Directive-Based Partitioning and Pipelining for Graphics Processing Units

Abstract

The community needs simpler mechanisms to access the performance available in accelerators, such as GPUs, FPGAs, and APUs, due to their increasing use in state-of-the-art supercomputers. Programming models like CUDA, OpenMP, OpenACC and OpenCL can efficiently offload compute-intensive workloads to these devices. By default these models naively offload computation without overlapping it with communication (copying data to or from the device). Achieving performance can require extensive refactoring and hand-tuning to apply optimizations such as pipelining. Further, users must manually partition the dataset whenever its size is larger than device memory, which can be especially difficult when the device memory size is not exposed to the user. We propose a directive-based partitioning and pipelining extension for accelerators appropriate for either OpenMP or OpenACC. Its interface supports overlap of data transfers and kernel computation without explicit user splitting of data. It can map data to a pre-allocated device buffer and automate memory-constrained array indexing and sub-task scheduling. We evaluate a prototype implementation with four different applications. The experimental results show that our approach can reduce memory usage by 52% to 97% while delivering a 1.41× to 1.65× speedup over the naive offload model.