A Delay Model and Speculative Architecture for Pipelined Routers

This paper introduces a router delay model that accurately models key aspects of modern routers. The model accounts for the pipelined nature of contemporary routers, the specific flow control method employed, the delay of the flowcontrol credit path, and the sharing of crossbar ports across virtual channels. Motivated by this model, we introduce a microarchitecture for a speculative virtual-channel router that significantly reduces its router latency to that of a wormhole router. Simulations using our pipelined model give results that differ considerably from the commonlyassumed ‘unit-latency’ model which is unreasonably optimistic. Using realistic pipeline models, we compare wormhole [6] and virtual-channel flow control [4]. Our results show that a speculative virtual-channel router has the same per-hop router latency as a wormhole router, while improving throughput by up to 40%.