[Haskell-cafe] Parallel compilation and execution?

[Alex Mason]

Hi Michael,

OpenMP is a very different beast, and was developed to help get over the shortcomings that languages like C and FORTRAN have with respect to parallel and concurrent programming (pthreads were about all there was before OpenMP). OpenMP lets you specify regions of code that should be run in multiple threads at once, each with a unique ID.  Here is an example of (part of) a parallel merge sort I've been working on

> static void
> pmergesort(long int * in, long int * tmp, long int n, int nthread)
> {
>     long int nhalf = n/2;
> 
>     if(n <= N_small)
>     {
>         insertsort1(in, n);
>         return;
>     }
> 
>     if(nthread > 1)
>     {
>         #pragma omp parallel num_threads(2)
>         {
>             if(omp_get_thread_num() == 0)
>                  pmergesort(tmp,       in,         nhalf, nthread>>1);
>             else pmergesort(tmp+nhalf, in+nhalf, n-nhalf, nthread>>1);
>         }
>     } else {
>         mergesort3(tmp,       in,         nhalf);
>         mergesort3(tmp+nhalf, in+nhalf, n-nhalf);
>     }
> 
>     merge( tmp, in, nhalf, n);
> }


The approach that Control.Concurrent takes is very different, preferring a style where the programmer says what things might be advantageous to run in parallel, but the runtime makes no guarantees that they will be, allowing the programmer to break work down into smaller chunks, and letting the runtime sort out which parts should be run concurrently. This allows for a much easier style of parallel programming, but is only really possible in a pure language like Haskell. 

On a side note, the Cilk language, which adds a small number of keywords like fork and sync to the C language takes an approach closer to what Control.Parallel does, but it's not a graceful, and IMO not as easy to use.

Hope that helps. I've been having a lot of fun over the last few weeks playing with OpenMP for a university assignment, and I've got to say I greatly prefer the haskell way of doing things.

Cheers,
Alex Mason

On 27/05/2011, at 10:23, michael rice wrote:

> Are the tools of Control.Parallel comparable to OpenMP?
> 
> Michael
> 
> --- On Thu, 5/26/11, michael rice <nowgate at yahoo.com> wrote:
> 
> From: michael rice <nowgate at yahoo.com>
> Subject: Re: [Haskell-cafe] Parallel compilation and execution?
> To: "David Virebayre" <dav.vire+haskell at gmail.com>
> Cc: "Daniel Fischer" <daniel.is.fischer at googlemail.com>, haskell-cafe at haskell.org
> Date: Thursday, May 26, 2011, 9:32 AM
> 
> Fair question. I copied the parallel version from:
> 
> http://www.haskell.org/ghc/docs/6.6/html/users_guide/lang-parallel.html
> 
> but pulled the non-parallel version from a text.
> 
> Michael
> 
> 
> --- On Thu, 5/26/11, David Virebayre <dav.vire+haskell at gmail.com> wrote:
> 
> From: David Virebayre <dav.vire+haskell at gmail.com>
> Subject: Re: [Haskell-cafe] Parallel compilation and execution?
> To: "michael rice" <nowgate at yahoo.com>
> Cc: haskell-cafe at haskell.org, "Daniel Fischer" <daniel.is.fischer at googlemail.com>
> Date: Thursday, May 26, 2011, 8:56 AM
> 
> 
> 
> 2011/5/26 michael rice <nowgate at yahoo.com>
> Thank, Daniel
> 
> Multiple threads are in evidence in my system monitor, but I wonder why I'm getting two different answers, one twice the other. The first is the parallel solution and the second is the non.
> 
> Why do you add n1+n2+1 in the parallel program, but only n1+n2 in the non-parallel one ?
>  
> 
> Michael
> 
> ===========
> 
> {-
> import Control.Parallel
> 
> nfib :: Int -> Int
> nfib n | n <= 1 = 1
>        | otherwise = par n1 (pseq n2 (n1 + n2 + 1))
>                      where n1 = nfib (n-1)
>                            n2 = nfib (n-2)
> -}
> 
> nfib :: Int -> Int
> nfib n | n <= 1 = 1
>        | otherwise = nfib (n-1) + nfib (n-2)
> 
> main = do putStrLn $ show $ nfib 39
> 
> =============
> 
> [michael at hostname ~]$ ghc --make -threaded nfib.hs
> [1 of 1] Compiling Main             ( nfib.hs, nfib.o )
> Linking nfib ...
> [michael at hostname ~]$ ./nfib +RTS -N3
> 204668309
> [michael at hostname ~]$ ghc --make nfib.hs
> [1 of 1] Compiling Main             ( nfib.hs, nfib.o )
> Linking nfib ...
> [michael at hostname ~]$ ./nfib
> 102334155
> [michael at hostname ~]$ 
> 
> 
> 
> -----Inline Attachment Follows-----
> 
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