Evaluation of Alternative Arbitration Policies for Myrinet Switches

Interconnection networks consist of a set of switches interconnected by point-to-point links, and hosts linked to those switches through a network interface card. These networks are becoming increasingly popular as a cost-effective alternative to parallel computers. One of the most wellknown example is the Myrinet network. These networks use techniques and incorporate components that have been successfully applied to interconnection networks for parallel computers. One of these techniques is the use of crossbar chips as the main component of switches, allowing to connect the input channel of any of their ports to the output channel of any of their ports for forwarding incoming packets. Of course, when several packets request the same output channel, certain criterion must be used for selecting one of them. This criterion may affect performance in terms of latency and throughput, specially in wormhole networks (Myrinet) where a packet occupies several resources while it is waiting for the output channel. In Myrinet, an arbitration policy based on round-robin criterion is used. However, there exist other arbitration policies that, when applied to Myrinet networks, could increase their performance. In this paper, several arbitration policies for selecting the packet for a free output channel have been considered and evaluated. We have found that strategies based on injection time and remaining distance to travel reduce the average and maximum packet latency of the current policy used in Myrinet.