LaPerm: Locality Aware Scheduler for Dynamic Parallelism on GPUs

Recent developments in GPU execution models and architectures have introduced dynamic parallelism to facilitate the execution of irregular applications where control flow and memory behavior can be unstructured, time-varying, and hierarchical. The changes brought about by this extension to the traditional bulk synchronous parallel (BSP) model also creates new challenges in exploiting the current GPU memory hierarchy. One of the major challenges is that the reference locality that exists between the parent and child thread blocks (TBs) created during dynamic nested kernel and thread block launches cannot be fully leveraged using the current TB scheduling strategies. These strategies were designed for the current implementations of the BSP model but fall short when dynamic parallelism is introduced since they are oblivious to the hierarchical reference locality. We propose LaPerm, a new locality-aware TB scheduler that exploits such parent-child locality, both spatial and temporal. LaPerm adopts three different scheduling decisions to i) prioritize the execution of the child TBs, ii) bind them to the stream multiprocessors (SMXs) occupied by their parents TBs, and iii) maintain workload balance across compute units. Experiments with a set of irregular CUDA applications executed on a cycle-level simulator employing dynamic parallelism demonstrate that LaPerm is able to achieve an average of 27% performance improvement over the baseline round-robin TB scheduler commonly used in modern GPUs.