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It is a well known fact that multiple virtual lanes can improve performance in interconnection networks, but this knowledge has had little impact on real clusters. Currently, a large number of clusters using In?niBand is based on fat-tree topologies that can be routed deadlock-free using only one virtual lane. Consequently, all the remaining virtual lanes are left unused. In this paper we suggest an enhancement to the fat-tree algorithm that utilizes virtual lanes to improve performance when hot-spots are present. Even though the bisection bandwidth in a fat-tree is constant, hot-spots are still possible and they will degrade performance for ?ows not contributing to them due to head-of-line blocking. Such a situation may be alleviated through adaptive routing or congestion control, however, these methods are not yet readily available in In?niBand technology. To remedy this problem, we have implemented an enhanced fat-tree algorithm in OpenSM that distributes traf?c across all available virtual lanes without any con?guration needed. We evaluated the performance of the algorithm on a small cluster and did a large-scale evaluation through simulations. In a congested environment, results show that we are able to achieve throughput increases up to 38% on a small cluster and from 221% to 757% depending on the hot-spot scenario for a 648-port simulated cluster.

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