On the Impact of Routing Algorithms in the Effectiveness of Queuing Schemes in High-Performance Interconnection Networks

Abstract

In High-Performance Computing (HPC) systems, the design of the interconnection network is crucial. Indeed, the network topology, the switch architecture and the routing scheme determine the network performance and ultimately the system one. As the number of endnodes in HPC systems grows, and the supported applications become increasingly demanding for communication, the use of techniques to deal with network congestion and its negative effects gains importance. For that purpose, routing schemes such as adaptive or oblivious try to balance the network traffic in order to prevent and/or eliminate congestion. On the other hand, there are deterministic routing schemes that balance the number of paths per link with the aim of reducing the head-of-line blocking derived from congestion situations. Furthermore, other techniques to deal with congestion are based on queuing schemes. This approach is based on storing separately different packet flows at the ports buffers, so that the head-of-line blocking and/or buffer-hogging are reduced. Existing queuing schemes use different policies to separate flows, and they can be implemented in different ways. However, most queuing schemes are often used and designed assuming that the network is configured with deterministic routing, while actually they could be combined also with adaptive or oblivious routing.This paper analyzes the behavior of different queuing schemes under different routing algorithms: deterministic, adaptive or oblivious. We focus on fat-tree networks, configured with the most common routing algorithms of each type suitable for that topology. In order to evaluate these configurations, we have run simulation experiments modeling large fat-trees built from switches with radices available in the market, and supporting several queuing schemes. The experiments results show how different the performance of the queuing schemes may be when combined with either deterministic or oblivious/adaptive routing. Indeed, from these results we can conclude that some combinations of queuing schemes and routings are counterproductive.