顺序一致性和原子性之间有什么区别? [英] What is the difference between sequential consistency and atomicity?

问题描述

我读到java volatile是顺序一致的，但不是原子的. 对于原子性，java提供了不同的库.

I read that java volatile are sequential consistent but not atomic. For atomicity java provides different library.

有人能用简单的英语解释两者之间的区别吗?

Can someone explain difference between two, in simple english ?

(我认为问题范围包括C/C ++，因此添加了这些语言标签以吸引更大的受众.)

(I believe the question scope includes C/C++ and hence adding those language tags to get bigger audience.)

推荐答案

想象一下类中的这两个变量:

Imagine those two variables in a class:

int i = 0;
volatile int v = 0;

那两种方法

void write() {
    i = 5;
    v = 2;
}

void read() {
    if (v == 2) { System.out.println(i); }
}

易失性语义保证read不会打印5或什么都不打印(假设没有其他方法在修改字段).如果v不稳定，则read可能也会打印0，因为i = 5和v = 2可能已重新排序.我想这就是顺序一致性的意思，它具有更广泛的含义.

The volatile semantics guarantee that read will either print 5 or nothing (assuming no other methods are modifying the fields of course). If v were not volatile, read might as well print 0 because i = 5 and v = 2 could have been reordered. I guess that's what you mean by sequential consistency, which has a broader meaning.

另一方面，挥发物不能保证原子性.因此，如果两个线程同时调用此方法(v是相同的volatile int):

On the other hand, volatile does not guarantee atomicity. So if two threads call this method concurrently (v is the same volatile int):

void increment() {
    v++;
}

您不能保证v会增加2.这是因为v++实际上是三个语句:

you have no guarantee that v will be incremented by 2. This is because v++ is actually three statements:

load v;
increment v;
store v;

并且由于线程交织，v只能增加一次(两个线程将加载相同的值).

and because of thread interleaving v could only be incremented once (both thread will load the same value).

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