VMware is offering a 3 page PDF whitepaper titled "Improving Guest Operating System Accounting for Descheduled Virtual Machines in ESX Server 3.x Systems" that explains how to install and monitor VMDesched on Linux and Windows guest operating systems. It also describes timer interrupt virtualization issues resolved by VMDesched and how VMDesched works.

When installed, the VMDesched component provides two key benefits:

• Improved accuracy for guest operating system CPU time accounting when physical CPU resources are overcommitted.
• Improved guest operating system timekeeping with respect to real time.

The experimental VMware Descheduled Time Accounting component, VMDesched, is an optional new component of VMware Tools. VMDesched is available starting with ESX Server 3.0.

How VMDesched Works

The VMDesched VMware Tools component coordinates with the VMware virtualization layer to determine when there is a backlog of timer interrupts. The virtualization layer arranges to deliver backlogged timer interrupts to the virtual machine only while the guest operating system is executing the VMDesched process. The backlog is cleared as quickly as allowed by the guest operating system.

Because all backlogged timer interrupts are delivered in the context of the VMDesched process, the guest operating system effectively charges VMDesched for consuming all of the time when the virtual machine was descheduled. This allows performance monitoring tools—such as top in Linux and Task Manager in Windows—to report virtual machine descheduled time without distorting the CPU time charged to actual guest processes. This also enables third-party performance tools and workload management software to detect virtual machine descheduled time and to incorporate this information when generating reports or alerts related to utilization.

Read the whitepaper, here.