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    <p>Since nobody else seems to understand my question I try to explain what I found after some nights of debugging:</p> <p>The current implementation of ForkJoinTasks works well if all fork/join calls are strictly paired. Illustrating a fork by an opening bracket and join by a closing one a perfect binary fork join pattern may look like this: </p> <p><strong>{([][]) ([][])} {([][]) ([][])}</strong></p> <p>If you use invokeAll() you may also submit list of subtasks like this: </p> <p><strong>{([][][][]) ([][][][]) ([][][][])}</strong></p> <p>What I did however looks like this pattern: </p> <p><strong>{([) ([)} ... ]]</strong></p> <p>You may argue this looks ill or is a misuse of the fork-join framework. But the only constraint is, that <a href="http://docs.oracle.com/javase/7/docs/api/java/util/concurrent/ForkJoinTask.html" rel="nofollow">the tasks completion dependencies are acyclic</a>, else you may run into a deadlock. As long as my <strong>[]</strong> tasks are not dependent on the <strong>()</strong> tasks, I don't see any problem with it. The offending <strong>]]</strong>'s just express that I do not wait for them explicitly; they may finish some day, it does not matter to me (at that point).</p> <p>Indeed the current implementation is able to execute my interlocked tasks, but only by spawning additional helper threads which is quite inefficient.</p> <p>The flaw seems to be the current implementation of join(): joining an <strong>)</strong> expects to see its corresponding <strong>(</strong> on top of its execution queue, but it finds a <strong>[</strong> and is perplexed. Instead of simply executing <strong>[]</strong> to get rid of it, the current thread suspends (calling wait()) until someone else comes around to execute the unexpected task. This causes a drastic performance break down.</p> <p>My primary intend was to put additional work onto the queue to prevent the worker thread from suspending if the queue runs empty. Unfortunately the opposite happens :-( </p>
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