1. A system for ray tracing, comprising: a memory storing an acceleration structure for a 3-D scene that is composed of primitives and in which rays are to be traced, the acceleration structure comprising a plurality of nodes, each defining a surface bounding a volume in the 3-D scene, the acceleration structure comprising leaf nodes whose surfaces bound respective selections of the primitives, and non-leaf nodes that each respectively bound a plurality of other nodes; a plurality of test cells, each configurable to test a ray for intersection with a node of the acceleration structure; and a controller configured for traversing a plurality of rays through the acceleration structure by iteratively loading subsets of the nodes into the test cells, and providing one or more rays to be concurrently tested with the nodes loaded into the test cells, the controller further configured, for leaf nodes, for collecting each ray into respective collections associated with each leaf node determined to be intersected by that ray, and for initiating testing of the rays of a collection with the primitives bounded by its associated leaf node surface by distributing the collected rays among the test cells and cycling the primitives through the test cells.