[Haskell-cafe] repa parallelization results

Thomas Miedema thomasmiedema at gmail.com
Thu Jan 14 19:19:01 UTC 2016

Anatoly: I also ran your benchmark, and can not reproduce your findings.

Note that GHC does not make effective use of hyperthreads (
https://ghc.haskell.org/trac/ghc/ticket/9221#comment:12). So don't use -N4
when you have only a dual core machine. Maybe that's why you were getting
bad results? I also notice a `NaN` in one of your timing results. I don't
know how that is possible, or if it affected your results. Could you try
running your benchmark again, but this time with -N2?

On Sat, Mar 14, 2015 at 5:21 PM, Carter Schonwald <
carter.schonwald at gmail.com> wrote:

> dense matrix product is not an algorithm that makes sense in repa's
> execution model,

Matrix multiplication is the first example in the first repa paper:
http://benl.ouroborus.net/papers/repa/repa-icfp2010.pdf. Look at figures 2
and 7.

    "we measured very good absolute speedup, ×7.2 for 8 cores, on multicore

Doing a quick experiment with 2 threads (my laptop doesn't have more cores):

$ cabal install repa-examples    # I did not bother with `-fllvm`

$ ~/.cabal/bin/repa-mmult -random 1024 1024 -random 1024 1204
elapsedTimeMS   = 6491

$ ~/.cabal/bin/repa-mmult -random 1024 1024 -random 1024 1204 +RTS -N2
elapsedTimeMS   = 3393

This is with GHC 7.10.3 and repa- (and dependencies from
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