Network Performance Under Bimodal Traffic Loads

In actual multicomputer networks, communications consist of hybrid traffic in which messages exhibit a variety of sizes. However, to date, most studies on network performance are based on traffic loads of uniformly-sized messages. We investigate the performance of wormhole-routed networks under bimodal traffic distributions, a mix of short and long messages. Our studies show that the presence of long messages degrades network performance for short messages dramatically, qualitatively changing network behavior. We present an analytical model for wormhole-routed networks which not only models network performance under uniformly sized message loads more accurately than existing models, but also can be extended to support bimodal traffic distributions. The model is validated against detailed simulation of routing networks, over a variety of message size distributions and message lengths. In virtually all cases, the model accurately predicts both network throughput and average message latency to within 8%. Because the impact of long messages can be severe, we consider three techniques-packetization, virtual lanes, and adaptive routing-to alleviate their impact. Packetization reduces the blocking time of long messages, improving network performance in most cases. Virtual lanes and adaptive routing together provide sufficient routing freedom to eliminate much of the blocking, producing performance comparable or even superior to that produced by packetization. Together, all three techniques are complementary, providing robust performance over a variety of traffic mixes and message sizes.