Sampling-based Steal Time Accounting under Hardware Virtualization

Virtualization enables the efficient sharing of hardware resources among multiple virtual machines (VMs). Because the physical resources are limited, the scheduler must often suspend one VM to allow some other VM to run. The operating system in a VM is typically unaware of the suspension and accounts periods of suspension as CPU time to the executing application thread. This misrepresentation of resource usage makes it difficult to tell whether a performance problem is caused by an actual bottleneck in the application or by the virtualization infrastructure. We present a novel approach to compute to what degree the threads of an application in a virtual machine are affected by suspension. Our approach does not require any modifications to the operating system or to the virtualization software. It periodically samples the system-wide amount of "steal time" that is reported by the virtualization infrastructure, and divides it among the monitored threads according to their CPU usage. With a prototype implementation, we demonstrate that our approach accounts accurate amounts of steal time to application threads, that it can be used to compute the true resource usage of an application, and that it incurs only negligible performance overhead.