SMT-SA: Simultaneous Multithreading in Systolic Arrays

Systolic arrays (SAs) are highly parallel pipelined structures capable of executing various tasks such as matrix multiplication and convolution. They comprise a grid of usually homogeneous processing units (PUs) that are responsible for the multiply-accumulate (MAC) operations in the case of matrix multiplication. It is not rare for a PU input to be zero-valued, in which case the PU becomes idle and the array becomes underutilized. In this paper we consider a solution to employ the underutilized PUs via simultaneous multithreading (SMT). We explore the design space of a SMT-SA variant and evaluate its performance, area efficiency, and energy consumption. In addition, we suggest a tiling method to reduce area overheads. Our evaluation shows that a 4-thread FP16-based SMT-SA achieves speedups of up to 3.6 as compared to conventional SA, with 1.7 area overhead and negligible energy overhead.