Adaptive routing in RS/6000 SP-like bidirectional multistage interconnection networks

The IBM RS/6000 SP is one of the most successful commercially available multicomputers. SP owes its success partially to the scalable, high bandwidth, low latency network. In this paper, we present the adaptive routing scheme used in the new SP network switch chip called the Switch2. We show that the adaptive routing methods outperform the oblivious routing methods on SP like multistage networks. It is shown that the adaptive routing increases the network throughout by up to 229% over oblivious routing in some cases. We also study the effect of output selection, functions on the network performance. We present six different output selection functions and study their performance for different system parameters and communication patterns through extensive simulation. The results show that three of these selection functions perform similar to each other and outperform the other selection functions consistently. Therefore, unlike previous research findings for meshes and tori, there is no need to use multiple (or hybrid) selection functions to obtain the best performance for the SP like bidirectional multistage interconnection networks. We also provide an analysis of the cost-effectiveness of different selection functions with respect to the complexity of their hardware implementations.