2x ATI 5670 & 3 Monitors - How?

[Kano]

Well don't forget that you would need DP then for some...

[bridgman]

Yep, but adapters are pretty easy to find for normal resolutions.

My admin system showed up with a PC that only had VGA and DP connectors, along with a display that only supported DVI and HDMI (my fault, I asked for a bigger display ). There was a bit of cursing when I unpacked it and started plugging in cables.

I borrowed a passive DP-to-DVI dongle and everything seems to work fine.

[rohcQaH]

Hopefully Wayland will give Linux the edge it really needs. X is antiquated.

I love the uninformed "Wayland will make everything better!"-hype

Xinerama is a very old attempt at multi-GPU-rendering from the last millenium, where multi-GPU cards didn't exist yet. It was designed to work in the use cases common during that time.
Basically your desktop is divided by monitors, and each GPU is responsible for all drawing operations that occur on it's own monitor.
This works well in cases where your applications are mostly restricted to a single monitor, but still allows you to drag a window to a different monitor when needed - the application and all it's textures and pixmaps are moved to the other GPU and it continues to run there.

There are limits to the solution Xinerama chooses: applications spanning multiple monitors, like fullscreen games or even a window sitting between two monitors. Now all rendering commands must be performend by both GPUs. Each GPU will only render half the window, but fillrate is rarely the bottleneck. Using Xinerama, these use cases are always *slower* with 2 GPUs than with one, which somewhat defeats the purpose of a modern multi-GPU system.

Then along came compositing: now your application does not live on a monitor any more, it's just a texture somewhere in video memory, waiting to be copied by the compositor. Which of the GPUs should be responsible?
And the compositor is a fullscreen application with all the caveats listed in the previous paragraph.


Xinerama was never meant to satisfy these use cases, it is the wrong solution, and it's not surprising that nobody really cares to maintain it or to fix bugs that arise from compositing. In fact, Xinerama is officially deprecated.
(that said, your specific bug is already fixed.)


Back my favourite topic, Wayland. Wayland will magically solve everything, bring world peace and give us free puppies.
Unfortunately, it doesn't even have a multi-GPU solution. None at all. Works even less than Xinerama.


Yes, linux needs a good multi-GPU solution that satisfies modern needs, but this is anything but trivial. There have been some attempts to hack something up, but it's a lot of work to get all the corner cases right, and AFAIK it's not high on the priority lists of any major contributor.
It's likely that any multi-GPU solution would end up somewhere in the DRI stack, thus outside of X or Wayland. If a multi-GPU solution arrives in the future, it can be expected to work equally well for both display systems.


Now you know why your specific setup cannot work as well as you hope, why it won't work as well as you hope for the near future, and why wayland really shouldn't give you any hope at all. Did I manage to spread some christmas cheer


(seriously: get an active (!) DP->DVI adapter and toss out the second GPU, it'll make things way easier for you.)

[gtrawoger]

@bridgman: My cards are two Sapphire HD5670's, with 1x D-SUB, 1x DVI-I and 1x HDMI out. Not sure if I can make a Display Port setup on that work. Then again, I got these cards for $75 each, so I am not complaining.

@rohcQaH: Boy, you sure know how to crush someones dreams.

Just kidding. I do appreciate the informative response. I kinda know that Wayland isn't even close to being ready to replace X. I read up on Xinerama as well with all this, and noticed that even at it's height it was just a stopgap solution.

But my comment about Wayland was more directed at the hope that if the developers are going to go through all this work, they may look at this issue and find a more elegant solution. Here is to hoping! Maybe 5 years from now I get to have 3 monitors and compiz at the same time.

[bridgman]

Yep, you're right. I was about to type something along the lines of "I thought all the Evergreen cards had a DP connector" but then I remembered that I picked exactly the same card (Sapphire 5670 with VGA/DVI/HDMI) for my home system.

Guess you can ignore everything we said about display port adapters

[rohcQaH]

oh yeah, most single-slot cards forfeit the DP connector due to space constraints. Because VGA just won't die

In that case, a single HD5750 + DP-adapter may be the solution you're looking for, if you can still return or re-sell the 5670s.

[gtrawoger]

Well, considering that I got it working (albeit without compiz) with the open source driver and xinerama I am going to leave it for the time being. If I can figure out a way to make use of these two cards while getting a DP setup I will do that.

Thanks again for all the help.

[gtrawoger]

Hmmm... looking over the boxing week flyers this deal I got for these two doesn't look so good anymore. Especially if I could get all three monitors running on one card with better performance.

[gtrawoger]

Ok, after an endless array of crashes I decided to try the eyefinity setup. Went out and got a 6850, with an ACTIVE display port to DVI adapter (important!).

What can I say... It's wonderful! Works flawless and got the desktop effects back on.

So, if you are interested in more than 2 monitors, eyefinity is the way to go. Avoid Xinerama.