I have started more and more to come across virtualised performance test environments. The problem is that many of these environment are over provisioned that is the sum of Virtual Machines vCPU in the ESX farm is greater than the number of avialable threads. That means that your virtual machine may have to wait to be scheduled onto a running thread. So, this pushes performance testing into the off hours when less of the other VM will be active (remember even an idle VM will want some CPU occessionally). But how can I tell if my VMs are waiting to be run?
Ready time is a percentage of time a virtual machine waits for a CPU to become available. In VMWare v3 is a percentage of time spent waiting, the sample period is 20 seconds so a 10% ready time means that your VM waited 2 seconds in total in the 20 second sample period. The higher the ready time the less valid the test.