Cache restoration for highly partitioned virtualized systems

Abstract

The economics of server consolidation have led to the support of virtualization features in almost all server-class systems, with the related feature set being a subject of significant competition. While most systems allow for partitioning at the relatively coarse grain of a single core, some systems also support multiprogrammed virtualization, whereby a system can be more finely partitioned through time-sharing, down to a percentage of a core being allotted to a virtual machine. When multiple virtual machines share a single core however, performance can suffer due to the displacement of microarchitectural state. We introduce cache restoration, a hardware-based prefetching mechanism initiated by the underlying virtualization software when a virtual machine is being scheduled on a core, prefetching its working set and warming its initial environment. Through cycle-accurate simulation of a POWER7 system, we show that when applied to its private per-core L3 last-level cache, the warm cache translates into 20% on average performance improvement for a mixture of workloads on a highly partitioned core, compared to a virtualized server without cache restoration.