Virtual Snooping: Filtering Snoops in Virtualized Multi-cores

Virtualization has been rapidly expanding its applications in numerous server and desktop environments to improve the utilization and manageability of physical systems. Such proliferation of virtualized systems opens a new opportunity to improve the scalability of future multi-core architectures. Among the scalability bottlenecks in multi-cores, cache coherence has been a critical problem. 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. However, in real virtualized systems, virtual machines cannot partition the cores perfectly without any data sharing across the snoop partitions. This paper investigates three factors, which break the memory isolation among virtual machines: data sharing with a hyper visor, virtual machine relocation, and content-based data sharing. In this paper, we explore the design space of virtual snooping with experiments on real virtualized systems and approximate simulations. The results show that virtual snooping can reduce snoops significantly even if virtual machines migrate frequently. We also propose mechanisms to address content-based data sharing by exploiting its read-only property.