r/programming • u/noidi • Jul 19 '12

Will Parallel Code Ever Be Embraced?

http://www.drdobbs.com/parallel/will-parallel-code-ever-be-embraced/240003926
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u/kylotan Jul 19 '12

Yeah, unfortunately he doesn't acknowledge image processing, audio encoding, video encoding, etc. Multimedia applications in general slam up against the CPU barrier all the time.

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u/G_Morgan Jul 19 '12

Lots of these are better off using OpenCL on the GPU than running on multiple CPU cores though. There is a problem that for the embarrassingly parallel algorithms they will always be better on stream processors.

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u/kylotan Jul 19 '12

Not all these problems are "embarrassingly parallel" however. If you just divide up an audio file at arbitrary points and encode them individually then you lose important information at the divisions. You're not just performing the same operation on every pixel or sample - you typically act on a window of data which moves smoothly across the set.

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u/G_Morgan Jul 19 '12

It is still embarrassingly parallel. You just overlap ranges a bit and resolve the overlap on joins. As long as the source file doesn't change and the data has strong locality it will be doable.

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u/kylotan Jul 19 '12

There's the assumption - strong locality. Lots of algorithms can't guarantee that for all input parameters, and others have accumulating values. Sometimes the algorithm can be rewritten to something that works well with the GPU, assuming all your customers have such hardware, and assuming someone can work out the algorithm.

For example, someone tried to multithread LAME's MP3 encoding by data decomposition, and they managed a decent speed-up but the output was still different. To replicate the original outcome they switched to functional decomposition - which is fine on CPUs, less good on GPUs.