Arch Linux

alexandernst

Member

Registered: 2010-04-02

Posts: 61

4 monitors and 2 identical GPUs

I have two NVIDIA GT610 cards on a ASUS X79 Sabertooth board and they are not SLI connected (they actually don't even support SLI). I also have 4 monitors, 2 connected to each card.

Theoretically speaking, this should just work. But reality is another thing.

I just found that NVIDIA removed +3 monitors support in basic mosaic mode (bit***s!), so that leaves me with nouveau.

Is nouveau able to use 2 GPUs and 4 monitors at the same time? And all that, in the same X server (so I can freely drag windows from one monitor to another).

If not, what else can I do to use my 4 monitors?

Btw, right now I'm using nouveau and 2 of my monitors are working fine, but  the other 2 won't even list as available in xrandr.

This is the output I get when listing providers:

[alexandernst@stupidbox 0s ~]$ xrandr --listproviders
Providers: number : 1
Provider 0: id: 0xbe cap: 0x7, Source Output, Sink Output, Source Offload crtcs: 2 outputs: 3 associated providers: 0 name:nouveau
[alexandernst@stupidbox 0s ~]$ 

And this is lshw output:

[alexandernst@stupidbox 0s ~]$ sudo lshw -numeric -c video
[sudo] password for alexandernst: 
  *-display               
       description: VGA compatible controller
       product: GF119 [GeForce GT 610] [10DE:104A]
       vendor: NVIDIA Corporation [10DE]
       physical id: 0
       bus info: pci@0000:01:00.0
       version: a1
       width: 64 bits
       clock: 33MHz
       capabilities: pm msi pciexpress vga_controller bus_master cap_list rom
       configuration: driver=nouveau latency=0
       resources: irq:57 memory:fa000000-faffffff memory:d0000000-d7ffffff memory:d8000000-d9ffffff ioport:e000(size=128) memory:fb000000-fb07ffff
  *-display
       description: VGA compatible controller
       product: GF119 [GeForce GT 610] [10DE:104A]
       vendor: NVIDIA Corporation [10DE]
       physical id: 0
       bus info: pci@0000:02:00.0
       version: a1
       width: 64 bits
       clock: 33MHz
       capabilities: pm msi pciexpress vga_controller bus_master cap_list rom
       configuration: driver=nouveau latency=0
       resources: irq:60 memory:f8000000-f8ffffff memory:c0000000-c7ffffff memory:c8000000-c9ffffff ioport:d000(size=128) memory:f9000000-f907ffff
[alexandernst@stupidbox 2s ~]$ 

Why is xrandr not detecting my second GPU?

Last edited by alexandernst (2015-02-07 15:33:55)

alexandernst

Member

Registered: 2010-04-02

Posts: 61

Re: 4 monitors and 2 identical GPUs

Reading my X log I realized that the second GPU gets removed -> https://paste.kde.org/peoyuxfam/lrspo4#line-755 but why?

alexandernst

Member

Registered: 2010-04-02

Posts: 61

Re: 4 monitors and 2 identical GPUs

Hold on! I got xrandr to show 2 GPUs!
I had to remove "video=vesa:off", "vga=...." and all related kernel parameters from GRUB.

Now, the problem is that KDE5's display kcm module isn't letting me configure the second GPU.

alexandernst

Member

Registered: 2010-04-02

Posts: 61

Re: 4 monitors and 2 identical GPUs

I discovered that  Fedora 21 (Xorg 1.16.1, xrandr 1.4.3) is able to automatically detect and use all my 4 monitors, but Arch (Xorg 1.16.4, xrandr 1.4.3) isn't! Debian 8 testing isn't able to do that either.
How can I discover what kind of dark magic is Fedora doing?

alexandernst

Member

Registered: 2010-04-02

Posts: 61

Re: 4 monitors and 2 identical GPUs

Turns out that Fedora was patched with http://pkgs.fedoraproject.org/cgit/xorg … atch?h=f21 and that is why all my 4 monitors were working correctly. This patch isn't available in mainstream X until 1.17.
It was merged 13 months after it's proposal: https://freedesktop.org/patch/14207/

Anyways, seems that when 1.17 is packaged for Arch I'll finally be able to use my 4 monitors

alexandernst

Member

Registered: 2010-04-02

Posts: 61

Re: 4 monitors and 2 identical GPUs

As expected, everything works with Xorg 1.17.
After running `xrandr --setprovideroutput 1 0`  (you may need to change the numbers according to the output of `xrandr --listproviders`) I can see all my 4 monitors in KDE's monitors tool and I can arrange them.

Anyways, that's all the good news I have. Bad news: It turns out that what this does is basically render everything using only one of the GPUs, and then "split" the rendered image in two and send one of the parts to the other GPU (so that other GPU can then send that part of the image to the monitors).

As you can imagine, this is rather slow and you need a really good GPU to render everything on all 4 monitors.

And that's why I just ended up ditching the dual NVIDIA cards and got a single NVIDIA GTX750 (which has 2 x DVI and 2 x HDMI) and switched back to NVIDIA's binary driver.

I hope this can help anybody that would possibly get here from a search result

johni

Member

Registered: 2012-02-03

Posts: 102

Re: 4 monitors and 2 identical GPUs

So, does this mean the nvidia binary drivers don't force you to only 3 monitors as long as they are one card?

alexandernst

Member

Registered: 2010-04-02

Posts: 61

Re: 4 monitors and 2 identical GPUs

Yes, that is correct. NVIDIA's driver doesn't limit the number of monitors per card, but it does limit the number of total monitors in a multi-gpu setup to 3.
I'm not sure if that limitation does exist also for muli-gpu in SLI mode. It would be nice if somebody with such a setup could post here.