An extension to the SCI flow control protocol for increased network efficiency

The scalable coherent interface (SCI) is a newly established IEEE standard that provides bus-like services to a network of nodes without actually using a physical bus. Rather, it utilizes very fast, unidirectional links that can be arranged into a variety of topologies. SCI provides a novel fairness protocol that equally allocates bandwidth to all nodes on the network. When global fairness is not required, it is often desirable to relax its enforcement; in exchange for increased network utilization and better load balancing. This paper presents an extension to the SCI flow control protocol that increases network utilization and, hence, attainable system throughput by enforcing fairness only among nodes whose transmissions interfere with one another. The enhanced protocol grants a node as much bandwidth as it requires, as long as its transmissions do not inhibit those of other nodes. Although this is commonly known as mar-min fairness, we more descriptively refer to it as relaxed fairness. We present extensive simulation results and compare them to the standard SCI protocol and to our desired performance, which has been previously derived in the literature. The comparisons show that, on average, this enhancement can provide a substantial increase in performance over the original protocol.