Butterfly vs. Unidirectional Fat-Trees for Networks-on-Chip: not a Mere Permutation of Outputs∗

Bidirectional topologies are usually preferred over unidirectional ones. However, recent works have demonstrated that RUFT, a traffic-balancing routing algorithm on a Unidirectional Multistage Network (UMIN), can perform as well as a Bidirectional Multistage Network, but significantly reducing both implementation and operating costs. RUFT is a simplification of the k-ary n-tree topology, the most widely-used implementation of the Fat-Tree topology. RUFT resembles the classical unidirectional Butterfly topology, but with a different connection pattern between switches from the last stage and cores. This work provides a comparison in terms of performance and implementation costs between both topologies, RUFT and unidirectional Butterfly for on-chip interconnects. Our high level and post-layout analysis shows that RUFT is a much more convenient alternative than traditional Butterfly when laying out on silicon.