https://en.wikipedia.org/wiki/Bounding_volume_hierarchy#:~:text=BVHs%20are%20often%20used%20in,intersected%20by%20the%20current%20ray.

Bounding volume hierarchies are used to support several operations on sets of geometric objects efficiently, such as in collision detection and ray tracing. A bounding volume hierarchy (BVH) is a tree structure on a set of geometric objects. All geometric objects, which form the leaf nodes of the tree, are wrapped in bounding volumes.

BVHs are often used in ray tracing to eliminate potential intersection candidates within a scene by omitting geometric objects located in bounding volumes which are not intersected by the current ray. BVH is a crucial component in ray tracing rendering engines like Arnold, as it helps accelerate ray intersection tests and reduce resource costs.

Users do not have control over RAM consumption of the BVH. Here are some tips to optimize Arnold renders when BVH is the bottleneck:

Optimize Your Scene Geometry. Simplify or optimize your 3D models and scene geometry. Complex geometry can lead to larger BVH structures and longer BVH build times. Consider using LODs (Level of Detail) or proxy objects for distant geometry to reduce the BVH complexity.

Use Arnold Stand-ins and Proxies. Arnold Stand-ins and proxies allow you to load complex geometry only when needed, reducing the BVH complexity during the initial BVH build. This can be particularly useful for scenes with a lot of high-poly assets.

Denoising. Applying denoising to your final render can help reduce the number of rays required and consequently, the BVH intersection tests.

Render in Layers. If your scene has many elements, consider rendering it in layers. This allows you to optimize each layer individually, potentially reducing BVH build times.

Distribute Rendering. If you have access to a render farm or multiple machines, distribute the rendering workload. This can significantly reduce rendering time as each machine can handle a portion of the BVH calculations.