The effect of the router arbitration policy on the scalability of ServerNet™ topologies

In this paper we extend a previously introduced method for optimizing the arbitration policy employed by ServerNet routers and we evaluate the method's effect on scalability. The ServerNet System Area Network (SAN), developed by Tandem Computers Inc., is a wormhole-routed, packet-switched, point-to-point network, with special attention paid to reducing latency and to assuring reliability. The ServerNet SAN uses multiple high-speed, low-cost routers to rapidly switch data directly between data sources and destinations. The ServerNet router arbitration algorithm has the ability to assign different priorities to different links thus controlling the distribution of available bandwidth among channels. We propose an analytical method that allows us to determine the correct priorities for channels so as to achieve fair arbitration thus increasing throughput and scalability, minimizing latency, and eliminating tree saturation. In our study we use data generated by a simulation tool to validate the proposed analytical model for determining the weights of router ports and to evaluate the resulting performance improvements.