Virtual Snooping Coherence for Multi-Core Virtualized Systems

Proliferation of virtualized systems opens a new opportunity to improve the scalability of multi-core architectures. Among the scalability bottlenecks in multi-cores, cache coherence has been one of the most critical problems. Although snoop-based protocols have been dominating commercial multi-core designs, it has been difficult to scale them for more cores, as snooping protocols require high network bandwidth and power consumption for snooping all the caches. In this paper, we propose a novel snoop-based cache coherence protocol, called virtual snooping, for virtualized multi-core architectures. Virtual snooping exploits memory isolation across virtual machines and prevents unnecessary snoop requests from crossing the virtual machine boundaries. Each virtual machine becomes a virtual snoop domain, consisting of a subset of the cores in a system. Although the majority of virtual machine memory is isolated, sharing of cachelines across VMs still occur. To address such data sharing, this paper investigates three factors, data sharing through the hypervisor, virtual machine relocation, and content-based sharing. In this paper, we explore the design space of virtual snooping with experiments on emulated and real virtualized systems including the mechanisms and overheads of the hypervisor. In addition, the paper discusses the scheduling impact on the effectiveness of virtual snooping.