Ray-Trace Acceleration (mental ray Renderer)

The mental ray renderer provides three different ray-tracing methods of accelerating the process of ray tracing. The methods are:

BSP (Binary Space Partitioning). This method (the default) performs best for most purposes.
Grid. This method can perform better on multiprocessor systems.
Large BSP. This method can perform better with large scenes and with distributed bucket rendering.

You set the raytrace acceleration method on the Render Scene Dialog > Renderer panel > Rendering Algorithms rollout.