ForkJoinPool stalls during invokeAll/join

Question

I try to use a ForkJoinPool to parallelize my CPU intensive calculations. My understanding of a ForkJoinPool is, that it continues to work as long as any task is available

Answer 1

You’re not using this framework for the very narrow purpose for which it was intended.

The framework started life as the experiment in the 2000 research paper. It’s been modified since then but the basic design, fork-and-join on large arrays, remains the same. The basic purpose is to teach undergraduates how to walk down the leaves of a balanced tree. When people use it for other than simple array-processing weird things happen. What it is doing in Java7 is beyond me; which is the purpose of the article.

The problems only get worse in Java8. There it’s the engine to drive all stream parallel work. Have a read in part two of that article. The lambda interest lists are filled with reports of thread stalls, stack overflow, and out of memory errors.

You use it at your own risk when you don’t use it for pure recursive decomposition of large data structures. Even then, the excessive threads it creates can cause havoc. I’m not going to pursue this discussion any further.