Performance of the SCI Ring

The Scalable Coherent Interface (SCI) is an emerging IEEE standard that provides computer-bus-like services to a set of nodes via fast, unidirectional links. This paper presents the first detailed performance study of the SCI ring, using both analytical models and simulation. Performance is analyzed for uniform and non-uniform traffic, and the effect of the ring's flow control protocol is studied. The queueing model is based on an M/G/1 queue, augmented to include the effect of packet trains on the mean and variance of the source transmission time. The model is validated against simulation results, and shown to be quantitatively accurate for uniform workloads, and at least qualitatively accurate for non-uniform workloads. The flow control mechanism is shown to effectively prevent node starvation and reduce the ability of nodes to unfairly consume ring bandwidth, but at the cost of decreased overall ring utilization. The SCI ring is also compared to a standard bus, modeled with a simple MIG/1 queue, and shown to provide substantially higher throughputs and lower latency than a bus with realistic clock speeds.