Performance Optimization of SU3_Bench on Xeon and Programmable Integrated Unified Memory Architecture
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SU3_Bench is a microbenchmark developed to explore performance portability across multiple programming models/methodologies using a simple, but nontrivial, mathematical kernel. This kernel has been derived from the MILC lattice quantum chromodynamics (LQCD) code. SU3_Bench is bandwidth bound and generates regular compute and data access patterns. Therefore, on most traditional CPU and GPU-based systems, its performance is mainly determined by the achievable memory bandwidth. Although SU3_Bench is a simple kernel, experience says its subtleties require a certain amount of tweaking to achieve peak performance for a given programming model and hardware, making performance portability challenging. In this paper, we share some of the challenges in obtaining the peak performance for SU3_Bench on a state-of-the-art Intel Xeon machine, due to the nuances of variable definition, the nature of compiler-provided default constructors, how memory is accessed at object creation time, and the NUMA effects on the machine. We discuss how to tackle those challenges to improve SU3_Bench's performance by 2× compared to the original OpenMP implementation available at Github. This provides a valuable lesson for other similar kernels. Expanding on the performance portability aspects, we also show early results obtained porting SU3_Bench to the new Intel Programmable Integrated Unified Memory Architecture (PIUMA), characterized by a more balanced flops-to-byte ratio. This paper shows that it is not the usual bandwidth or flops, rather the pipeline throughput, that determines SU3_Bench's performance on PIUMA. Finally, we show how to improve performance on PIUMA and how that compares with the performance on Xeon, which has around one order of magnitude more flops-per-byte.
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