Pragmatic concurrency Re: [Haskell-cafe] multiple computations, same input

Tomasz Zielonka tomasz.zielonka at
Thu Mar 30 10:05:30 EST 2006

On Wed, Mar 29, 2006 at 12:50:02PM +0100, Jon Fairbairn wrote:
> [...]
> but add [a] pragma[s] to the effect that evaluation should
> be input driven, and that ll, ww, and cc are to be given
> equal time. Something like {-# STEPPER cs; ROUND_ROBIN
> ll,ww,cc #-} (please do not take this as a suggestion of
> real syntax!).
> The way I would implement this is to add a new primitive,
> STEP, which is like seq except that it only evaluates its
> argument until it encounters another STEP. (It really isn't
> much different to seq).
> [...]
> It seems to me that this wouldn't take much effort to
> implement, but it would provide a simple means of removing
> space leaks from a whole bunch of programmes without
> mangling the source code much.

Actually, it may require no effort from compiler implementors.
I just managed to get the desired effect in current GHC! :-)

I implemented your idea of stepper by writing the function stepper that
rewrites the list invoking "yield" every 500 processed elements. This
way I can concurrently consume the list without the space leak - when a
thread evaluates too many list elements, it gets preempted. I think it
suffices if RTS employs a round-robin scheduler. I am not sure it's

The code isn't as beautiful as the naive wc implementation. That's
because I haven't yet thought how to hide newEmptyMVar, forkIO, putMVar
i takeMVar. Perhaps someone will come up with a solution to this.

    import Control.Concurrent
    import Control.Monad
    import System.IO.Unsafe (unsafePerformIO)

    stepper l = s n l
        n = 500
        s 0 (x:xs) = unsafePerformIO $ do
            return (x : s n xs)
        s i (x:xs) = x : s (i-1) xs
        s _ []     = []

    main = do
        cs <- liftM stepper getContents
        ll <- newEmptyMVar
        ww <- newEmptyMVar
        cc <- newEmptyMVar
        forkIO $ putMVar ll $! length (lines cs)
        forkIO $ putMVar ww $! length (words cs)
        forkIO $ putMVar cc $! length cs
        takeMVar ll >>= print
        takeMVar ww >>= print
        takeMVar cc >>= print

See how well it works:

    $ cat words words words words | ./A +RTS -sstderr
    ./A +RTS -K8M -sstderr
    3725868                                             <- that's the size of cs
    643,015,284 bytes allocated in the heap
     72,227,708 bytes copied during GC
        109,948 bytes maximum residency (46 sample(s))  <- no space leak!

           2452 collections in generation 0 (  0.33s)
             46 collections in generation 1 (  0.00s)

              2 Mb total memory in use                  <- no space leak!

      INIT  time    0.00s  (  0.01s elapsed)
      MUT   time    1.25s  (  1.27s elapsed)
      GC    time    0.33s  (  0.36s elapsed)
      EXIT  time    0.00s  (  0.00s elapsed)
      Total time    1.58s  (  1.64s elapsed)

      %GC time      20.9%  (22.0% elapsed)

      Alloc rate    514,412,227 bytes per MUT second

      Productivity  79.1% of total user, 76.2% of total elapsed

Thanks for your idea, Jon! :-)

Best regards

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