CoMeFa: Deploying Compute-in-Memory on FPGAs for Deep Learning Acceleration

Abstract

Block RAMs (BRAMs) are the storage houses of FPGAs, providing extensive on-chip memory bandwidth to the compute units implemented using Logic Blocks (LBs) and Digital Signal Processing (DSP) slices. We propose modifying BRAMs to convert them to CoMeFa (Compute-In-Memory Blocks for FPGAs) RAMs. These RAMs provide highly parallel compute-in-memory by combining computation and storage capabilities in one block. CoMeFa RAMs utilize the true dual-port nature of FPGA BRAMs and contain multiple configurable single-bit bit-serial processing elements. CoMeFa RAMs can be used to compute with any precision, which is extremely important for applications like Deep Learning (DL). Adding CoMeFa RAMs to FPGAs significantly increases their compute density, while also reducing data movement. We explore and propose two architectures of these RAMs: CoMeFa-D (optimized for delay) and CoMeFa-A (optimized for area). Compared to existing proposals, CoMeFa RAMs do not require changing the underlying SRAM technology like simultaneously activating multiple wordlines on the same port, and are practical to implement. CoMeFa RAMs are especially suitable for parallel and compute-intensive applications like DL, but these versatile blocks find applications in diverse applications like signal processing, databases, etc. By augmenting an Intel Arria-10-like FPGA with CoMeFa-D (CoMeFa-A) RAMs at the cost of 3.8% (1.2%) area, and with algorithmic improvements and efficient mapping, we observe a geomean speedup of 2.55x (1.85x) across microbenchmarks from various applications and a geomean speedup of up to 2.5x across multiple Deep Neural Networks. Replacing all or some BRAMs with CoMeFa RAMs in FPGAs can make them better accelerators of DL workloads.