Virtual machines: a whole new world for performance analysis

This article addresses a problem of performance monitoring inside virtual machines (VMs). It advocates focused monitoring of particular virtualized programs, explains the need for and the importance of such an approach to performance monitoring in virtualized execution environments, and emphasizes its benefits for virtual machine manufacturers, virtual machine users (mostly, software developers) and hardware (processor) manufacturers. The article defines the problem of in-VM performance monitoring as the ability to employ modern methods and hardware performance monitoring capabilities inside virtual machines to an extent comparable with what is being done in real environments. Unfortunately, there are numerous reasons preventing us from achieving such an ambitious goal, one of those reasons being the lack of support from virtualization engines; that is why a novel method of 'cooperative' performance data collection is disclosed. The method implies collection of performance data at physical hardware and simultaneous tracking of software states inside a virtual machine. Each statistically visible execution point of the virtualized software may then be associated with information on real hardware events. The method effectively enables time-based sampling of virtualized workloads combined with hardware event counting, is applicable to unmodified, commercially available virtual machines, and has competitive precision and overhead. The practical significance and value of the method are further illustrated by studying a parallel workload and uncovering virtualization-specific performance issues of multithreaded programs.