VRay RT Slows down with Very High Render Resolution

[jandrewg]

I was doing some tests at work comparing Vray RT to Vray Advanced and I came across this.
I had a simple scene, 100,000 polys, with no lights, a default Vray Mtl, and GI Environment turned on.

Render Times
_Resolution _V-Ray Adv _V-Ray RT
3840x2160 2m53.9s 1m50.0s
7680x4320 10m52.5s 9m42.4
15360x8640 42m59.0s 92h59m24.3s

System Specs
3ds Max 2018 with V-Ray 3.6 (I also tested on Max 2016 with V-Ray 3.54 and got largely the same results)
CPU Xeon E5-2643 3.3ghz
RAM 32 GB
GPU PNY Geforce 1080 founders edition with 8gb of vram

[radoslav.platikanov]

Did you use bucket or progressive sampling with V-Ray Adv?
With progressive sampling and RT, rendering a 15360x8640 resolution without any additional render elements will take more than 11 GB of memory just for the sampler, and more if you have render elements. Most likely you've run out of RAM, which would make the rendering extremely slow.

[jandrewg]

Thanks for the fast response. Yes you were correct I was using bucket for Advanced. Would using hybrid help this out at all or would the solution be to have a videocard with more memory?

[jandrewg]

I did some more testing. I didn't realize before when you said running out of RAM you meant system RAM (which is exactly what it was, my misunderstanding). I also forgot to mention that I had denoiser on which was cranking up the memory requirements to well over 30gb which of course was why the render times were exceedingly slow (I assume it was caching information that couldn't fit into memory onto the HD). I turned that off and it appears to be much better now. Again, thanks for the help.

[radoslav.platikanov]

Doesn't matter if you use CPU, GPU or hybrid. The GPU memory is generally not relevant to the rendering resolution as the sampling and frame buffers are stored in the system memory.
In your case, 32 GB RAM is probably not enough for such large resolutions and progressive sampling/RT.

[jandrewg]

You the man Radoslav. Thank you so much again. Luckily we are upgrading shortly. I'll make sure we have plenty of system memory.

[radoslav.platikanov]

Also keep in mind that if the denoiser is updating during rendering (post effects rate more than 0), it will probably make things very slow by itself, denoising large resolutions takes time.