Nvidia 9800GX2 - connect both GPUs to one screen [SOLVED]

[SerialPrecision]

I recently bought a computer with an Nvidia 9800GX2 and I was using one of its DVI ports to plug my screen in. My screen has two DVI outputs and I'm not sure if I'm supposed to plug both into my computer. I am using the 169.09-r1 drivers (64 bit) and in the Nvidia settings it counts it as two GPUs with 512MB each. For GPU0 it says its connected to my screen and for GPU1 it says no screen. At first I generated my xorg.conf by running "Xorg -configure" and then I tried the one generated by running "nvidia-xconfig"

Here is the 1st one (I changed some mouse stuff and added a few font paths, but also I tried having two screen sections, pointing to "both" of my GPUs and I removed the parts where it thought I had two monitors)

[asturm]

It definitely won't help plugging into both DVIs. The proprietary nvidia driver should cope with SLI acceleration (I had to look that up, it actually seems to work with Linux) but it's not something xorg-x11 will be aware of imo.

You could search your Xorg.0.log for SLI initialisation, like that:

[SerialPrecision]

Hmm I found some information that told me to enable "MultiGpu" in xorg.conf instead of SLI like this: nvidia-xconfig --multigpu=on

This generated a new xorg.conf and then I commented out the second screen section. Now it looks like this:

[gzunk]

I don't have experience specifically with the 9800GX2, but have with SLI'd nvidia. Here's my take on the situation.

You don't connect each GPU seperately to a display. Yes there are two GPU's in a 9800GX2, but that fact is "hidden" from you. They are used in an SLI fashion, such that OpenGL / DirectX rendering can be split between the two and then combined together and displayed on your screen.

There isn't much support for SLI in Linux, and to be honest, you don't really need it unless you're running games. So for a standard desktop (or even a 3D compiz enhanced desktop) you would not need SLI. this means that only one of the GPU's is actually getting used. This means that your card will be cooler, since the other GPU is doing nothing.

The DVI ports at the back of the card are not connected to each GPU. They are both connected to the "master" GPU. Depending upon your monitor size you may need to connect multiple DVI ports in order to drive it at it's full resolution. This, however, is unlikely. Each of the DVI ports on the GX2 is a dual link DVI port, each of which can drive I think up to 2560x1600 at 60Hz.

When I was running SLI, I had 4 DVI ports at the back (two for each card) of which I was only using two - the two on the "master" card. The two on the other card were not connected at all, since the data is transferred over the SLI link between the two cards and then combined on the master card and sent to the monitor.

Your second GPU is not on a seperate card (like mine was) but still communicates only with the primary GPU (via a really quick bus), and it's the primary GPU that sends the signal to the monitor. The secondary GPU will never be connected to a screen.

To cut a long story short, unless you're playing games under Linux, then don't worry about enabling SLI to bring the other GPU on-line. It won't make a difference to your desktop experience.
_________________
Intel Core 2 Quad Q9650 | Asus P5K | 8GB DDR-2 800
Sapphire Radeon 4870 X2 | 30" Samsung SM305T+
(4xSATA) 3 TB | 2 x DVD+-RW DL (Pioneer 112D 115D)
Logitech X-530 5.1 | DiNovo Edge | MX Revolution

[SerialPrecision]

Thank you for the explanation, however I was actually planning to try out Enemy Territory Quake Wars and would like to take advantage of both GPUs. The MultiGpu option is supposed to provide this but I'm just not sure of the syntax in xorg.conf to connect both GPUs to my screen.

[Gef]

Does "# update-pciids" helps showing the name of your card in "# lspci" ?

(Anyway, i can'y help with multigpu stuff ; my laptop runs a nVidia 8400M G).
_________________
Laptop : Gentoo ~amd64
(remote) Server : Gentoo amd64

[Monkeh]

[SerialPrecision]

Well I'm not 100% sure that its working but maybe someone can confirm, this is what the nvidia settings look like now.

I'm guessing this means the screen is using both cards?? The DVI cable is plugged into the master card (the one with the HDMI port on it). The update-pciids doesn't seem to do anything:

[Monkeh]

That looks correct to me.

[SerialPrecision]

Cool, thanks for the help. I think when you're setting up your SLI, you just need to s/MultiGPU/SLI in xorg.conf

[Monkeh]