A Study of Pointer-Chasing Performance on Shared-Memory Processor-FPGA Systems

The advent of FPGA acceleration platforms with direct coherent access to processor memory creates an opportunity for accelerating applications with irregular parallelism governed by large in-memory pointer-based data structures. This paper uses the simple reference behavior of a linked-list traversal as a proxy to study the performance potentials of accelerating these applications on shared-memory processor-FPGA systems. The linked-list traversal is parameterized by node layout in memory, per-node data payload size, payload dependence, and traversal concurrency to capture the main performance effects of different pointer-based data structures and algorithms. The paper explores the trade-offs over a wide range of implementation options available on shared-memory processor-FPGA architectures, including using tightly-coupled processor assistance. We make observations of the key effects on currently available systems including the Xilinx Zynq, the Intel QuickAssist QPI FPGA Platform, and the Convey HC-2. The key results show: (1) the FPGA fabric is least efficient when traversing a single list with non-sequential node layout and a small payload size; (2) processor assistance can help alleviate this shortcoming; and (3) when appropriate, a fabric only approach that interleaves multiple linked list traversals is an effective way to maximize traversal performance.