GPU using CPU as well?

[padre.ayuso]

Hi there, I thought there was another thread but could not find it. When rendering a scene with GPU (two GTX 970s) I downloaded NVinspector to see what my GPUs were doing. I put my scene to render with GPU at 3000 HDTV, and it froze. Now I could see that my GPUs were sitting there, idling away, but my CPU was using all 24GBs of memory it had and thus reached limit and got stuck. I'm using VRay Displacement Mod for the grass and when rendering at 800 HD it worked fine, just at 3000 it did not.

I saw also another post where it says that GPU rendering can do any size and it will not affect the performance of the GPU, but why is it affection the CPU? Me not understand...

Thank you!

[savage309]

Can you share the log (or even better, the scene) ? I suspect that the view-based displacement that we do on the CPU before we send it to the GPU for some reason takes too long and your GPU render is not starting at all (probably you are out of CPU/host memory and your machine has started to swap) ?
Best,
Blago.

[padre.ayuso]

Thanks Blago! I put it on the dropbox, I just need your email address to give you access.

Best
alex

[savage309]

It is blagovest.taskov@chaosgroup.com
While we take a look, please make sure that you are not running out of CPU memory.

[padre.ayuso]

It is blagovest.taskov@chaosgroup.com
While we take a look, please make sure that you are not running out of CPU memory.

Hi Blago,
I added your email address to drop box. Now, if I understand what you are saying, yes, I'm running out CPU memory, and so GPU never starts. But I only run out of CPU memory when I send on GPU, not when I send it as a CPU rendering, if that makes sense. So my question is rather, why would it use all that CPU memory to run on GPU? I have rarely had a scene run over 15GB of memory usage and while I have not used much GPU rendering, I'd think it uses the GPU not the CPU, anyhow, this is where I think I'm getting lost.

[savage309]

It happens that RT GPU will require more CPU memory compared to using Adv / RT CPU, because we need to prepare additional stuff for the GPU. Using BF/BF it took ~19GB of CPU memory (peak) to start the GPU render and I run out of mem (24GB) if I start building LC.
It is pretty much expected, but I will make a note to double check that it is not a bug, just in case.
Best,
Blago.

[padre.ayuso]

It happens that RT GPU will require more CPU memory compared to using Adv / RT CPU, because we need to prepare additional stuff for the GPU. Using BF/BF it took ~19GB of CPU memory (peak) to start the GPU render and I run out of mem (24GB) if I start building LC.
It is pretty much expected, but I will make a note to double check that it is not a bug, just in case.
Best,
Blago.

I figured it was not a bug, but I can seem to understand now where my GPU problems come from when trying to do big scenes. Interesting that it takes less memory to use BF/BF than when using LC and I did not think of that, but for an exterior scene, I supposed you don't really need LC. I even like the response in smaller rendering settings for GPU but I do need higher settings, so if this is something that can be improved it would be awesome. That scene, is a small one compared to some of the scenes I have to do, so I barely use my GPU except to test lighting and materials at times, which does speed up the process I admit. Having said that, thanks Blago for taking a look at this.

[savage309]

It is mainly because of the heavy displacement used for grass, LC on top of that can use a lot memory. If you turn off any of them it will use much less. Probably more efficient (and generally better?) way to the that will be using V-Ray Fur. It will be supported in RT GPU in V-Ray 3.30 (both the betas and the stable build that is coming soon).