Performance analysis of buffer coherency policies in a multisystem data sharing environment

Abstract
Six buffer coherency policies for a multisystem transaction processing environment are compared. These policies differ in their basic approaches on how and when the invalidated pages are identified or if the updated pages are propagated to the buffers of the remote nodes. They can be classified as detection, notification (of invalid pages), and (update) propagation oriented approaches. The policies trade off CPU overhead of coherency messages with buffer hit probability in different ways, resulting in a tradeoff of response time and maximum throughput. The main contribution is to develop analytical models to predict buffer hit probabilities under various buffer coherency policies assuming the LRU replacement policy and the independent reference model (IRM). The buffer models are validated using simulation models and show excellent agreement. Integrated analytic models capturing buffer hit probability and CPU overhead are developed to predict the overall response times under these coherency policies. The difference in buffer hit probabilities amongst various policies are found to be very sensitive to the skewness of the data access.

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