IMAGine: An In-Memory Accelerated GEMV Engine Overlay

Processor-in-Memory (PIM) overlays and alternative reconfigurable tile fabrics have been proposed to eliminate the von Neumann bottleneck and enable processing performance to scale with BRAM capacity. The performance of these FPGA-based PIM architectures has been limited due to a reduction of the BRAMs maximum clock frequencies and less than ideal scaling of processing elements with increased BRAM capacity. This paper presents IMAGine, an In-Memory Accelerated GEMV engine, a PIM-array accelerator that clocks at the maximum frequency of the BRAM and scales to 100% of the available BRAMs. Comparative analyses are presented showing execution speeds over existing PIM-based GEMV engines on FPGAs and achieving a $2.65 \times-3.2 \times$ faster clock. An AMD Alveo U55 implementation is presented that achieves a system clock speed of 737 MHz, providing 64 K bit-serial multiply-accumulate (MAC) units for GEMV operation. This establishes IMAGine as the fastest PIM-based GEMV overlay, outperforming even the custom PIM-based FPGA accelerators reported to date. Additionally, it surpasses TPU v1-v2 and Alibaba Hanguang 800 in clock speed while offering an equal or greater number of multiply-accumulate (MAC) units.