Hybrid rendering slower and noisier than GPU only

[Alex_M]

I had already mentioned this in another thread a week ago but I thought I should make a separate one about it to keep things more organized and easy to find. Here's essentially a copy-paste:

I did various tests with the hybrid CUDA rendering and I saw some pretty bizarre things. Here are my findings from the tests I did:

1. Render times didn't improve in any of the tests I conducted. In all of them the render time was either the same or longer. In one of the scenes (an interior) the render was 1.7 times longer.
2. I get noisier images when I add the CPUs to the mix and I didn't change anything in the render settings.
3. The more CPU render nodes I add to the rendering, the slower the rendering is.

Note: The CPU was not busy with other tasks while rendering with Vray.

I think there's something wrong with my setup or I missed some steps in order to make hybrid rendering work correctly? I have emailed some of the scenes to Blagovest Taskov and Peter Matanov for inspection but it happens with any scene.
Here are some screenshots:

[savage309]

We have those scenes and we have been able to reproduce the results with them here, thanks.

Best,
Blago,

[Alex_M]

Very interesting. It would be less strange if it happened in a specific scene only but so far every scene I have tested is slower and noisier with hybrid and I have tested a wide variety from studio shots to interiors and exteriors. I read people saying their render times have increased. Any guess why this might be? Maybe something to do with my particular software/hardware setup?

[super gnu]

ive seen some minor improvements on certain scenes and increases on other scenes..

[savage309]

We haven't had a scene with slowdown before the release, the same goes for all the beta users. But the use cases and the scenes can vary a lot. We are looking into the scene that you have provided.

Best,
Blago.

[GregCooper]

Thought I'd add to this. I've never had a scene that renders faster with Hybrid rendering than without. Any time I've tested it using my 4x 1080ti's its always faster than 4x 1080ti's + CPU (12core threadripper). Is this somehow faster when you have less GPUs maybe? I generally just don't use hybrid rendering because of this.

[savage309]

CPU has to feed the GPUs with data. If they are very fast (and many) compared to the CPU, it might be the case that there is close to no speedup.

[Micha]

And if your CPU needs to simulate the GPU Cuda mode, than it is not so efficient and the CPU power is missing to feed the GPU. So, could it be that your GPU doesn't run at 100% anymore if you let the CPU work in Cuda mode?

[glorybound]

I reported this almost immediately after Hybrid rendering was released.

[GregCooper]

Right thanks Blago, I had wondered if it was something like that.

[^Lele^]

And if your CPU needs to simulate the GPU Cuda mode, than it is not so efficient and the CPU power is missing to feed the GPU. So, could it be that your GPU doesn't run at 100% anymore if you let the CPU work in Cuda mode?

https://forums.chaosgroup.com/forum/...776#post957776