Adaptive Prefetching for Fine-grain Communication in PGAS Programs

Applications that require distributed-memory systems and exhibit irregular memory access patterns present both productivity and performance challenges. This is largely due to the fine-grain communication that arises from irregular memory accesses to distributed data. The Partitioned Global Address Space (PGAS) model provides a globally shared address space and one-sided communication, making it well suited for implementing irregular codes. However, while the PGAS model provides productivity advantages, irregular applications have difficulty achieving high performance due to the cost of fine-grain remote accesses. One way to improve the performance of such PGAS programs is to prefetch remote data before it is needed, thereby hiding communication latency. The challenges of applying prefetching are computing the prefetch distance (i.e., how far ahead to issue prefetches), determining when prefetching will be profitable, and modifying the program to perform prefetching. In this work, we present an adaptive prefetching optimization that can be applied to PGAS programs with irregular memory access patterns. We target the Chapel parallel programming language, which implements a PGAS model. Our optimization leverages runtime information to adjust the prefetch distance and to pause/resume prefetching when that is likely to provide performance gains. Furthermore, we develop compiler support to automatically apply the optimization without requiring user intervention. We evaluate the optimization across five different distributed-memory systems and four workloads. We observe runtime speed-ups of 0.5 – 377x for the Chapel workloads when compared to not prefetching, and 3 – 215x when compared to implementations written in UPC, OpenSHMEM, and one-sided MPI.