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Dragonfly networks have been recently proposed for the interconnection network of forthcoming exascale supercomputers. Relying on large-radix routers, they build a topology with low diameter and high throughput, divided into multiple groups of routers. While minimal routing is appropriate for uniform traffic patterns, adversarial traffic patterns can saturate intergroup links and degrade the obtained performance. Such traffic patterns occur in typical communication patterns used by many HPC applications, such as neighbor data exchanges in multidimensional space decompositions. Non-minimal traffic routing is employed to handle such cases. Adaptive policies have been designed to select between minimal and nonminimal routing to handle variable traffic patterns.However, previous papers have not taken into account the effect of saturation of intra-group (local) links. This paper studies how local link saturation can be common in these networks, and shows that it can largely reduce the performance. The solution to this problem is to use nonminimal paths that avoid those saturated local links. However, this extends the maximum path length, and since all previous routing proposals prevent deadlock by relying on an ascending order of virtual channels, it would imply unaffordable cost and complexity in the network routers.In this paper we introduce a novel routing/flow-control scheme that decouples the routing and the deadlock avoidance mechanisms. Our model does not impose any dependencies between virtual channels, allowing for on-the-fly (in-transit) adaptive routing of packets. To prevent deadlock we employ a deadlock-free escape subnetwork based on injection restriction. Simulations show that our model obtains lower latency, higher throughput, and faster adaptation to transient traffic, because it dynamically exploits a higher path diversity to avoid saturated links. Notably, our proposal consumes traffic bursts 43% faster than previous ones.

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Abstract-High-radix hierarchical networks are costeffective topologies for large scale computers. In such networks, routers are organized in supernodes, with local and global interconnections. These networks, known as Dragonflies, outperform traditional topologies such as multi-trees or tori, in cost and scalability. However, depending on the traffic pattern, network congestion can lead to degraded performance. Misrouting (non-minimal routing) can be employed to avoid saturated global or local links. Nevertheless, with the current deadlock avoidance mechanisms used for these networks, supporting misrouting implies routers with a larger number of virtual channels. This exacerbates the buffer memory requirements that constitute one of the main constraints in high-radix switches.In this paper we introduce two novel deadlock-free routing mechanisms for Dragonfly networks that support on-thefly adaptive routing. Using these schemes both global and local misrouting are allowed employing the same number of virtual channels as in previous proposals. Opportunistic Local Misrouting obtains the best performance by providing the highest routing freedom, and relying on a deadlock-free escape path to the destination for every packet. However, it requires Virtual Cut-Through flow-control. By contrast, Restricted Local Misrouting prevents the appearance of cycles thanks to a restriction of the possible routes within supernodes. This makes this mechanism suitable for both Virtual Cut-Through and Wormhole networks.Evaluations show that the proposed deadlock-free routing mechanisms prevent the most frequent pathological issues of Dragonfly networks. As a result, they provide higher performance than previous schemes, while requiring the same area devoted to router buffers.