[Haskell-cafe] Using DPH

[Ben Lippmeier]

On 12/04/2011, at 7:32 PM, Wilfried Kirschenmann wrote:

> Hi,
> 
> In order to do a performance comparison beetween different approaches for our application, I make different implementation of a simple example (computing the norm of a vector expression.
> I rely on Repa to do this. 
> However, when I tried to build the parallel version (-threaded -fvectorise -rtsopts), I got an error specifying that dph-par was not available. Indeed, It wasn't.

Repa and DPH are different projects. The compilation mechanism and approach to parallelism is quite different between them. You only need -fvectorise to turn on the vectoriser for DPH code. You don't need (or want) -fvectorise for Repa programs. DPH is also still at the "research prototype" stage, and not yet at a point where you'd try to use it for anything real.

With your example code, you also need to use R.force at appropriate points, and add matches against @(Array _ [Region RangeAll (GenManifest _)]). The reasons for both of these are explained in [1]. Hopefully the second will be fixed by a subsequent GHC release. You must also add {-# INLINE fun #-} pragmas to polymorphic functions or you will pay the price of dictionary passing for the type class overloading.


With the attached code:

desire:tmp benl$ ghc --version
The Glorious Glasgow Haskell Compilation System, version 7.0.3

desire:tmp benl$ ghc-pkg list |grep repa
    repa-2.0.0.2
    repa-algorithms-2.0.0.2
    repa-bytestring-2.0.0.2
    repa-io-2.0.0.2

desire:tmp benl$ ghc -rtsopts -threaded -O3 -fllvm -optlo-O3 -fno-liberate-case --make haskell.hs -XBangPatterns -fforce-recomp

desire:tmp benl$ /usr/bin/time ./haskell
[3.3645823e12]
725188000000
        6.62 real         6.39 user         0.22 sys


This runs but doesn't scale with an increasing number of threads. I haven't looked at why. If all the work is in R.sum then that might be the problem -- I haven't put much time into optimising reductions, just maps and filters.

Cheers,
Ben.

[1] http://www.cse.unsw.edu.au/~benl/papers/stencil/stencil-icfp2011-sub.pdf


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