On the Performance of Tagged Translation Lookaside Buffers: A Simulation-Driven Analysis

Recent virtualization-driven CPU architectural extensions involve tagging the hardware-managed Translation Look aside Buffer (TLB) entries to avoid TLB flushes during context switches, thereby sharing the TLB among multiple address spaces. While tagged TLBs are expected to improve the performance of virtualized workloads, a systematic evaluation of this improvement, its dependence on TLB and workload related factors and the performance implications of the contention arising from TLB sharing are yet to be investigated. This paper undertakes these investigations using a simulation-driven approach. We develop a simulation model for the tagged TLB and integrate it into a full-system simulation framework. Using this model, we show that the performance impact of using tagged TLBs ranges from 1% to 25% and is highly dependent on the size of the TLB, the TLB miss penalty and the nature of the workload and the type of tag used. The performance of consolidated workloads is also simulated and the observations from these simulations are used to highlight the performance variation due to resource contention in the shared TLB. Isolating the TLB behavior of one application in a consolidated workload from these variations due to the TLB contention by means of a static TLB usage control scheme is also explored. Furthermore, we show that the performance improvement due to tagged TLBs can be further increased by 1.4X for selected high-priority applications, by restricting the TLB usage of other low-priority workloads, in a consolidated workload scenario.