A Case for Uni-directional Network Topologies in Large-Scale Clusters

Designing low-latency network topologies of switches is a key objective for next-generation large-scale clusters. Low latency is preconditioned on low hop counts, but existing network topologies have hop counts much larger than theoretical lower bounds. To alleviate this problem, we propose building network topologies based on uni-directional graphs that are known to have hop counts close to theoretical lower bounds. A practical difficulty with uni-directional topologies is switch-by-switch flow control, which we resolve by using hot-potato routing. Cycle-accurate network simulation experiments for various traffic patterns on uni-directional topologies show that hot-potato routing achieves performance comparable to that of conventional deadlock-free routing. Similar experiments are used to compare several uni-directional topologies to bi-directional topologies, showing that the former achieve significantly lower latency and higher throughput. We quantify end-to-end application performance for parallel application benchmarks via discrete-even simulation, showing that uni-directional topologies can lead to large application performance improvements over their bi-directional counterparts. Finally, we discuss practical issues for uni-directional topologies such as cabling complexity and cost, power consumption, and soft-error tolerance. Our results make a compelling case for considering uni-directional topologies for upcoming large-scale clusters.