On the use of virtual channels in networks of workstations with irregular topology

Networks of workstations are becoming increasingly popular as a cost-effect ive alternative to parallel computers. Typically, these networks connect wo rkstations using irregular topologies, providing the wiring flexibility, sc alability, and incremental expansion capability required in this environmen t. Recently. we proposed two methodologies for the design of adaptive routi ng algorithms for networks with irregular topology, as well as fully adapti ve routing algorithms for these networks. These algorithms increase through put considerably with respect to previously existing ones, but require the use of at least two virtual channels. In this paper, we propose a very effi cient flow control protocol to support virtual channels when link wires are very long and/or have different lengths. This flow control protocol relies on the use of channel pipelining and control flits. Control traffic is min imized by assigning physical bandwidth to virtual channels until the corres ponding message blocks or it is completely transmitted. Simulation results show that this flow control protocol performs as efficiently as an ideal ne twork with short wires and flit-by-flit multiplexing. The effect of additio nal virtual channels per physical channel has also been studied, revealing that the optimal number of Virtual channels Varies with network size. The u se of virtual channel priorities is also analyzed. The proposed flow contro l protocol may increase short message latency, due to long messages monopol izing channels and hindering the progress of short messages. Therefore, we have analyzed the impact of limiting the number of flits (block size) that a virtual channel may forward once it gets the link. Simulation results sho w that limiting the maximum block size causes the overall network performan ce to decrease.