Fair and efficient packet scheduling in wormhole networks

Abstract

Most switch architectures for parallel systems are designed to eliminate only the worst kinds of unfairness such as starvation scenarios in which packets belonging to one traffic flow may not make forward progress for an indefinite period of time. However stricter fairness can lead to a more predictable and better performance, in addition to improving isolation between traffic belonging to different users. This paper presents a new easily implementable scheduling discipline, called Elastic Round Robin (ERR), for the unique requirements of wormhole switching, popular in interconnection networks for parallel systems. Despite the constraints of wormhole switching imposed on the design, our scheduling discipline is at least as efficient as other scheduling disciplines, and more fair than scheduling disciplines of comparable efficiency proposed for any other kind of network, including the Internet. We prove that the work complexity of ERR is O(1) with respect to the number of flows. We analytically prove the fairness properties of ERR, and show that its relative fairness measure has an upper bound of 3 m, where m is the size of the largest packet that actually arrives during an execution of ERR. Finally, we present simulation results comparing the fairness and performance characteristics of ERR with other scheduling disciplines of comparable efficiency.