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Old 07-09-07, 05:25 AM   #1
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Default Serving two users with one GPU?


I'm using a GF 7600GT with two connected monitors, amd64 smp, kernel 2.6.20, xorg 7.2, driver version 8776 (gentoo stable).

I'd like to be able to have two persons using one machine, each getting his own mouse, keyboard and monitor.
The computer is usually configured for TwinView, which works fine and everything, so there are no apparent flaws in my system or hardware.

At first I tried configuring my card without TwinView (getting :0.0 and :0.1). Configuration worked, I had two seperate screens, both running a copy of my desktop environment. Well, both screens being run under just one user account isn't perfect, but wouldn't be a huge problem either. It's just that I still had only one mouse pointer (which wasn't limited to one screen) and only one keyboard focus.
I didn't find anything on 'man xorg.conf', it's either 'send core events' or 'be completely ignored by all my applications', but I couldn't find a way to assign an input device to a specific screen.

I guess there's a '1 X server = 1 person' policy somewhere (which would be reasonable), so my second attemt was to start two instances of xorg, one screen on :0 and the other on :1.

well, after creating two ServerLayouts and everything, both worked individually, I could even start both X servers simultaneously. But only one screen was visible at a time, the other was always blank and didn't accept keyboard or mouse input. I had to switch between them using Ctrl-Alt-F7 / F8.
My xorg.conf from this attempt is attached, but I tried so many different variations that it seems pointless. Please note that I did not yet attach a second keyboard, since I currently don't have a second one, but I doubt that's the problem (or is it?).

I noticed that glxgears running on one X server will still continue drawing while switched to the second X server, so there doesn't seem to be a problem with simultanous access to the GPU.
The problem seems that each X server will use it's own Ctrl-Alt-F? slot (VT?), thus blocking input and blanking output when switched away.

Is there a way to get this working as I intended?
Would installing a second GPU help? (I doubt it, but I do have an old 6600GT around)
Or is even that useless, as linux will only allow one active X server no matter what?

thanks for reading,
Attached Files
File Type: txt xorg.conf.txt (2.3 KB, 133 views)
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Old 07-09-07, 09:49 AM   #2
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Default Re: Serving two users with one GPU?

The situation is that the driver performs a GPU context switch when
changing to different X-servers on different VTs. Means: you can't
have two X-servers simultaneously generate output, only one can
be operational at a time.

Unfortnuatly, there doesn't exist an OOTB solution for what you
like to achive. A somewhat simple solution would be to define
two seperate screens and to start a Xnest-Server (or Xepher)on
screen 0:1. However, Xnest/Xepher would need to be extended to receive
keyboard and mouse input events from external devices rather
than taking forwarded events from the accelerated X-server. This
is probably just a couple of days of implementation effort, but
the drawback is that the Xnest screen wouldn't have 3D acceleration ...



Edit: just run into that one:
Seems that somebody already did the necessary modifications to Xephyr.
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Old 07-09-07, 11:02 AM   #3
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Default Re: Serving two users with one GPU?


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Old 07-09-07, 01:41 PM   #4
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Default Re: Serving two users with one GPU?

thank you, "multiseat" was an excellent keyword for further research.

unfortunately, starting the second X server on the same card kills the first one in a bad way (no matter if NoInt10 is set or not), resulting in a garbled screen when you try to touch or exit the first server after the second started.
Is this a driver bug, an xorg bug or just a result of me shooting myself in the foot by using experimental configuration options (i.e. a bug noone cares to fix )?

As per comment 4.1 from Chris Tylers on the blog entry you linked, what I'm trying is not (yet) possible on a single GPU. I'll need to recalculate my power consumption and check if my PSU is strong enough before mounting the second card though, so I cannot yet report on any results.

Xephyr would work if it weren't for the fact that I need 3d acceleration. Honestly, the most frequent use case of this setup would be multiplayer gaming with friends. I don't need a second workstation, since noone else works here.
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Old 07-09-07, 03:45 PM   #5
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Default Re: Serving two users with one GPU?

Concurrent use of multiple X-servers talking to the same nVidia card is not supported by nVidia's driver. You can have two X-servers talking to the same card, but you just can't have both of them active at one time, per Zander@nvidia

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