Re: [Haskell-cafe] repa parallelization results

Read that paper I linked. Anything else I say will be a rehash of that paper. :) On Mar 15, 2015 4:21 PM, "Anatoly Yakovenko" wrote:

Ok, so whats the difference between the sequence and parallel versions? does the parallel one contain a thunk for every element in the output?

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Read what I linked. You are benchmarking repa for exactly the pessimal workload that it is bad at.

Repa is for point wise parallel and local convolution parallel programs. The way repa can express matrix multiplication is exactly the worst way to implement a parallel matrix mult. Like, pretty pessimal wrt a memory traffic / communication complexity metric of performance.

Benchmark something like image blur algorithms and repa will really shine.

Right now your benchmark is the repa equivalent of noticing that random access on singly linked lists is slow :)

On Mar 15, 2015 2:44 PM, "Anatoly Yakovenko" wrote:

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I am not really focusing on matrix multiply specifically. So the real problem is that the implementation using parallelized functions is slower then the sequential one, and adding more threads makes it barely as fast as the sequential one.

So why would i ever use the parallelized versions?

On Sat, Mar 14, 2015 at 9:24 AM, Carter Schonwald wrote:

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http://www.cs.utexas.edu/users/flame/pubs/blis3_ipdps14.pdf this

On Sun, Mar 15, 2015 at 12:44 PM, Carter Schonwald wrote: paper

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(among many others by the blis project) articulates some of the ideas i allude to pretty well (with pictures!)

On Sat, Mar 14, 2015 at 12:21 PM, Carter Schonwald wrote:

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dense matrix product is not an algorithm that makes sense in repa's execution model, in square matrix multiply of two N x N matrices, each result entry depends on 2n values total across the two input matrices. even then, thats actually the wrong way to parallelize dense matrix product! its worth reading the papers about goto blas and the more recent blis project. a high performance dense matrix multipy winds up

needing

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to do some nested array parallelism with mutable updates to have efficient sharing of sub computations!

On Fri, Mar 13, 2015 at 9:03 PM, Anatoly Yakovenko wrote:

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you think the backed would make any difference? this seems like a runtime issue to me, how are the threads scheduled by the ghc

runtime?

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On Fri, Mar 13, 2015 at 4:58 PM, KC wrote: > How is the LLVM? > > -- > -- > > Sent from an expensive device which will be obsolete in a few > months! > :D > > Casey > > > On Mar 13, 2015 10:24 AM, "Anatoly Yakovenko" > > wrote: >> >> https://gist.github.com/aeyakovenko/bf558697a0b3f377f9e8 >> >> >> so i am seeing basically results with N4 that are as good as

using

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>> sequential computation on my macbook for the matrix multiply >> algorithm. any idea why? >> >> Thanks, >> Anatoly >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe@haskell.org >> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe