CPU running at sub 100% while rendering in Arnold

[KyleG87]

Hi guys. So I'm running into a bit of a weird issue here. My Arnold renderer within Maya is running normal most of the time, but on this project I'm working on, I've got a shader where I've got many masks and diffuse textures etc plugged into it. Now, it renders fine, but as soon as I plug displacement maps into it, Arnold runs very slowly and does not actually render with full CPU usage. Now conversely, if I plug the displacements into a fresh aiStandard Shader, Arnold runs fine as well, utilizing 100% CPU usage. It's only specifically when I'm plugging in displacements to this shader (with a hefty amount of 8k textures already plugged into it) that I'm getting this issue. Is this normal? I understand longer render times etc, but what about this would cause my actual CPU usage to run at a LESSER level? When I plug the displacements into that shader, the CPU usage runs at about 30-70% usage, where as normally it runs at 100% usage until the task is done. Anyone know if this is normal? Thanks.

Images here: summary of node tree I've got going on. Those textures and masks are being used to control the diffuse being shown on the surface of the armor pieces on the model. The image shown rendering at 100% CPU usage is without displacements being plugged into that shader. The one where it's rendering at 47% CPU usage is with the displacements plugged into it. And to reiterate, the problem is being caused by a combination of the shader + the displacements. The shader without the displacements renders fine at 100% CPU usage, and the displacements alone plugged into a fresh aiStandard Shader render fine at 100% CPU usage. It's only when I plug the displacements into that specific shader am I getting this weird problem of sub-100% CPU rendering.

[KyleG87]

Nobody's seen this?.......

[mspeer · Accepted solution]

Hi!

There are several tasks and computations that are required to prepare a certain frame, that may throttle the CPU load, like disk I/O, animation, dynamics,..

and

- creating the final high resolution geometry for a lower geometry mesh based on a displacement map.

Based on the time needed to "prepare" a frame in the first step and the time needed to "render" a frame in the second step the impact on CPU load can be higher or lower. So for example, if there is a noticeable time required to create a displaced object, compared to the overall render time of a certain frame , then the overall CPU load shown for this frame will noticeable lower.

[KyleG87]

Okay thank you. So this is a normal occurrence then? I was just worried because I had tested several other projects and this did not happen. It was only when I plugged a displacement into this specific shader with many maps attached to it. I just wanted to make sure things were working as intended.

[mspeer]

Hi!

Yes, this is a normal behavior.

However without access to your project I can't check what exactly is causing this.

For further troubleshooting I recommend to disable subdivisions, replace or remove certain textures, ...

[KyleG87]

Yes, when I turn off "render subdivisions", "render displacements", it renders almost right away. Is it possible that there's something wrong with the texture files themselves? They are maps created from Mari. Maybe if they channels were set incorrectly when exported or something? Maybe the texture nodes are set the wrong way? I'm using ACES too so it's all kind of complicated. I'm not sure if this is just normal behavior because I've got like 10+ 8k maps plugged into a material, or maybe something could be going wrong with the shader network/textures/both to make it this laggy?

[Stephen.Blair]

Subdivision and displacement take time, it doesn't happen instantly. If you check a detailed Arnold log, you'll see the time spent on subdivision and displacement. 

[KyleG87]

Hey Stephen. No I get that totally. I have other projects with Subdivisions and they render pretty quickly. I've got a threadripper CPU so rendering doesn't take me very long compared to the average. With this specific project in general though there's times where I'll hit render and the renderview won't even start to update for over 2 minutes, where as I have other scenes that begin rendering with subdivisions almost immediately, which led me to believe maybe something else was going wrong.

[Stephen.Blair]

An Arnold log will show us what's happening...

[KyleG87]

Oh sorry. Okay just tried it.

[KyleG87]

It saved as a mel file? I had to put it into a zip to link it in here. Sorry I'm not the most tech savvy, lol.

[KyleG87]

Hey Stephen, I posted this in another thread too. Maybe this is the problem? I'm using ACES for my color space based on a tutorial I'm following. These are the settings I've got. Could this be causing the problem?

[Anonymous]

Try it out