High Processor Count Kills Caustics Speed

V-Ray V-Ray for 3ds Max Issues
1

Anyone else notice that FEWER processors seems to calculate caustics MUCH faster than machines with a lot of procs (like 32-96 procs)?

Is there anything that can be done about this to limit the number of CPUs specifically for caustics? Thanks.

Note: It would be EXCEEDINGLY helpful to have something like VRAY_NUM_THREADS​ that applies only to caustics photon mapping calculations. 96 core machines are reduced to slower than a 7950X without it.

2

Hi, thanks for posting.

Anyone else notice that FEWER processors seems to calculate caustics MUCH faster

Are you using progressive caustics? If you are that is a limitation that the progressive caustics may have in some scenes.
From the docs:

  • Depending on the scene, the performance might not scale linearly with the number of threads/cores, resulting in inefficient CPU utilization.

​

3

Thanks for the reply, Vladamir.

Actually, this is Photon mapped caustics. I was avoid progressive because it says you should use a time limit, and we have a heterogeneous farm; so it is not possible to set a time limit that would be consistent during an animation rendered with different types of nodes.

And what I am seeing is that caustics photon calculation in scaling INVERSELY proportional to the number of cores. More cores == slower performance than few cores.

4

Hi @Joelaff

What CPUs are you using?
I’m comparing the 64-core 3990X and 7950X, the scaling between the 2 processors seems to be ok on my side. What are your Caustics settings and sampler settings?

Best,
Muhammed

5

Comparing a 7950X with AMD EPYC 9R14 Processor​ for instance. The beast take more than 4x as long, if not longer, than the gaming CPU.

It seems like the real slowdown begins around 32-64 cores on up. The more the cores the longer it takes.

Caustics settings at default. Bucket rendering, Photon mapping.

6

Hello,

I’m testing on Threadripper 3970X and 3990X, the slowdown seems to be minimal so far
I’m comparing to 5950X and 7950X, all of the CPUs I have access to locally. I will ask my colleagues to test on EPYC, I think we have one machine at office. We will update you shortly
If we still cannot reproduce, we will ask for a scene.

Best,
Muhammed