What can cause a stuck lightcache?

[Neilg]

I have a scene I have been battling with for 2 days now and I cannot get it to render. Tried locally and now on an ec2 instance and i'm getting nowhere.

It gets through a few lines worth of lightcache at the top, then cpu usage drops off a cliff and it does nothing. It spent 12 hours like this on a local machine last night.

Displacement is turned off, there are no sss materials, the scene geometry has been optimised with proxies bringing the max file down from 700mb to 100mb, every single bitmap in the scene has been reduced to 1,000px for testing, I have tried converting all the maps to using the HDRI loader. same thing every time.
This does not happen with an override material.

e: also forgot to say, original scene had a forest pro rug in it - deleted that long ago. This is a somewhat straightforward yet detailed interior shot of a living room. Not even remotely complicated scene and the kind of thing I would expect vray to chew through in a matter of hours. Every single asset in it was either modeled from scratch or bought & checked in great detail, we intentionally tried to keep these files as clean as possible.

Any other suggestions of things to try?

[Morbid Angel]

Its most likely bad piece of geometry which is getting either gi ray or reflected/refracted ray precalculated with lc map stuck. I recall some issue like that long time ago, but it was fixed. I would check a few things - try rendering BF/BF. If that works then render with lc and retrace off and leak prevention off and use lc for glossy rays off.

[Neilg]

Yeah it's odd, it's been multiple years since i've seen anything like this.

BF/BF is unsurprisingly fine, but too noisy/slow.
I have had a small test going and after 16 hours stuck on the same part of the light cache, it broke free and finished the rest of the image in a normal time for how low res it was.
We have 19 images to finish, a 16 hour delay before each will start rendering is not going to work... There's one part very early on where CPU usage plummets to 2% and it takes forever to resolve whatever is the cause.

Switching off retrace, leak prevention and glossy rays did not help unfortunately.

[Morbid Angel]

Can you send scene to investigate or is it too complex?

[tricky]

Have you tried a simple override material to check if that improves things?
Switch off all lights and see if that helps.

[Neilg]

Have you tried a simple override material to check if that improves things?
Switch off all lights and see if that helps.

With a grey override it doesnt happen - renders in minutes.

Switching off reflections/refraction and glossy rays does not help, it still gets stuck.
Switching off all maps and map filtering does not help, it still gets stuck.

I'm now making multiple versions with chunks of geometry deleted.

[Neilg]

Something incredibly dumb happened to this file. An object had a blend material in which i'd switched the top layer off. At some point, that top layer had ended up with a raytrace material in it.
3 file versions back and it's not there.
The file is now working, but the mystery has changed course and I need to find out how this got here.

Appreciate everyone who threw out some ideas. Cant believe how much of a headache this gave me. If only i'd bothered to render locally and read the messages...

[themaxxer]

I can only recommend "forensic" (it's free) from SiNi software to get rid of most of the problems in a scene... because sometimes it's a standard material or no material on a object...

best regards
themaxxer