memory with volumetric light cache concerns

[psanitra]

I`m rendering simple density volume with raytraced GI on the volume grid shader.

Memory usage in that case is fine, for 100mil voxels VDB it takes like 10GB.
But when i turn on volumetric lightcache to speed up things, i`m seeing unreasonable memory consumption, 10x more. Grid that takes 10GB ram with actually go to 128GB RAM and crash.
7GB RAM volume with no LC, will turn into 90GB with LC.

Is this expected behaviour? does not sound right...

[Svetlin.Nikolov]

Hey,

It would use more memory when you have more render elements. Do you have any?

Cheers!

[psanitra]

I do, but i don`t see how would volume light cache checkbox(not main LC) make the RE memory usage skyrocket 1000x more....,with all elements on it only takes 10GB as i wrote before, from that mem usage, elements are couple hundred MB. Memory increase from RE is normaly "neglectable" on HD res(in MB)...
I`m rendering with buckets btw, grid is in geometry mode.I`m talking about this checkbox:

[kostadin.petkov]

Hello psanitra,

Volume Light Cache, as you know stores the already computed lighting to be used again if needed (which happens a lot with volumes). To compute some render elements however it is not enough to have only the final lighting cached, some would need a bit more data.
For example if we have Light Select RE enabled, the Volume Light Cache would need to store not only the lighting, but it would need to know the Lights that emitted the light that reached the voxels form the volume. This could be quite a lot a data for a single voxel.
An important note is that the data cached for Render Elements doesn't scale with the resolution, it scales with number of voxels that need lighting cached. Light Cache refers to position in the 3D space not to a pixel (as something that is not in view of the camera, might contribute to the lighting and render elements).

Currently Volume Light Cache Stores additional data for Shadow, RawShadow, GI, RawGI, and LightSelect render elements, with the LightSelect RE being the biggest memory consumer, it actually consumes more memory then the lighting itself.

[psanitra]

just wow..., naturally one would expect some extra memory usage for caches, internal structures to help the stuff render more efficiently, but 100GB RAM? for volume that renders fine with 10GB? Thanks for explanation, i had no idea..

[Svetlin.Nikolov]

This is what we know on our part.

Can you please double check if turning off one by one the render elements Kostadin mentioned would make a difference, so we'd know if there isn't anything broken?

Of course we'd want to reduce this memory usage over time, but let's first make sure there isn't anything more to your case...

[psanitra]

It`s coming from RE, that`s for sure. I have basic passes + light select + denoiser, so here is how it went, 1280x720pixel

no LC and all RE 10GB
with LC and all RE 90GB

-turning off just denoiser reduced it to 80GB

-turning off denoiser and basic passes(diffuse,spec,refl,ref,gi,selfillum), down to 60GB.

-turning of lightselect, 6 different RE/lights. no RE left now, 25GB.

So turning on volumetric LC the increase in RAM is 10GB->25GB, the remaining 75GB RAM consumptions goes to handling the RE...

still wow , hope that can be significantly improved in future..., i have the same setup in Renderman with same elements, including light selects and it goes just fine with 27GB..., food for thoughts...

[Svetlin.Nikolov]

Oh yes, it can definitely be improved.

As a side note, additionally we hope that the entire volume light cache concept can go away at some point - over time, more and more scenes, especially those with more complex lighting, more occlusion, huge res grids rendering in small resolution, etc, are showing less performance gain, or even slowdowns with the volume light cache. V-Ray GPU for example does not use it at all, Corona's grid volume implementation left it out, so it's an old concept and a solution for old problems which are going to be ever less relevant.

Cheers!

[david.anastacio]

Hi Svetlin.Nikolov,

Any update on this matter? We here also ran into this issue, light cache on the volume grid shader is a ram hogging machine with large volumes. (In some cases 97gb to 12gb).

Best,
David.

[Svetlin.Nikolov]

Hey, there are two thing you can do currently:
- make sure that you are not using any render elements you dont need as they are the major ram consumer with volume light cache. Make sure to check specifically the elements mentioned above
- if you are using the very latest V-Ray or Phoenix, there are some significant speedups when you enable Probabilistic Shading from the atmosphere options, which is an alternative to the volume light cache and would allow you to turn it off. You have to take care and adjust the subdivs of the Probabilistic Shading - check here for more info https://docs.chaosgroup.com/pages/vi...ageId=50990522

Cheers!

[david.anastacio]

Thanks.

That's the aproach we are having now too. It greatly improves the render times and memory. Have you guys considered changing the volume grid shader default to not use the LC?

Best,
David.

[Svetlin.Nikolov]

Yes, this is the future The Volume Light Cache was a concept that worked okay 7-8 years ago when GI was something exotic for volumes, but now it's a matter of time until it gets shuffled out. E.g. V-Ray GPU or Corona do not use that at all when rendering the Phoenix volume shader, which is in fact the volume grid.

We are doing some changes that would allow probabilistic volumetrics to work correctly with all the modes and combinations of options in the volume shader, so it's coming...

[david.anastacio]

We are doing some changes that would allow probabilistic volumetrics to work correctly with all the modes and combinations of options in the volume shader, so it's coming...

That's awesome news, thanks for the update.
Keep those goodies comming

Best,
David.