BE-NPU: A Bandwidth-Efficient Neural Processing Unit With Adaptive Processing Schemes for Reduced Off-Chip Bandwidth Demand

Existing neural processing units (NPUs) mainly focus on the optimized multiply-accumulate (MAC) arrays for efficient inference of convolutional neural networks (CNNs). However, off-chip data transmission usually keeps NPUs waiting during CNN inference, causing up to 38.4GB/s off-chip bandwidth (OCB) demand for mobile AI devices. And none of the previous benchmarks quantitatively evaluate the bandwidth efficiency of different NPU architectures. In addition, CNNs exhibit distinct characteristics of off-chip data transmission when applied to different fields, and it has become a challenging task for NPUs to support different CNNs efficiently with reasonable OCB demand. To address the aforementioned issues, this paper proposes the Bandwidth-Peak Performance Ratio for n percentages of ideal frame rate (BPPR-n%) to demonstrate the normalized OCB demand of different NPU architectures. A bandwidth-efficient NPU (BE-NPU) is introduced with adaptive processing schemes to reduce the OCB demand during inference of different CNNs. The adaptive processing schemes include both instruction-level and thread-level schemes. For the instruction-level scheme, decoupled execute/access is introduced into depth-first (DF) and layer-first (LF) schemes to improve the concurrency between NPU calculation (CAL) and direct memory access (DMA) instructions. For the thread-level scheme, DF and LF threads are hybridly processed to further improve overall NPU efficiency. Compared with state-of-the-art works, BE-NPU achieves 48.1%~80.6% reduction of BPPR-80% and 67.0%~95.1% reduction of BPPR-95%. The proposed architecture is synthesized with TSMC 28nm technology node. BE-NPU utilizes 14.3% additional logic gates compared with baseline implementation.