[Haskell] repa not running in parallel?

[Alexander Herz]

Hi,

I'm new to haskell and I tried to reproduce the perfomance values from 
the paper "Regular, Shape-polymorphic, Parallel Arrays in Haskell".

I modified the repa-mmult example from the repa-example package to use 
the mmultP from Data.Array.Repa.Algorithms.Matrix.

Then I compile it with "ghc -threaded -fglasgow-exts -O Main.hs" but 
running it with two large matrices :

"./Main -random 4096 4096 -random 4096 4096" I can see that only one of 
my 4 cpus is utilized.

What am I doing wrong?

Note: The unmodified repa-example also does not run in parallel.

Thx,
Alex