Ring Road: A Scalable Polar-Coordinate-based 2D Network-on-Chip Architecture

Networks-on-chip (NoCs) are scaled out to build large-scale multi-chip networks to meet the growing demand for computing. However, the traditional router-based NoC architecture has significant limitations: 1) The overhead of routers is high, especially for long-distance traffic that traverses numerous chips and hops; 2) On/off-chip traffic is mixed up, so the local NoC performance is dragged down by the cross-chip traffic; 3) The routing design of intra/inter-chip networks is also entangled. The alternative router-less solution based on isolated multi-ring (IMR) can address some of these issues; however, the complete lack of routers leads to wiring overhead and limited scalability thus not suitable for large-scale multi-chip networks. Therefore, we are motivated to design a new network architecture that can reduce router/wiring overhead, isolate on/off-chip traffic, and decouple inter/intra-chip routing design by combining the advantages of both routers and router-less rings. In this paper, we propose Ring Road, in which high-speed isolated multi-rings are integrated with compact routers, thus achieving flexible traffic delivery with less router usage and overhead. A polar-coordinate-based description is presented to describe the topology, and corresponding deadlock-free routing algorithms are discussed. As a standalone NoC, Ring Road is low-cost, low-latency, and high-performance. It can also be scaled out into multi-chip networks without redesigning the on-chip routing regardless of inter-chip topology. With low-cost isolated ring channels, no matter how heavy the cross-chip traffic is, the local NoC performance remains consistent. Circuit implementation and post-synthesis analysis show that Ring Road achieves $1.7\sim 2\times$ bandwidth-per-area and $1.6\sim 1.9\times$ bandwidth-per-power compared with the traditional mesh router. Cycle-accurate simulation shows that Ring Road achieves better performance and energy efficiency at various workloads and configurations.