Effects of injection pressure on network throughput

©2006 IEEE. Personal use of this material is permitted. However, permission to reprint/republish this material for advertising or promotional purposes or for creating new collective works for resale or redistribution to servers or lists, or to reuse any copyrighted component of this work in other works must be obtained from the IEEE.Recent parallel systems use multiple injection ports and various injection policies, but little is known about their impact on network performance. This paper evaluates the influence that these injection interfaces have on maximum sustained throughput in adaptive cut-through torus networks by modeling the number of injection queues (1 or 4), and the allocation of new packets to those queues. Network evaluations for medium to large size 2D tori show that designs with multiple injection ports do not improve performance under uniform traffic. On the contrary, they result in more pressure from the injection interface to acquire the scarce network resources of an already clogged system. Interestingly, for small networks, a single injection FIFO queue, with the HOLB it entails, indirectly provides the much needed injection control. For networks with thousands of nodes and multiple injection channels, as those being implemented in current massively parallel processors, this implicit form of congestion control is not enough. In such systems, restrictive injection policies are required to prevent routers from being flooded with new packets for loads beyond saturation.C. Izu, J. Miguel-Alonso, J.A. Gregori

Load Unbalance in k-ary n-cube Networks

Abstract. This paper studies the effect that HOL (Head-of-Line) blocking in the packet injection queue has on the performance of bidirectional k-ary ncubes, for values of k over a certain threshold (around 20). The HOL blocking causes an unbalanced use of the channels corresponding to the two directions of bidirectional links, which is responsible for a drop in the network throughput and a rise in the network delay. Simulation results show that this anomaly only appears in those rings where most injections are performed (normally, those in the X axis), and that the elimination of the HOL blocking in the injection queue enables the network to sustain peak throughput after saturation.