Multi-gpu, multi-monitor setup with GeForce RTX 3090s

Graphics / Linux Linux
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I have been unable to get a multi-monitor setup working, and the information online for this topic is sparse and often conflicting. Hopefully someone here can provide a decisive answer.

I would like to run a single X server with a single X screen that spans across 4 monitors attached to 2 identical GPUs (3090s). Graphics applications should see a single display with a resolution equal to the sum of the four monitors (in this case, 7680x4320), and I should be able to move my mouse and draw windows seamlessly across the 4 displays.

I’m seeing options like Mosaic, Surround, and MultiGPU.

My question is:
Do any of these work for Linux with my hardware? And if so, how would I go about setting up my X11 environment (xorg.conf) to achieve this configuration?

I am referencing these guides without success:

Configuration:
Ubuntu Server 20.04
Driver 515.76
2x GeForce RTX 3090
4x 4K, 60hz monitors
No desktop manager yet, just vanilla X.org

I appreciate any help with this topic

2

You should be able to use BaseMosaic for this if your mainboard supports it (or you turn off iommu), but I actually don’t see any sense in doing so. Why don’t you just hook up all four monitors to one gpu?
Having all 4 monitor be recognized as one is a bit tricky, depending on window manager used. The modern way would be using randr monitor objects but rarely any WM is supporting this. The second, older method would be disabling the randr extension in the Xserver and then use a simple WM which doesn’t rely on it.