Is It Worth the Flexibility Provided by Irregular Topologies in Networks of Workstations?

Abstract

Networks of workstations (NOWs) are becoming a cost-effective alternative for small-scale parallel computing. Usually, NOWs present an irregular topology as a consequence of the needs in a local area network. Routing algorithms used in NOWs are inherently different from those used in regular networks, mainly due to the irregular connections between switches. In these algorithms, routing is considerably restricted in order to avoid deadlocks. Recently, a general methodology for the design of adaptive routing algorithms for irregular networks has been proposed by the authors. The resulting algorithms increase the maximum achievable throughput while reducing message latency. In this paper, we study how much network performance we are losing due to the irregular topology of NOWs. We analyze the performance of the up*/down* routing algorithm in a 2D mesh topology and compare it with the performance achieved by the XY routing scheme in the same network, in order to answer the following two questions: 1) in a 2D mesh, which of the two routing algorithms achieves better performance?, and 2) where does the up*/down* routing algorithm work better, in a 2D mesh or in an irregular network?. Simulation results show that the up*/down* routing strategy performs better in a regular network than in an irregular one. On the other hand, the XY routing algorithm considerably outperforms the up*/down* scheme. However, when the adaptive routing algorithm proposed by the authors is used, differences in performance are much smaller. Thus, the higher performance of a regular topology could not compensate for the loss in wiring flexibility with respect to irregular networks, or their capability of adding a single switch at any moment.