The Cost of Adaptivity and Virtual Lanes in a Wormhole Router

We examine the cost in router complexity of adaptivity and virtual lanes in wormhole routers, using f-flat adaptive routers (based on a generalization of planar-adaptive routing) which include routers with a range of routing freedom. Our studies show that adaptivity is expensive because it requires additional virtual channels and much larger crossbar switches for both adaptivity and deadlock prevention. Increases of 50 to 100% in channel utilization are required to justify additional degrees of routing freedom.

Three internal router architectures for virtual lanes are examined and the fully expanded crossbar is found to be most effective because it gives simplest control and minimal internal blocking. Examining router designs with from 1–16 virtual lanes indicates that 30% improvements in channel utilization are required to justify each additional virtual lane. These studies combined with published simulation results indicate that only modest numbers of virtual lanes are likely to be cost effective.