Cycles uses a lot of VRAM, no?

[z01ks]

I’m just playing around with the Octane version of blender, testing render speeds and VRAM limitations on both Cycles GPU compute and Octane Render.

I put a 4k exr map as displacement map (adaptive subdivision) on a simple plane and cranked the default 1920 x 1080 render resolution to 400%. I hit render and Cycles instantly used up all 24 GB of my GPU.

With Octane, a similar scene used around 4-7 GB I believe.

So I was kind of surprised that I was able to run into the GPU VRAM limitation so easily with Cycles. All it took was the combination of a single 4k displacement map and 7680 x 4320 px render resolution. Can anyone explain the math behind why this is happening in Cycles?

[SterlingRoth]

Here is a post that has some info on cycles memory use

Blender Object Mode Performance

It would nice to know a bit more about the geometry, just so my math is correct. Assuming you have 171 million triangles, with closed meshes, and no subdivision, that would be around 114 million vertices, so given how Cycles stores geometry, that would be around 7.8 Gb. With those stats, and by using a different layout, we could save around 3.1 Gb. That would not change the amount of memory used by the BVH, which I expect would be more that double the memory of the geometry. OptiX can compress …

Were you using tiling? how about denoising?

[DNorman]

@z01ks

adaptive subdivision

Adaptive subdivision is still (years later) experimental because they have not been able to optimise performance.

[deadpin]

Not only performance, but there’s a variety of features that do not work with it yet and other things subtly broken: https://developer.blender.org/T53901

For now, you can try the following and see if it helps since there’s a bug where it will double-subdivide the mesh leading to extremely high memory usage https://developer.blender.org/T85030 :

• Disable adaptive subdivision on the modifier
• Set both Viewport and Render to 0
• Re-enable adaptive subdivision
• See if that helps a bit (if it doesn’t, you can try also limiting the level of subdivision in Cycle’s Render → Subdivision → Max Subdivision setting)