Violence isn’t the answer.

We want to be able to stop a running thread or task when we discover that we no longer need or want to finish it. As we saw in the last two columns, in a simple parallel search we can stop other workers once one finds a match, and when speculatively running two alternative algorithms to compute the same result we can stop the longer-running one once the first finds a result. [1,2] Stopping threads or tasks lets us reclaim their resources, including locks, and apply them to other work.

But how do you stop a thread or task you longer need or want? Table 1 summarizes the four main ways, and how they are supported on several major platforms. Let’s consider them in turn. …