Janus: A Flexible Processing-in-Memory Graph Accelerator Towards Sparsity

Graph application is ever-growing in relational data analysis. However, the memory access patterns become the performance bottleneck in graph analytics and graph neural network (GNN) suffering from single-side and dual-side sparsity, separately. Existing RRAM-based processing-in-memory accelerators reduce data movements but fail to handle both types of sparsity in graph data. To address these issues, our work introduces Janus, a flexible highly-compact architecture that is capable of being configured to enable single-sparse mode and dual-sparse mode, to accelerate graph analytics and GNN workloads in compressed mapping, respectively. Upon performing graph analytics with single-side sparsity, Janus employs a tandem-isomorphic-crossbar design both to remove zero-stored footprint, and to eliminate redundant search and sequential indexing. To address the challenge of dual-side sparsity in GNN, Janus still takes a random index access mechanism to gather data rapidly and uses a semi-SPM2 compute paradigm to boost the RRAM-based analog multiplication-and-accumulation in the compressed format. Compared with the state-of-the-art works, Janus outperforms them in both performance and energy efficiency for graph analytics and GNN, respectively.