[Haskell-cafe] multithreading speedup

[Fawzi Mohamed]

I was trying to speed up a program that I wrote and so I thought  
about using multiple threads.
I  have a quite easy parallel program and I did the following

do
       subRes <- MVar.newMVar []
       putStrLn "starting threads"
       subV <- flip mapM [0 .. (nThreads - 1)] $
               ( \i -> do
                   subR <- MVar.newEmptyMVar
                   let writeRes r = do { MVar.putMVar subR r }
                   forkOS (do
                            let r=eval (startData !! i)
                            writeRes $! r
                            putStr $ "writtenRes")
                   return subR
               )
       putStrLn "started threads"
       toFold <- mapM MVar.takeMVar subV
       putStrLn "about to fold"
       return $ foldl1 mergeRes toFold

I know that the threads really calculate what I want, and as soon as  
they are finished I get the result.
The problem is that I have no speed up, the time is basically the sum  
of the time for the two threads.
I thought that ghc now would take advantage of the two cpus if I  
compiled with -threaded.
Am I wrong, do I need some special flag, a newer version of the  
compiler (I have 6.6.20070129), or it is just normal?

Thanks
Fawzi