The Performance of the CedarMultistage Switching Network 1

While multistage switching networks for vector multiprocessors have been studied extensively , detailed evaluations of their performance are rare. Indeed, analytical models, simulations with pseudo-synthetic loads, studies focused on average-value parameters, and measurements of networks disconnected from the machine, all provide limited information. In this paper, instead , we present an in-depth empirical analysis of a multistage switching network in a realistic setting: we use hardware probes to examine the performance of the omega network of the Cedar shared-memory machine executing real applications. The machine is conngured with 16 vector processors. The analysis suggests that the performance of multistage switching networks is limited by traac non-uniformities. We identify two major non-uniformities that degrade Cedar's performance and are likely to slow down other networks too. The rst one is the contention caused by the return messages in a vector access as they converge from the memories to one processor port. This traac convergence penalizes vector reads and, more importantly, causes tree saturation. The second non-uniformity is the uneven contention delays induced by even a relatively fair scheme to resolve message collisions. Based on our observations, we argue that intuitive optimizations for multistage switching networks may not be the most cost-eeective ones. Instead, we suggest changes to increase the network bandwidth at the root of the traac convergence tree and to delay traac convergence up until the nal stages of the network.