Treelet Prefetching For Ray Tracing

Yuan-Hsi Chou, Tyler Nowicki, Tor M. Aamodt

Abstract: Ray tracing is traditionally only used in offline rendering to produce images of high fidelity because it is computationally expensive. Recent Graphics Processing Units (GPUs) have included dedicated accelerators to bring ray tracing to real-time rendering for video games and other graphics applications. These accelerators focus on finding the closest intersection between a ray and a scene using a hierarchical tree data structure called a Bounding Volume Hierarchy (BVH) tree. However, BVH tree traversal is still very costly due to divergent rays accessing different parts of the tree, with each ray following a unique pointer-chasing sequence that is difficult to optimize with traditional methods. To address this, we propose treelet prefetching to reduce the latency of ray traversal. Treelets are smaller subtrees created by splitting the BVH tree. When a ray visits a treelet root node, we prefetch the corresponding treelet, enabling deeper levels of the tree to be fetched in advance. This reduces the latency associated with pointer-chasing during tree traversal. Our approach uses a hardware prefetcher with a two-stack treelet based traversal algorithm, maximizing the benefits of treelet prefetching. Our simulation results show treelet prefetching on average improves performance of the baseline RT Unit in Vulkan-Sim by 32.1% while maintaining the same power consumption.