public boolean stop;

  public void run(){
     if(stop)return;
     for(;;)
         println("hello");
  }
}

This optimization is called 'loop invariant code motion'. The reason this optimization is done is to reduce the overhead of reading 'stop'; what is the point of reading it, if it doesn't change in the loop.

It is best not to think in terms of caches because:

  • unless you know what you are talking about, probably you don't understand it correctly
  • it depends on the CPU architecture.

If you want to understand Java concurrent, then you need to understand the Java Memory Model.

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  • 本文由 发表于 2020年8月1日 15:51:28
  • 转载请务必保留本文链接:https://go.coder-hub.com/63202993.html
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