[Haskell-cafe] Haskell vs GC'd imperative languages, threading,
parallelizeability (is that a word? :-D )
sjanssen at cse.unl.edu
Fri Aug 10 16:51:24 EDT 2007
On Friday 10 August 2007 03:51:49 Hugh Perkins wrote:
> Well, managed to shave 25% of C# execution time by writing my own bit
> array. For now, I will concede that, under the conditions of the shoot,
> bitarrays in c# are slower than bitarrays in Haskell. I'll let you know if
> I get any new ideas on this.
> Getting back to the original problem, which is: threading. Donald, one of
> the things that is very interesting about Haskell is it's potential for
> automatic threading, ie you write a trivial algorithm that looks like it
> runs in a single thread, and the runtime splits it across multiple cores
> It's fairly safe to say that maps, foldrs, foldls, and their derivatives
> are safe to parallelize? (For example, hand-waving argument, a foldr of
> (/) on [1,5,7,435,46,2] can be split into a foldr on [1,5,7] and a foldr on
> [435,46,2], then their results combined).
Yes, the semantics of Haskell allow us to run pretty much any operation in
parallel. However, the major problem is that lists are a fundamentally
sequential data structure. It's quite expensive to chop up parts of a list
to eg. send them to a parallel map function. I think the upcoming NDP arrays
have a much better chance here.
> To what extent is the technology you are using in your algorithm
> parallizable? (I actually cant tell, it's a genuine question). In the
> case that it is parallelizable, to what extent is it trivial for a runtime
> to know this? (Again, I dont have enough information to tell)
The program doesn't have much chance for parallelism as written. It uses the
imperative ST monad and destructive update extensively.
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