EPU: An Energy-Efficient Explainable AI Accelerator With Sparsity-Free Computation and Heat Map Compression/Pruning

Abstract

Deep neural networks (DNNs) have recently gained significant prominence in various real-world applications such as image recognition, natural language processing, and autonomous vehicles. However, due to their black-box nature in system, the underlying mechanisms of DNNs behind the inference results remain opaque to users. In order to address this challenge, researchers have focused on developing explainable artificial intelligence (AI) algorithms. Explainable AI aims to provide a clear and human-understandable explanation of the model's decision, thereby building more reliable systems. However, the explanation task differs from well-known inference and training processes as it involves interactions with the user. Consequently, existing inference and training accelerators face inefficiencies when processing explainable AI on edge devices. This article introduces explainable processing unit (EPU), the first hardware accelerator designed for explainable AI workloads. The EPU utilizes a novel data compression format for the output heat maps and intermediate gradients to enhance the overall system performance by reducing both memory footprint and external memory access. Its sparsity-free computing core efficiently handles the input sparsity with negligible control overhead, resulting in a throughput boost of up to 9.48x. It also proposes a dynamic workload scheduling with a customized on-chip network for distinct inference and explanation tasks to maximize internal data reuse hence reducing external memory access by 63.7%. Furthermore, the EPU incorporates point-wise gradient pruning (PGP) that can significantly reduce the size of heat maps by a factor of 7.01x combined with the proposed compression format. Finally, the EPU chip fabricated in a 28 nm CMOS process achieves a remarkable heat map generation rate of 367 frames/s for ResNet-34 while maintaining the state-of-the-art area and energy efficiency of 112.3 GOPS/mm(2) and 26.55 TOPS/W, respectively.