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Date: Tue, 10 Apr 2012 12:58:44 +0200
From: Samuele Giovanni Tonon <>
Subject: Re: JtR:Multi-GPU Setups

On Fri, Apr 06, 2012 at 03:55:43AM -0400, SAYANTAN DATTA wrote:
> Hi,
> The AMD card needs to be running X, but the nvidia card only needs you
> > to run "nvidia-smi" and it will create the /dev nodes you need to use
> > the card. Don't waste resources on a second X server.
> >
> Did you run any openCL application? If so, which vendor's openCL did you
> use?   Does it detect both the cards?

Hello Sayantan,
sorry for my late reply to your question.

I hope with this one to give you some good advice and extend those advice to
all the people interested in GPU computing for john.

= Single GPU hints

As already told by many people AMD/ATI need to work an Xserver running AND a
user logged in and use 'ssh -X' (X11 protocol enabled in sshd)

this means that if that card is your only card but you plan to work on that
server from remote (ssh), you must set up a lightdm session with an autologin
this should help you in doing that:

Nvidia on the other hand doesn't need any xserver running, so you can use OpenCL
through a normal ssh.

both nvidia and amd gives you libraries for opencl and the driver to use it, that
driver is the proprietary one; if you find yourself stuck with some errors it's most likely
because you are using open source driver (radeon, vesa) instead of proprietary (nvidia, fglrx)

What card to choose: i suggest move to ati hd 6xxxx family which is a good tradeoff of 
price per speed; ati hd 5xxxx family are good too but maybe a bit outdated and with different
"bandwith" pci speed from cpu -> gpu and vice versa.

Nvidia are generally more expensive than amd but they offer a bit more stability of driver and
issue .

Everybody atm is "going wild" for the amd hd 7970; i still don't know if it's worth getting one, i'd wait a bit more

= CPU mode

if you have a AMD card and AMD cpu you might change your OpenCL code to either run on CPU or on GPU.
while this can help on tracking problem debug with gdb

it could lead to some misbehaviours due to using a different architecture (and not having problem with
the pci-e bandwith) 

= Dual GPU system:

As already told in other messages this at the moment is a bit messy: linux distribution are not yet
ready with packages for nvidia and AMD so mixing both drivers and libraries could lead you to strange
side effects.

Nothing to worrry about, but you could end spending a lot of time trying to figure out which correct
order of libraries to specify when using nvidia and which one to use when using AMD.

Having dual AMD or dual Nvidia is simpler but not so useful: at the moment jtr still need a good interface
design for GPU to have it scale with more than one GPU so at the moment is not so *needed* to have more
than one GPU.

I hope i have clarified a bit more the situation of OpenCL developing and current jtr opencl status.


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