[Haskell-cafe] transparent parallelization
derek.a.elkins at gmail.com
Tue Sep 18 18:23:16 EDT 2007
On Tue, 2007-09-18 at 18:13 +0200, Thomas Girod wrote:
> Hi there. Beeing rather new to the realm of Haskell and functional
> programming, I've been reading about "how is easier it is to
> parallelize code in a purely functional language" (am I right saying
> that ?).
> My knowledge of parallelization is also very weak, but I've been
> thinking about this and I have a question. Let's say we take a small
> piece of code, like the quicksort algorithm.
> > qsort  = 
> > qsort (x:xs) = qsort lesser ++ [x] ++ qsort greater
> > where lesser = filter (<x) xs
> greater = filter (>=x) xs
> (actually I didn't test this code, this is just for the example)
> Here, computing "lesser" and "greater" are two disjoint actions - we
> don't need the result from one to compute the other, and haskell does
> not allow one to alter data so that would change the behaviour of the
> other. So I guess those could be run in parallel.
> Would it be fairly simple to automatically determine parts of code
> that can be run in parallel, and parts that cannot (at least in
> non-monadic code) ?
The problem with Haskell is not finding opportunities to parallelize,
they are legion. Actually, quite the opposite, there's so much that your
code ends up slower than a sequential realization. The hard part is
making a good cost-model and a good way to create coarser chunks of
work. It's not worthwhile to spawn a thread (even a very lightweight
one) for virtually every subexpression.
Automatic parallelization is easy, efficient parallelization is hard.
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