Design Exploration of Multi-tier Interconnection Networks for Exascale Systems

Interconnection networks are one of the main limiting factors when it comes to scale out computing systems. In this paper, we explore what role the hybridization of topologies has on the design of an state-of-the-art exascale-capable computing system. More precisely we compare several hybrid topologies and compare with common single-topology ones when dealing with large-scale applicationlike traffic. In addition we explore how different aspects of the hybrid topology can affect the overall performance of the system. In particular, we found that hybrid topologies can outperform state-of-the-art torus and fattree networks as long as the density of connections is high enough--one connection every two or four nodes seems to be the sweet spot--and the size of the subtori is limited to a few nodes per dimension. Moreover, we explored two different alternatives to use in the upper tiers of the interconnect, a fattree and a generalised hypercube, and found little difference between the topologies, mostly depending on the workload to be executed.