[Haskell-cafe] Concurrency performance problem

[Edward Z. Yang]

If you just pass -N, GHC automatically sets the number of threads
based on the number of cores on your machine. Do you mean -threaded?

Excerpts from Łukasz Dąbek's message of Mon Mar 04 11:39:43 -0800 2013:
> 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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>