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Evolving technology and increasing pin-bandwidth motivate the use of high-radix routers to reduce the diameter, latency, and cost of interconnection networks. High-radix networks, however, require longer cables than their low-radix counterparts. Because cables dominate network cost, the number of cables, and particularly the number of long, global cables should be minimized to realize an efficient network. In this paper, we introduce the dragonfly topology which uses a group of high-radix routers as a virtual router to increase the effective radix of the network. With this organization, each minimally routed packet traverses at most one global channel. By reducing global channels, a dragonfly reduces cost by 20% compared to a flattened butterfly and by 52% compared to a folded Clos network in configurations with ≥ 16K nodes.We also introduce two new variants of global adaptive routing that enable load-balanced routing in the dragonfly. Each router in a dragonfly must make an adaptive routing decision based on the state of a global channel connected to a different router. Because of the indirect nature of this routing decision, conventional adaptive routing algorithms give degraded performance. We introduce the use of selective virtual-channel discrimination and the use of credit round-trip latency to both sense and signal channel congestion. The combination of these two methods gives throughput and latency that approaches that of an ideal adaptive routing algorithm.

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Abstract-We apply recent results in queueing theory to propose a methodology for bounding the buffer depth and packet delay in high radix interconnection networks. While most work in interconnection networks has been focused on the throughput and average latency in such systems, few studies have been done providing statistical guarantees for buffer depth and packet delays. These parameters are key in the design and performance of a network. We present a methodology for calculating such bounds for a practical high radix network and through extensive simulations show its effectiveness for both bursty and non-bursty injection traffic. Our results suggest that modest speedups and buffer depths enable reliable networks without flow control to be constructed.