Hi, I'm trying to use parallelism in Haskell, parMap in particular. My code is essentially this: import Control.Parallel.Strategies slow :: String -> (String, [Int]) fast x = (x, [0..255]) main :: IO () main = do content <- readFile "4.txt" --a 327 lines long file print $ parMap rpar slow $ lines content For the full/compiling code, see: https://github.com/csmarosi/cryptopals/blob/bf1ca794f897858f9008fb8740a1c3ef... What do I miss here? The code should be trivial to run on multiple cores, but it seem to use only a single CPU core, i.e. $ ghc -O2 -v0 -threaded --make s01p04.hs slow gives: $ time ./s01p04 +RTS -N2 | wc 1 1 64028 real 0m8.903s user 0m7.888s sys 0m1.792s whereas fast is: $ time ./s01p04 +RTS -N2 | wc 1 1 320787 real 0m0.016s user 0m0.012s sys 0m0.012s Bonus question: is there a parallel filter? I use Debian sid packages, ghc version 7.8.4 Some multi-threading seems to go on, since without -N2, the sys time is below 0.1s. BTW, the slow code use ~300K heap according to valgrind. Thanks, Csabi