Enhancements to the Scalable Coherent Interface Cache Protocol

As the number of NUMA system\u27s cache coherency protocols based on the IEEE Std. 1596-1992, Standard for Scalable Coherent Interface (SCI) Specification increases, it is important to review this complex protocol to determine if the protocol can be enhanced in any way. This research provides two realizable extensions to the standard SCI cache protocol. Both of these extensions lie in the basic confines of the SCI architectures. The first extension is a simplification to the SCI protocol in the area of prepending to a sharing list. Depending if the cache line is marked Fresh or Gone , the flow of events is distinctly different. The guaranteed forward progress extension is a simplification to the SCI protocol in this area; making the act of prepending to an existing sharing list independent of whether the line is in the Fresh or Gone state. In addition, this extension eliminates the need for SCI command, as well as distributes the resource requirements of supplying data of a shared line equally among all nodes of the sharing list. The second extension addresses the time to purge (or invalidate) an SCI sharing list. This extension provides a realizable solution that allows the node being invalidated to acknowledge the request prior to the completion of the invalidation while maintaining the memory consistency model of the processors of the system. The resulting cache protocol was developed and implemented for Sequent Computer System Inc. NUMA-Q system. The cache protocol was run on systems ranging from eight to sixty four processors and provided between 7% and 20% reduction in time to invalidate an SCI sharing list