Why is Cycles' viewport slower with a second GPU?

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1

I’ve been running blender with a single 3090 for a while and I wanted faster viewport resolve, with and without denoising, for heavier scenes.
I then when and got a still very expensive 2nd 3090 thinking my problems would be solved.

To my surprise, viewport is SLOWER. It seems there is a distinct lag or delay that was not present with the single 3090. If I activate OptiX denoising then, oh boy that is the worst. Very slow resolve, with a few artifacts here and there.

I have both GPU ticked under the OptiX tab.

Is this supposed to happen with Cycles?
Does Cycles not support multiple GPUs?
Do I have to link the GPUs with nvlink so it behaves properly?

So many questions…The gpus are running fine as I tested them with Octane. So this is absolutely something related to Blender/Cycles alone.

Anyone willing to enlighten me here?
And if not here, is there a better place to ask for this kind of technical issue?

2

AFAIR there was an additional setting about render and viewport using multiple GPU ?? (I don’t have multiple GPU)…
Or even https://docs.blender.org/manual/en/latest/render/cycles/gpu_rendering.html pre-last paragrph??

Maybe the speedup is eaten up by feeding both GPUs for your special scene ?

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3

I’m running 2x RTX 2080 Ti on my Workstation, with no NVLink and most of the time, i dont even need denoiser in viewport, everthing works well
My rendercube works with 2x RTX A6000 + NVLink and same. Cyles = Realtime.

Do you tried to run the new/second GPU solo in Blender?
Newest Driver? Any Addons?

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4

i think that could be it…maybe I needed 3 gpus so one alone is exclusively for the display.

Do you know if the tiles part affects viewport performance or just final rendering?

5

weird. maybe my motherboard is not enough for the 2x 3090s then.

What motherboard do you have for the dual A6000?

6

To be honest: i even never heard about viewport performance with more than one card… cylces does support multiple GPU… yes for rendering… :face_with_raised_eyebrow:…

Of course the second slot for your GPU should also support 16xPCI speed… i think :thinking:.

7

with dual gpu my motherboard does pcie 4.0 x8 instead of 4.0 x16. But the difference should be negligible according to benchmarks and reviews I found online. damn…

8

But it’s half speed… !! Maybe also on the same channel… ?? Barely counterbalanced by a second GPU…

9

The bigger machine has a server Motherboard with 2x CPU. I’m in homeoffice and my server is in the company. The board is not the newest one, we upgraded just the GPUs last year and put the “old” one in my workstation.
But in viewport, there are running both without any problems or taskmanager is lying to me and the numbers say they are 95% sleeping :wink: