Virtual Hierarchies to Support Server Consolidation

Server consolidation is becoming an increasingly popular technique to manage and utilize systems. This paper develops CMP memory systems for server consolidation where most sharing occurs within Virtual Machines (VMs). Our memory systems maximize shared memory accesses serviced within a VM, minimize interference among separate VMs, facilitate dynamic reassignment of VMs to processors and memory, and support content-based page sharing among VMs. We begin with a tiled architecture where each of 64 tiles contains a processor, private L1 caches, and an L2 bank. First, we reveal why single-level directory designs fail to meet workload consolidation goals. Second, we develop the paper's central idea of imposing a two-level virtual (or logical) coherence hierarchy on a physically flat CMP that harmonizes with VM assignment. Third, we show that the best of our two virtual hierarchy (VH) variants performs 12-58% better than the best alternative flat directory protocol when consolidating Apache, OLTP, and Zeus commel workloads on our simulated 64-core CMP.