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Date: Thu, 26 Apr 2012 00:55:42 +0300
From: Milen Rangelov <>
Subject: Re: New RAR OpenCL kernel

Older GPUs are more OK with smaller ndranges cause they are more limited in
resources. They would probably get the same (or even slower) speed with
higher ndranges. For faster GPUs it makes much more sense though because
you can easily get to the point where there are not enough wavefronts/warps
to feed the CUs busy.

On Thu, Apr 26, 2012 at 12:43 AM, magnum <> wrote:

> On 04/25/2012 11:11 PM, Milen Rangelov wrote:
> > The values for LWS (worksize) and KPC (ndrange) seem quite too unoptimal
> to
> > me. Worksize of 256 is just not healthy for such a kernel, for a number
> of
> > reasons. I would stick to 64 (or 32 for nvidia), even hardcoded, no need
> to
> > customize that at all...but well that's just my opinion :)
> Thanks, I'll hardcode it as 64 then - I got worse speed with 32 on
> nvidia. I spent some time in nvidia's profiler today and I did not use
> all SM's and threads or whatever they called it unless I used LWS <= 64
> and GWS >= 1536. The funny thing is I got the same 4400 c/s anyway. It
> got better in theory (less suggestions from the tool) but in practice it
> stayed the same. For GTX580 I'm using 8192 now since higher figures
> don't make any difference.
> > NDRange of just 256 is just bad though. It's not enough to keep the CUs
> > busy enough to 'hide' memory access latencies. It needs to be at least
> > several thousands. You are underusing the GPU that way.
> With the older cards I've tested (like 9600GT and Cedar) it did not
> matter: I'm just seeing longer and longer runtime, and the same c/s
> (after a best gws at lws<<5 or something like that, lws being very low).
> Does this imply any specific problem with my kernel?
> magnum

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