Wayland + Multi GPU

[nPHYN1T3]

This is a tad "taking the back road" but what I've read is Cosmic will be Wayland only. Last I was able to find any info on things Wayland "allows" Multi GPU about the same way you can go sky diving without a parachute i.e. you can do it, but it doesn't workout so well. So will Cosmic + Wayland allow proper multi GPU (where each GPU runs their own screens and workloads) or will it do what I last heard about Wayland which is dump all rendering on a "primary" GPU and turn all other GPU's into idle power wasting, heat spewing output splitters that get passed frame buffer data by the now over taxed "primary" GPU?

[mmstick]

There is no primary GPU in COSMIC. It will do as described.

[nPHYN1T3]

I described two different scenarios of operation so which one is it? I spoke with a Wayland dev a few years ago and his explanation of multi GPU on Wayland was that having a machine with 4 Titan cards would perform the same as 1 Titan and 3 ATI 8Mb 3D rage cards. I see discussions on how to select or set primary on Wayland through various udev hackery and silliness but nothing on how to choose GPUs.

Under X DISPLAY=:0.X denotes the GPU via it's assigned XScreen so how will this work in Cosmic that differs from what I've been told about Wayland in general. If there is no "primary" then does Cosmic try to lump all GPU's into a collective workload like xrandr providers or an Xinerama setup which will annihilate performance? (Performance might be OK if all GPU's are matched, that is a scenario I've not been able to test.)

[mmstick]

As above, there is no primary GPU. Displays connected to GPU1 will be rendered by GPU1, and displays connected to GPU2 will be rendered by GPU2. The compositor handles multi-GPU environments.

[nPHYN1T3]

OK but again so how do you say Hey blender run on GPU3 and hey Inkscape run on GPU 1 then? Is there a mechanism to do this with Cosmic that is unique to Cosmic or is there a Wayland equivalent to running with DISPLAY=:0.3 and DISPLAY=:0.1?

I know this is perhaps on the line of not being Cosmic specific, as I said initially this is a "back roads" way of asking since this might be more Wayland questions than Cosmic but again my discussions with Wayland Devs all seemed like the old ways are being ditched with no equivalent built into the shiny new Wayland. However I've been curious about COSMIC since most DE are K or G based and GTK nuked multi GPU (XScreen enumeration) way back around v3.1.

[mmstick]

By default, the GPU that would be selected is based on what GPU is driving the display that is focused. Otherwise, it's based on certain environment variables set before running the application. Right clicking the app icon in the dock or app library will show alternative GPUs. Similar can be seen with switcherooctl.

[nPHYN1T3]

Well I just grabbed the iso to test rather than speculate but it's a no go. Default drivers (nv555) are broken on my GPU array, can't open (or even find) display settings, term won't launch, file manager won't launch, text editor won't launch hah. The only things that worked was Firefox launched and the nVidia settings. So I could see my GPU use but while Firefox was open (default settings are for HW accel) it was a slide show despite GPU use being low.

Given I could drag the Firefox window between screens / GPU's I'm guessing the compositor is tripping up over how to switch rendering when one GPU has x to z features but the other doesn't. This is still kinda the xrandr providers issue but with less overhead. I still don't know how this "clumped" approach will ever work unless all the cards are matched but what do I know. It was just a question ahead of the alpha test release, I've tested, for the 0.00000000001 others out there running MultiGPU that might be COSMIC curious maybe this helps someone...for now it still seems like the "clumped" approach while nice and seamless is flawed by latency and differences in 3D acceleration features a card may or may not have compared to another card it's rendering with. The segregated XScreen per GPU is the only way to ensure each card does what it can and doesn't try to clumsily hand off to the others...as far as I've found. It's nice COSMIC has the dedicated GPU selection UI but it seems like Wayland kinda undoes the point of that.

[zacpod]

Did you ever get this working? I'm having the same issue with 2 GPUs (3080ti and 4060ti)
I get desktops on the 4 monitors plugged in to the 3080, and just a black screen with a functioning mouse cursor on the screens on the 4060.

[nPHYN1T3]

Unless the Wayland team addresses the lack of feature pairity to X for MultiGPU this will never be fixed. (I had hoped Cosmic was doing something to address this but they clearly aren't.) Given there isn't a ton of multiGPU users out there (other than for Ai/ML/Comp/Mining all of which don't use their outputs) this will probably never be addressed.