CPU Ready time, in ESX land, is the amount of time a VM was waiting to be serviced by a phsyical processor core. So, if you have a VM that wanted to do stuff, but had to wait 200ms to be scheduled onto a processor, it has 200ms of ready time. Its a sign of scheduling contention.
To achieve best performance in a consolidated environment, you must consider ready time -
the time a virtual machine must wait in a ready-to-run state before it can be scheduled on a
CPU. This paper provides information to help you understand the factors that influence ready
time on an ESX Server 3.0 system
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Processor Utilization versus Ready Time
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The first test (Figure 1) shows how ready time changes with utilization. Six virtual machines were
running on the test server. All of them were pinned to a single CPU. Load was generated by the
CPU burner program run with a setting of approximately 15 percent, so that if it were the only
thing running, the virtual machine would consume close to 15 percent of the capacity of a
single CPU. Idle virtual machines also consumed some CPU capacity to perform guest operating
system maintenance tasks.