[Haskell-cafe] Problems with threading?

Don Stewart dons at galois.com
Mon Jun 7 17:50:48 EDT 2010

> While working on the Shootout, I noticed the following benchmarks:
> http://shootout.alioth.debian.org/u64/program.php?test=chameneosredux&lang=ghc&
> id=3
>  http://shootout.alioth.debian.org/u64q/program.php?test=chameneosredux&lang=
> ghc&id=3
> The same program becomes almost 4 times slower when compiled with --threaded
> and run with +RTS -N5 -- even though the multi-core benchmark really only ever
> uses one processor.

Using -N5 sounds suspicious. There are only 4 cores on the machine.
> Other languages seem to have found a way of arranging these threads in a way
> such that parallelism actually happens, but as it stands, compiling this
> benchmark without --threaded actually makes Haskell competitive against the
> genuinely parallel alternatives in other languages...which is unusual by
> itself.
> I wanted to throw this out for people to discuss, because I'd like to see it
> improved.  As it stands, I'm going to submit a version which asks not to be
> compiled with --threaded (and has a few other improvements).

What parallelization did you try? Is it a good algorithm?

-- Don

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