“部分订购"；和发生前关系java [英] "Partial Ordering" and Happens-before relation java

问题描述

我正在阅读 Java并发实践

我对事前发生关系的具体解释感到困惑.

它表明，

操作是通过偏序进行排序的，该事件称为事前发生

部分订购" 到底是什么意思?

(书中有一个解释，但我不清楚)

解决方案

部分排序意味着并非每对操作都具有关系happens-before.

实际上，并非每对操作都具有这种关系这一事实使您可以同时执行操作.

例如，假设您有操作A，B，C和A； D.

我们可以定义部分排序:A must happen before B and C.

然后，A和B以及A和C具有happens-before关系. 但是，A和D没有这种关系，因此D可以在A之前，A之后或在A执行期间执行.

另一方面，如果happens-before是完整顺序，例如A happens-before B happens-before C happens-before D(请注意，在这种情况下，对于每一对操作，您都知道哪个发生在另一个操作之前，因此它是完整的排序)，那么这些操作必须是串行执行的，并且并发是不可能的.

I am reading Java Concurrency in Practice

I am confused with the specific explanation regarding happens-before relationship.

It states that,

operations are ordered by a partial ordering called happens-before

What exactly does this mean by "partial ordering"?

(There is an explanation in the book but its not clear to me )

解决方案

Partial Ordering means that not every pair of operations has the relation happens-before.

Actually, the fact that not every pair of operations has that relation enables you to perform operations concurrently.

For example, suppose you have operations A, B, C & D.

We can define a partial ordering: A must happen before B and C.

Then A and B have the happens-before relation, as do A and C. However, A and D don't have that relation, so D can be executed either before A, after A or while A is being executed.

If, on the other hand, happens-before was a full ordering, such as A happens-before B happens-before C happens-before D (note that in this case, for each pair of operations you know which one happens-before the other, hence it is a full ordering), then the execution of the operations would have to be serial, and no concurrency would be possible.

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