[Haskell-beginners] parallel map/filter

[Csaba Marosi]

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/bf1ca794f897858f9008fb8740a1c3ef1997482b/s01p04.hs

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