Reestablishing Page Placement Mechanisms for Nested Virtualization

Page placement mechanisms have long been used to reduce cache conflict misses. They become more important in clouds where the emerging way-based cache partitioning is used for better workload isolation but at a cost of increased cache conflicts. However, page placement mechanisms become ineffective in virtualized environments, such as clouds, because the real locations of memory pages (i.e., their host physical addresses) are hidden from guest OSs. The paper proposes <sc>xPlace</sc> as a solution to reestablish page placement mechanisms under the nested virtualization configuration. To keep high portability and low overhead, <sc>xPlace</sc> follows an approach that creates a synergy between the host and guest VMs, such that the page placement mechanism inside each guest VM becomes effective even if its page placement decisions are made based on the guest physical addresses of memory pages. The paper addresses the technical issues for implementing this approach in the nested virtualization setting, particularly how to create the synergy with the obstacle created by guest hypervisors sitting between the host and guest VMs. Evaluation based on the prototype implementation and diverse real world applications shows that <sc>xPlace</sc> can greatly reduce cache conflicts and improve application performance in the nested environment.