Re: [Haskell-cafe] Concurrency performance problem

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 :

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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" wrote:

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What do you exactly mean? I have included link to full source listing: http://hpaste.org/83460.

-- Łukasz Dąbek

2013/3/4 Don Stewart :

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Depends on your code...

On Mar 4, 2013 6:10 PM, "Łukasz Dąbek" wrote:

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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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