Hardware support for spin management in overcommitted virtual machines

Multiprocessor operating systems (OSs) pose several unique and conflicting challenges to System Virtual Machines (System VMs). For example, most existing system VMs resort to gang scheduling a guest OS's virtual processors (VCPUs) to avoid OS synchronization overhead. However, gang scheduling is infeasible for some application domains, and is inflexible in other domains. In an overcommitted environment, an individual guest OS has more VCPUs than available physical processors (PCPUs), precluding the use of gang scheduling. In such an environment, we demonstrate a more than two-fold increase in runtime when transparently virtualizing a chip-multiprocessor's cores. To combat this problem, we propose a hardware technique to detect several cases when a VCPU is not performing useful work, and suggest preempting that VCPU to run a different, more productive VCPU. Our technique can dramatically reduce cycles wasted on OS synchronization, without requiring any semantic information from the software. We then present a case study, typical of server consolidation, to demonstrate the potential of more flexible scheduling policies enabled by our technique. We propose one such policy that logically partitions the CMP cores between guest VMs. This policy increases throughput by 10–25% for consolidated server workloads due to improved cache locality and core utilization, and substantially improves performance isolation in private caches.