In a large multiprocessor server platform using multicore chips, the scheduler often migrates a thread or process, in order to achieve better load balancing or ensure fairness among competing scheduling entities. Each migration incurs a severe performance impact from the loss of cache and Translation Lookaside Buffer (TLB) footprints and subsequent higher cache misses and page walks. Such impact is likely to be more severe in virtualized environments, where high over-subscription of CPUs is very common for server consolidation workloads or virtual desktop infrastructure deployment, causing frequent migrations and context switches. We demonstrate the performance benefit of preserving a portion of L2 cache—in particular, MRU cache lines—and warming the destination L2 cache by prefetching those cache lines under different migration scenarios. We observed a 1.5-27% reduction in CPI (cycles per instruction) following a migration. We also study the effectiveness of preserving TLB entries over a context switch or migration.
In a large multiprocessor server platform, using multicore chips, the scheduler often migrates a scheduling entity, i.e. a thread or process or virtual machine, in order to achieve better load balancing or ensure fairness. The migration impact is likely to be more severe in virtualized environments, where high over-subscription of logical CPUs is very common for server consolidation workloads or virtual desktop infrastructure deployment. We demonstrate the performance benefit of saving and restoring cached data during migration. In particular, we measure the efficiency (benefit per cache block) of saving various subsets of the cached data, in order to balance implementation cost and complexity with improvements in cycle time. We also describe an implementation that moves cached data when a thread migrates, and we show the benefits in terms of reduced misses and reduced processor cycles.
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