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Date: Sun, 14 Jul 2013 19:57:29 -0700
From: Jeremi Gosney <epixoip@...dshell.nl>
To: john-dev@...ts.openwall.com
Subject: Re: GPGPU SLI/CROSSFIRE

On 7/14/2013 7:23 PM, Sayantan Datta wrote:
> Hi marcus,
>
> On Mon, Jul 15, 2013 at 3:24 AM, marcus.desto <marcus.desto@...pl
> <mailto:marcus.desto@...pl>> wrote:
>
>     does SLI / CROSSFIRE increase performance during GPGPU
>     computations? Does it have any advanges for GPGPU computations at all?
>
>
>
> Of course it does, only you need to write programs that can do it.


Actually this is not true, at least not for AMD. Crossfire needs to be
disabled in order to use OpenCL.

In the "old days" you had to explicitly disable Crossfire in order to
run OpenCL apps. If Crossfire was enabled, the app would either 1. not
run, 2. would run very slowly, or 3. would only run on one GPU. But I
believe nowadays the driver just simply ignores any Crossfire configuration.

See the AMD APP FAQ [1]:

"ATI CrossFire technology allows multiple AMD GPUs to work together on a
single graphics-rendering tasks. This  does not apply to AMD APP
computational tasks because it is not compatible with the compute model
used for AMD APP applications."

"It is recommended that ATI CrossFire be turned off in most system
configurations so that AMD APP applications can access all available
GPUs in the system."


[1]: http://developer.amd.com/wordpress/media/2012/10/AMD_APP_SDK_FAQ.pdf



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