A Software-Programmable Neural Processing Unit for Graph Neural Network Inference on FPGAs

Graph neural networks (GNNs) are a widely-used class of deep learning (DL) models for learning latent representations of graph-structured data for a variety of node/graph-level prediction tasks, some of which require real-time low latency inference. Most existing GNN accelerators rely on preprocessing input graphs on a host/embedded CPU to parallelize computations on different sub-graphs, making them unsuitable for real-time use cases. Others are extremely specialized streaming pipelines for only a specific type of GNN and therefore suffer from long FPGA bitstream compile times when the model is updated and cannot be used in applications that combine GNNs with other classes of DL models. In this work, we enhance the neural processing unit (NPU) FPGA overlay architecture, instruction set, and software stack to support a variety of GNN models. We achieve this without sacrificing the NPU flexibility; our enhanced NPU can be programmed purely through software to accelerate different GNNs or any of its originally supported DL workloads (e.g. MLPs, RNNs, GRUs, LSTMs). In addition, this flexibility enables our NPU software compiler to generate GNN kernels with different performance targets (throughput-optimized vs. latency-optimized) by exploiting different dimensions of compute parallelism on the same overlay architecture. Besides the flexibility benefits, our NPU implemented on an Intel Stratix 10 NX (14 nm) FPGA can process $7.8 \times$ more graphs per second at a similar latency on average compared to a state-of-the-art model-specific FPGA accelerator targeting real-time applications on an AMD Ultrascale+ same-generation FPGA. It also achieves 5.8 $\times$ higher throughput compared to an Nvidia RTX A6000 GPU (8 nm) and $2.6 \times$ lower latency than a state-of-the-art accelerator that combines CPU-based graph preprocessing with AMD Versal (7 nm) fabric and AI engine compute. Finally, we present a case study for using our enhanced NPU in real-time GNNbased multi-input multi-output (MIMO) antenna scheduling, highlighting that it meets the latency requirements of this task in 5G communication networks.