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Old 07-20-06, 09:35 PM   #1
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Question Xinerama and GLX with dual GPUs, compatibility question


First of all, I'm new here, I hope this is the right place to ask this.

I'm currently speccing out a new Linux-based workstation and would like to upgrade from my current dual-head setup to three monitors. I know this works fairly easily with Xinerama, but I'd also like to have OpenGL acceleration as I do a lot of (OpenGL) graphics programming and switching back to dual head for that would be rather inconvenient.

I've read the relevant topic in the nVidia driver readme, but before dropping a significant amount of money on the planned setup, I'd like to make sure it's actually going to work.

What I'm thinking of here is two cards, one with 2 DVI displays attached, the other with 1 DVI display. As far as I can tell, the card with the 2 displays needs to have TwinView enabled. Note that I'm not talking about SLI, I know that that only supports one display.

- First of all, I can't afford two GeForce 7900s, so I'd ideally want to use two different cards for this setup, the document linked above hints that this is possible. Given current consumer-level cards, what combinations are known to work? Specifically, I was looking at the GeForce 7900GT, 7600GS, and 7300GT. Will any combinations of those cards work in such a setup? Ideal case is of course 7900GT with 7300GT in terms of speed-price tradeoff - if it'll work.

- Assuming I get two different but compatible graphics cards, and I have an OpenGL window that doesn't cross monitor boundaries, will the speed of the rendering be affected by the other card? So if I have the OpenGL window on one of the faster card's screens, will it run at maximum speed for that card, or will the slower card slow it down in any way? (compared to two identical cards, say)

- Will there be problems with TwinView and GLX/Xinerama interacting? If so, what problems, and will using 3 separate cards (7900/7600, 7300, 7300?) solve that? (I've not got any components for the new system yet, so getting a motherboard with 3 PCIe graphics slots is possible, though I'd like to avoid it as they're rather expensive)

Any help or comments are appreciated.


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Old 07-21-06, 07:44 AM   #2
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Default Re: Xinerama and GLX with dual GPUs, compatibility question


I'm running a tripple head setup for about a year using two 7800GTX
and three 1600x1200 LC displays. LCD1 and LCD2 connected to card1
operate in twinview mode. LCD3 connected to card2 is tied in with
Xinerama. LCD1 and LCD2 could also be defined as seperate screens.
Both varaints have pros and cons.

Concerning OpenGL: the window width/hight limit is currently 4096
pixels. If the window is bigger, then fallback to soft-rendering is done
(at least for me).

My first experimental setup used a 7800GTX for LCD1 and LCD2 and
a 6600 card for LCD3. I didn't have done any benchmarks at that time,
but i had the impression that replacing the 6600 with a second 7800GTX
didn't made a big difference for UT2K4: for this game, the CPU is
typically the bottleneck. Even if you can use a slower card for LCD3,
i would recomend buying the same card as used for LCD1+2, because
then you can also use an SLI setup in case you need it.

Concerning overhead because of xinerama: i once did some benchmarks
and IIRC, a 2560x1024 twinview setup using only the first card was
about 20% slower than a 3840x1024 using both cards with xinerama.


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Old 07-21-06, 10:59 AM   #3
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Thumbs up Re: Xinerama and GLX with dual GPUs, compatibility question


Thanks for your input, it's reassuring to hear that it'll work. I assume that since it worked with a 6600 and a 7900, a 7300/7900 combination will work too. I might upgrade the second card at a later stage if I feel the need, as right now, that would just be too expensive.

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