[Haskell-cafe] Concurrency performance problem

[Łukasz Dąbek]

Thank you for your help! This solved my performance problem :)

Anyway, the second question remains. Why performance of single
threaded calculation is affected by RTS -N parameter. Is GHC doing
some parallelization behind the scenes?

--
Łukasz Dąbek.

2013/3/4 Don Stewart <dons00 at gmail.com>:
> Apologies, didn't see the link on my phone :)
>
> As the comment on the link shows, youre accidentally migrating unevaluated
> work to the main thread, hence no speedup.
>
> Be very careful with evaluation strategies (esp. lazy expressions) around
> MVar and TVar points. Its too easy to put a thunk in one.
>
> The strict-concurrency package is one attempt to invert the conventional
> lazy box, to better match thge most common case.
>
> On Mar 4, 2013 7:25 PM, "Łukasz Dąbek" <sznurek at gmail.com> wrote:
>>
>> What do you exactly mean? I have included link to full source listing:
>> http://hpaste.org/83460.
>>
>> --
>> Łukasz Dąbek
>>
>> 2013/3/4 Don Stewart <dons00 at gmail.com>:
>> > Depends on your code...
>> >
>> > On Mar 4, 2013 6:10 PM, "Łukasz Dąbek" <sznurek at gmail.com> wrote:
>> >>
>> >> Hello Cafe!
>> >>
>> >> I have a problem with following code: http://hpaste.org/83460. It is a
>> >> simple Monte Carlo integration. The problem is that when I run my
>> >> program with +RTS -N1 I get:
>> >> Multi
>> >> 693204.039020917 8.620632s
>> >> Single
>> >> 693204.039020917 8.574839s
>> >> End
>> >>
>> >> And with +RTS -N4 (I have four CPU cores):
>> >> Multi
>> >> 693204.0390209169 11.877143s
>> >> Single
>> >> 693204.039020917 11.399888s
>> >> End
>> >>
>> >> I have two questions:
>> >>  1) Why performance decreases when I add more cores for my program?
>> >>  2) Why performance of single threaded integration also changes with
>> >> number of cores?
>> >>
>> >> Thanks for all answers,
>> >> Łukasz Dąbek.
>> >>
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