Modularity, Routing and Fault Tolerance in Interconnection Networks

In interconnection networks, there are several problems related to deadlock. We shall address deadlocks in the context of modularity, shortest path routing in irregular networks and fault tolerance. Deadlock freedom in interconnection networks is not a modular issue. One cannot generally combine two or more deadlock free interconnection network configurations without introducing new deadlock problems. We show how flow control can be used to isolate the deadlock issues within each interconnection network configuration, allowing modular reasoning on deadlock freedom. Achieving shortest path routing in irregular interconnection networks is not trivial. Irregular topologies tend to have cycles of links, and routing shortest path within such cycles quickly causes deadlocks. We use a number of virtual channels divided into layers, and let the routing of a packet be restricted to one (or a few) layer. Each layer only allows shortest path routes which do not cause deadlocks. Dynamic fault tolerance in irregular interconnection networks can be handled by using a reconfiguration to address the new topology of the network. During reconfiguration, there is a time window where not all sources have operative routes to all destinations. Also, the dependencies between old and new routing functions make it difficult to allow only parts of the network to participate in the reconfiguration. We propose two methods, one that addresses the first problem by exploiting the inherent redundancy of the network, and a second which allows most of the network nodes to remain oblivious of a fault.