[Haskell-cafe] Significant slow-down in parallel code?

Neil Mitchell ndmitchell at gmail.com
Mon Jan 4 15:14:51 EST 2010

Hi Jamie,

First question, what version of GHC are you using? There are
significant performance improvements to parallel code in GHC 6.12, so
it's worth an upgrade. Once you've upgraded you might want to try out
threadscope which is designed to help track down these sorts of

If you are using 6.10, I recommend turning off parallel garbage
collection with the RTS flags (see the manual) as that can cause

Thanks, Neil

2010/1/4 Jamie Morgenstern <cantthinkthinkpink at gmail.com>:
> Hello;
>  I have a piece of code in which I employ the `par` construct to add some implicit parallelism
> to a theorem prover. However, when running the *same* code with
> +RTS -N1
> +RTS -N5
> +RTS -N10
> I see a huge slowdown (a factor of 50 with 5 processes and a factor of 100 for 10 on an 8-core machine).
> Very little time is being spent using the garbage collector. Any suggestions?
> Thanks,
> -Jamie
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