[Haskell-cafe] Concurrency performance problem

[Łukasz Dąbek]

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.