The Journal of Supercomputing, 37, 271–295, 2006 C  2006 Springer Science + Business Media, LLC. Manufactured in The Netherlands. Explanation of Performance Degradation in Turn Model SLAVKO GAJIN slavko.gajin@rcub.bg.ac.yu Belgrade University Computer Centre, University of Belgrade, Serbia and Montenegro ZORAN JOVANOVI ´ C zoran@rcub.bg.ac.yu Faculty of Electrical Engineering, University of Belgrade, Serbia and Montenegro Abstract. The Turn model routing algorithms for mesh interconnection network achieve partial adaptivity without any virtual channels. However, the routing performance measured by simulations is worse than with the simple deterministic routing algorithm. Authors have explained these results simply by uneven dynamic load through the network. However, this phenomenon has not been studied further. This paper investigates performance degradation with Turn model and drawbacks of partially adaptive routing in comparison with the deterministic routing, and it introduces some new concepts. Our simulations deal with individual channels and results are presented by 3D graphs, rather than by commonly used averages. An additional parameter—channel occupation, which is consistent with queuing theory commonly used in many proposed analytical models, is introduced. We also propose a new structure, the Channel Directions Dependency Graph (CDDG). It provides a new approach in analysis, helps in understanding of dynamic routing behaviour, and it can be generalized in other routing algorithms. Keywords: interconnection networks, multicomputers, wormhole, turn-model, deterministic routing, adap- tive routing, virtual channel 1. Introduction Communication between processing nodes in multicomputers is performed by message passing mechanism through the interconnection network. The nodes in the interconnec- tion network, named routers, support a certain number of input and output channels. Links between routers connect output channels of one router to input channels of its neighbour- ing routers, defining the network topology. Regular topologies are preferred because they are supported by generic mathematical models. The most popular topologies are orthog- onal structures: n-dimensional meshes and k-ary n-cubes with wrap-around channels. Messages are often divided into packets and each packet is serialized into a sequence of data units called flow control unit or flits. Channels receive and forward flits through the network in pipeline fashion providing queuing buffers typically organized in FCFS (First Come First Served) manner. The message passing mechanism generally deals with flow control and routing, which define how the flits traverse along the certain path. The most popular flow control tech- nique is wormhole, since it forwards the heading flit as soon as possible, while remaining flits follow it in the pipeline. As the result, one flit buffer per channel is sufficient in the router to provide wormhole technique.