Author Topic: Iray is much faster without CPU  (Read 2406 times)


Having just participated in the X-TAON challenge, I ended up with some fairly long rendertimes inside Painter with Iray. Was in a bit of a time crunch, so didn't have time to investigate, but now I have after the end of the challenge.

My Iray was set to utilize both CPU and GPU as standard, but I notiched that only my CPU was working when rendering. CPU load was at 100% while GPU load was in the area of 1-10%. My rendertime for a 4K image was just shy of 50 minutes.

I unchecked the CPU in the Iray settings and did another render. Naturally no CPU load this time, and the GPU load was still very low when looking at the monitor, but my rendertime got so much faster. The same image at 4K was now done in 8½ minutes. I've compared the renders and they are of equal quality.

My system info:
CPU: AMD Threadripper 2950X
GPU: Geforce RTX 2080 8GB

Render settings:
Resolution = 3840 x 2160
Min samples = 5
Max samples = 2500
Max time = 0
Caustic sampler = off
Firefly filter = on

I don't know if this is a bug, a hardware issue, or I'm just missing something. Can't seem to find much info on the subject but to me it has proved to be quite useful knowledge, so maybe others can benefit from this as well =)

IMPORTANT EDIT: Did some more testing, and at first it seemed to be fine, but now I'm getting very inconsistent results. For some of my test the GPU didn't seem to kick in at all, no matter what the Iray settings were. Now I have changed the program settings for Substance Painter in my Nvidia Control Panel, checking the CUDA CPU setting to always use all my GPU's (even if I only have one). It was previously set to use the global option (which was also set to use all GPU's), but this seems to have done the trick. Now it's working with both CPU and GPU checked in the Iray settings.

Will update further if this changes.
Last Edit: December 20, 2018, 01:03:14 am

Indeed, Iray performs much better when only the GPU is enabled. Mostly because you don't have to do memory transfert and share computation between the CPU and the GPU. Also GPU in general perform much better at rendering things. ;)
Don't forget your log file. It can be exported from the Help menu of the software.
Fabrice Piquet aka Froyok. Product Manager, Technical Artist and Documentation at Adobe.