Order/Radix Problem: Towards Low End-to-End Latency Interconnection Networks

We introduce a novel graph called a host-switch graph, which consists of host vertices and switch vertices. Using host-switch graphs, we formulate a graph problem called an order/radix problem (ORP) for designing low end-to-end latency interconnection networks. Our focus is on reducing the host-to-host average shortest path length (h-ASPL), since the shortest path length between hosts in a host-switch graph corresponds to the end-to-end latency of a network. We hence define ORP as follows: given order (the number of hosts) and radix (the number of ports per switch), find a host-switch graph with the minimum h-ASPL. We demonstrate that the optimal number of switches can mathematically be predicted. On the basis of the prediction, we carry out a randomized algorithm to find a host-switch graph with the minimum h-ASPL. Interestingly, our solutions include a host-switch graph such that switches have the different number of hosts. We then apply host-switch graphs to interconnection networks and evaluate them practically. As compared with the three conventional interconnection networks (the torus, the dragonfly, and the fat-tree), we demonstrate that our networks provide higher performance while the number of switches can decrease.