Because VMware products provide a virtual interface to the hardware, traditional performance instrumentation that is based on measuring hardware resources may not be accurate. As a result, Perfmon (in Windows) and top (in UNIX variants) will not provide accurate measurements of CPU utilization. The problems seen as a result of usage of traditional in-guest performance measurements come from three areas:

1. They are unaware of work being performance by the virtualization software, they will not have complete information on the resources being used by the virtualization software. This includes memory management, scheduling, and other support processes like the service console in ESX.
2. The way in which guest OSes account time is different and ineffective in a virtual machine.
3. Their visibility into available CPU resources is based on the fraction of the CPU that they have been provided by the virtualization software.

Items two and three are covered in more detail in a KB on the subject. Performance analysis on virtual deployments should always use host-based tools. On ESX Server, this means esxtop or VirtualCenter.