Analysis and Improvement of Valiant Routing in Low-Diameter Networks

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Valiant routing randomizes network traffic to avoid pathological congestion issues by diverting traffic to a random intermediate switch. It has received significant attention in recently proposed high-radix, low-diameter topologies, which are prone to congestion issues. It has been implemented obliviously, or as the basis of some non-minimal adaptive routing algorithms. An analysis of the original mechanism identifies two potential improvements regarding the selection of the intermediate switch. First, when traffic is local the randomization introduced by Valiant results in unnecessarily long paths. Instead, the introduced Restricted Valiant routing randomizes traffic within a local partition, avoiding congestion and generating shorter paths. Second, in certain cases the path to the selected random intermediate node can be blocked; a version with recomputation selects a new random intermediate node as long as the associated path remains stalled. The proposals are evaluated by simulation in a state-of-the-art Dragonfly network with different traffic patterns. Results show that Restricted Valiant is highly effective in cases of local traffic, with a small improvement under global patterns. Valiant with recomputation increases injection, further reducing average latency and increasing throughput. However, the higher injection increases congestion effects in some cases. Such problem is emphasized when more injection buffers are added, because of the increased pressure on the interconnect. Overall, the results are very relevant for routing in high-radix networks and might constitute the basis for other adaptive routing algorithms.