Work-conserving distributed schedulers for terabit routers

Buffered multistage interconnection networks offer one of the most scalable and cost-effective approaches to building high capacity routers. Unfortunately, the performance of such systems has been difficult to predict in the presence of the extreme traffic conditions that can arise in the Internet. Recent work introduced distributed scheduling, to regulate the flow of traffic in such systems. This work demonstrated, using simulation and experimental measurements, that distributed scheduling can deliver robust performance for extreme traffic. Here, we show that distributed schedulers can be provably work-conserving for speedups of 2 or more. Two of the three schedulers we describe were inspired by previously published crossbar schedulers. The third has no direct counterpart in crossbar scheduling. In our analysis, we show that distributed schedulers based on blocking flows in small-depth acyclic flow graphs can be work-conserving, just as certain crossbar schedulers based on maximal bipartite matchings have been shown to be work-conserving. We also study the performance of practical variants of these schedulers when the speedup is less than 2, using simulation.