[Haskell-cafe] Control.Concurrent.forkIO versus Control.Parallel.par

Mario Blažević mblazevic at stilo.com
Sun Jul 27 22:49:43 EDT 2008

    Hello. I have a question about parallel computation in Haskell. After browsing the GHC library documentation, I was left with impression that there are two separate mechanisms for expressing concurrency: Control.Parallel.par for pure computations and Control.Concurrent.forkIO for computations in IO monad.

    This dichotomy becomes a problem when one tries to use concurrency from a monad transformer, though I'm sure that's not the only such situation. One cannot assume that the base monad is IO so forkIO cannot be used, while Control.Parallel.par won't run monads. My first solution was to replace the base monad class for the monad transformer by the following ParallelizableMonad class:

class Monad m => ParallelizableMonad m where
   parallelize :: m a -> m b -> m (a, b)
   parallelize ma mb = do a <- ma
                          b <- mb
                          return (a, b)

instance ParallelizableMonad Identity where
   parallelize (Identity a) (Identity b) = Identity (a `par` (b `pseq` (a, b)))

instance ParallelizableMonad IO where
   parallelize ma mb = do va <- newEmptyMVar
                          vb <- newEmptyMVar
                          forkIO (ma >>= putMVar va)
                          forkIO (mb >>= putMVar vb)
                          a <- takeMVar va
                          b <- takeMVar vb
                          return (a, b)

I tested this solution, and it worked for IO computations in the sense that they used both CPUs. The test also ran slower on two CPUs that on one, but that's beside the point.

Then I realized that par can, in fact, be used on any monad, it just needs a little nudge:

parallelize :: m a -> m b -> m (a, b)
parallelize ma mb = let a = ma >>= return
                        b = mb >>= return
                    in a `par` (b `pseq` liftM2 (,) a b)

However, in this version the IO monadic computations still appear to use only one CPU. I cannot get par to parallelize monadic computations. I've used the same command-line options in both examples: -O -threaded and +RTS -N2. What am I missing?

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