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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