[concurrency-interest] ForkJoinPool cap on number of threads

Viktor Klang viktor.klang at gmail.com
Wed Mar 25 05:06:54 EDT 2015

Hi Luke,

the newest version of FJP has a tighter bound on max number of threads
(search for maxSpares here:

Another way of limiting is to have the ThreadFactory return null when no
more threads should be created.

On Tue, Mar 24, 2015 at 8:35 PM, Luke Sandberg <lukeisandberg at gmail.com>

> Hi,
> I'm trying to use ForkJoinPool and I am concerned that the maximum number
> of threads it will create is capped at 32767.  Whereas I would like to set
> a much lower cap (4-16).
> You can configure the FJP with a target parallelism but i don't know if it
> is possible to actually put a cap on the max parallelism.
> In my specific case i will just be using it as an ExecutorService
> (basically execute() and shutdown()) with the 'asyncMode' bit set to
> 'true', and will _not_ be using the Fork/Join framework.  (I want to see if
> my application can get better performanced via decreased contention and
> better cache locality).  Based on my reading of the docs i think this means
> that it will not create any compensating threads since i would not be using
> the managed blocking facilities.
> Does anyone have any experience with using this in production?  I am
> concerned that 'compensating' threads could cause my servers to run out of
> memory for thread stacks (at 1MB a piece they aren't cheap)' and my
> applications run in a shared hosting environment with strict caps on per
> process ram usage (If i go over my reservation, my server will be killed).
> Currently, we manage this with ThreadPoolExecutor by exposing our maxSizes
> to our configuration and using that to set reservations.
> Thanks,
> Luke
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