Personalized PageRank (PPR) is a graph algorithm that evaluates the importance of the surrounding nodes from a source node. Widely used in social network related applications such as recommender systems, PPR requires real-time responses (latency) for a better user experience. Existing works either focus on algorithmic optimization for improving precision while neglecting hardware implementations or focus on distributed global graph processing on large-scale systems for improving throughput rather than response time. Optimizing low-latency local PPR algorithm with a tight memory budget on edge devices remains unexplored. In this work, we propose a memory-efficient, low-latency PPR solution, namely MeLoPPR, with largely reduced memory requirement and a flexible trade-off between latency and precision. MeLoPPR is composed of stage decomposition and linear decomposition and exploits the node score sparsity: Through stage and linear decomposition, MeLoPPR breaks the computation on a large graph into a set of smaller sub-graphs, that significantly saves the computation memory; Through sparsity exploitation, MeLoPPR selectively chooses the sub-graphs that contribute the most to the precision to reduce the required computation. In addition, through software/hardware co-design, we propose a hardware implementation on a hybrid CPU and FPGA accelerating platform, that further speeds up the sub-graph computation. We evaluate the proposed MeLoPPR on memory-constrained devices including a personal laptop and Xilinx Kintex-7 KC705 FPGA using six real-world graphs. First, MeLoPPR demonstrates significant memory saving by 1.5x to 13.4x on CPU and 73x to 8699x on FPGA. Second, MeLoPPR allows flexible trade-offs between precision and execution time: when the precision is 80%, the speedup on CPU is up to 15x and up to 707x on FPGA; when the precision is around 90%, the speedup is up to 70x on FPGA.
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