[concurrency-interest] ForkJoinPool not designed for nested Java 8streams.parallel().forEach( ... )

Aleksey Shipilev aleksey.shipilev at oracle.com
Mon May 5 17:04:40 EDT 2014

On 05/06/2014 12:40 AM, Christian Fries wrote:
> It appears to me as if nested Java 8 parallel streams will create a
> very subtle „implicit coupling“ between the tasks submitted to the
> common pool, because tasks of an inner loop are joined with tasks on
> an outer loop.

What coupling are you talking about now? In my mental model, starting
the parallel() computation from the parallel() computation is "just"
invoking the FJTask from the already executing one, and that is just
forking-joining on a new task. Joining on inner FJTask will assist
executing it. Which means that FJThreads executing the outer loop will
assist executing the inner loop if they are "joining" on inner loop

> So maybe the problem is to have a common FJP at all?
> I still have the impression that the line ((t =
> Thread.currentThread()) instanceof ForkJoinWorkerThread) is not
> correct in the case of a nested loop.

Why? It checks if we are within the FJP and may proceed to awaitJoin to
*help* other tasks.


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