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 <[email protected]>: > > 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" <[email protected]> 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 <[email protected]>: > >> > Depends on your code... > >> > > >> > On Mar 4, 2013 6:10 PM, "Łukasz Dąbek" <[email protected]> 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. > >> >> > >> >> _______________________________________________ > >> >> Haskell-Cafe mailing list > >> >> [email protected] > >> >> http://www.haskell.org/mailman/listinfo/haskell-cafe > _______________________________________________ Haskell-Cafe mailing list [email protected] http://www.haskell.org/mailman/listinfo/haskell-cafe
