CPU Ready: Over Built VM or Over Utilized Host?

When talk performance with any virtualized server environment, CPU Ready is a common key indicator of how well your VM is performing. However, its not always a cut and dry explination as to why your CPU Ready times are high.

To start, for those that aren’t aware, CPU Ready is:

The amount of time a virtual machine waits in the queue in a ready-to-run state before it can be scheduled on a CPU.

This means that a VM is ready to process something, however, it has to wait because the CPU resources it requires are not available on the physical host.

Before we examine causes of high CPU Ready, lets try to look at what are acceptable values for CPU Ready time. Unfortunately, there is no hard set value to say ‘Yep, your CPU Ready has crossed the ‘its bad’ threshold.’ General rule of thumb is that your CPU Ready time not be higher than 200ms if being checked in the vCenter performance charts, or 5% if being checked using the esxtop command. Again, this isn’t a hard set value. Your VMs role may require less CPU Ready time for more critical functions, or may be more lenient to longer CPU Ready times as well. It all depends on your environment, and its up to you as an admin to determine what works in your individual environment.