Simulation study of memory performance of SMP multiprocessors running a TPC-W workload

The infrastructure to support electronic commerce is one of the areas where more processing power is needed. A multiprocessor system can offer advantages for running electronic commerce applications. The memory performance of an electronic commerce server, i.e. a system running electronic commerce applications, is evaluated in the case of shared-bus multiprocessor architecture. The software architecture of this server is based on a three-tier model and the workloads have been setup as specified by the TPC-W benchmark. The hardware configurations are: a single SMP running tiers two and three, and two SMPs each one running a single tier. The influence of memory subsystem on performance and scalability is analysed and several solutions aimed at reducing the latency of memory considered. After initial experiments, which validate the methodology, choices for cache, scheduling algorithm, and coherence protocol are explored to enhance performance and scalability. As in previous studies on shared-bus multiprocessors, it was found that the memory performance is highly influenced by cache parameters. While scaling the machine, the coherence overhead weighs more and more on the memory performance. False sharing in the kernel is among the main causes of this overhead. Unlike previous studies, passive sharing i.e. the useless sharing of the private data of the migrating processes, is shown to be an important factor that influences performance. This is especially true when multiprocessors with a higher number of processors are considered: an increase in the number of processors produces real benefits only if advanced techniques for reducing the coherence overhead are properly adopted. Scheduling techniques limiting process migration may reduce passive sharing, while restructuring techniques of the kernel data may reduce false sharing misses. However, even when process migration is reduced through cache-affinity techniques, standard coherence protocols like MESI protocol don't allow the best performance. Coherence protocols such as PSCR and AMSD produce performance benefits. PSCR, in particular, eliminates coherence overhead due to passive sharing and minimises the number of coherence misses. The adoption of PSCR and cache-affinity scheduling allows the multiprocessor scalability to be extended to 20 processors for a 128-bit shared bus and current values of main-memory-to-processor speed gap