VCache: Providing a Transparent View of the LLC in Virtualized Environments

Since most of the current multi-core processors use a large last-level cache (LLC), efficient use of an LLC is critical for the overall performance of multi-cores. To improve the caching efficiency, page coloring is a representative software-based approach to allow the OS to control placement of pages on an LLC to improve their cache utility and to avoid conflicts among cores. However, system virtualization, with additional address translation by the hypervisor, can make page coloring techniques used by the guest OS ineffective, as guest physical addresses used by the guest OS for coloring differ from real addresses used for cache indexing in the LLCs. In this paper, we propose a novel LLC architecture to provide the guest OS with a flexible control over LLC placement in virtualized systems. The proposed vCache architecture can preserve coloring information set by the guest OS. In addition to color preservation, vCache can potentially eliminate the traditional limitation of page coloring, the cost of dynamic color changes for memory pages. By using the pollute buffer mechanism, one of the color-based cache optimization techniques, vCache shows performance improvement of benchmark applications up to 33% without degrading the performance of another co-running application in the VM.