Opportunities and limitations of Quality-of-Service in Message Passing applications on adaptively routed Dragonfly and Fat Tree networks

Avoiding communication bottlenecks remains a critical challenge in high-performance computing (HPC) as systems grow to exascale. Numerous design possibilities exist for avoiding network congestion including topology, adaptive routing, congestion control, and quality-of-service (QoS). While network design often focuses on topological features like diameter, bisection bandwidth, and routing, efficient QoS implementations will be critical for next-generation interconnects. HPC workloads are dominated by tightly-coupled mathematics, making delays in a single message manifest as delays across an entire parallel job. QoS can spread traffic onto different virtual lanes (VLs), lowering the impact of network hotspots by providing priorities or bandwidth guarantees that prevent starvation of critical traffic. Two leading topology candidates, Dragonfly and Fat Tree, are often discussed in terms of routing properties and cost, but the topology can have a major impact on QoS. While Dragonfly has attractive routing flexibility and cost relative to Fat Tree, the extra routing complexity requires several VLs to avoid deadlock. Here we discuss the special challenges of Dragonfly, proposing configurations that use different routing algorithms for different service levels (SLs) to limit VL requirements. We provide simulated results showing how each QoS strategy performs on different classes of application and different workload mixes. Despite Dragonfly's desirable characteristics for adaptive routing, Fat Tree is shown to be an attractive option when QoS is considered.