A case for low frequency single cycle multi hop NoCs for energy efficiency and high performance

As the number of cores in a multi-core system increase, network on-chip (NoC) latency and transmission energy scale unfavorably, since they are directly proportional to the number of hops traversed. Designers often have to trade-off energy to get lower latency (for instance long-distance bypass links with high-radix multi-stage routers) or latency to get lower energy (e.g., scaling down voltage and frequency of NoC routers and links). This work offers an alternate design-space for latency-energy optimization that has previously been unexplored, by harnessing the fact that lower frequency links can actually be used to transmit over longer on-chip distances within a cycle. We leverage a recently proposed micro-architecture that enables the construction of single-cycle multi-hop paths on the fly over a regular mesh network, and augment it with support for dynamic voltage and frequency scaling by decoupling router frequency from link frequency. In essence, we enable packets to traverse only wires from the source to the destination (as if it had a dedicated connection) only getting buffered at routers if necessary (at turns or due to contention). We address the synchronization challenges of multi-hop bypass setup signals in a multi-frequency domain and propose novel static/dynamic router and link frequency assignment techniques. Across synthetic as well as full-system benchmarks, we demonstrate reduced energy with similar or better run-times.