Hoplite

Abstract

We can design an FPGA-optimized lightweight network-on-chip (NoC) router for flit-oriented packet-switched communication that is an order of magnitude smaller (in terms of LUTs and FFs) than state-of-the-art FPGA overlay routers available today. We present Hoplite, an efficient, lightweight, and fast FPGA overlay NoC that is designed to be small and compact by (1) using deflection routing instead of buffered switching to eliminate expensive FIFO buffers and (2) using a torus topology to reduce the cost of switch crossbar. Buffering and crossbar implementation complexities have traditionally limited speeds and imposed heavy resource costs in conventional FPGA overlay NoCs. We take care to exploit the fracturable lookup tables (LUT) organization of the FPGA to further improve the resource efficiency of mapping the expensive crossbar multiplexers. Hoplite can outperform classic, bidirectional, buffered mesh networks for single-flit-oriented FPGA applications by as much as 1.5 × (best achievable throughputs for a 10 × 10 system) or 2.5 × (allocating same amount of FPGA resources to both NoCs) for uniform random traffic. When compared to buffered mesh switches, FPGA-based deflection routers are ≈ 3.5 × smaller (HLS-generated switch) and 2.5 × faster (clock period) for 32b payloads. In a separate experiment, we hand-crafted an RTL version of our switch with location constraints that requires only 60 LUTs and 100 FFs per router and runs at 2.9ns. We conduct additional layout experiments on modern Xilinx and Altera FPGAs and demonstrate wide-channel chip-spanning layouts that run in excess of 300MHz while consuming 10--15% of overall chip resources. We also demonstrate a clustered RISC-V multiprocessor organization that uses Hoplite to help deliver the high processing throughputs of the FPGA architecture to user applications.