Process Migration Effects on Memory Performance of Multiprocessor Web-Server

In this work we put into evidence how the memory performance of a Web-Server machine may depend on the sharing induced by process migration. We considered a shared-bus shared-memory multiprocessor as the simplest multiprocessor architecture to be used for accelerating Web-based and commercial applications. It is well known that, in this kind of system, the bus is the critical element that may limit the scalability of the machine. Nevertheless, many factors that influence bus utilization, when process migration is permitted, have not been thoroughly investigated yet. We analyze a basic four-processor and a high-performance sixteen-processor machine. We show that, even in the four-processor case, the overhead induced by the sharing of private data as a consequence of process migration, namely passive sharing, cannot be neglected. Then, we consider the sixteen-processor case, where the effects on performance are more massive. The results show that even though the performance may take advantage of larger caches or from cache affinity scheduling, there is still a great amount of passive sharing, besides false sharing and active sharing. In order to limit false sharing overhead, we can adopt an accurate design of kernel data structures. Passive sharing can be reduced, or even eliminated, by using appropriate coherence protocols. The evaluation of two of such protocols (AMSD and PSCR) shows that we can achieve better processor utilization compared to the MESI case.