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What is CPU READY?

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Posted: 21 Feb, 2008
by: Troen L.
Updated: 12 Sep, 2009
by: Troen L.

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 is known as ready time.

The higher the ready time is, the slower the virtual machine is performing. The ready time should preferably be as low as possible. Virtual machines that are allocated multiple cpus or have high timer interrupts are more frequently seen with high ready time values.

Previously VMware warned about having virtual machines with 5% or higher values, but they now seem to no longer specify such a given value. Whether 5% is ok for a given service depends on the service, but it should preferably be as low as possible.

VMware has a very good paper on this subject here: http://www.vmware.com/pdf/esx3_ready_time.pdf

The Virtual Infrastructure client for vCenter 2.x will only show ready time values as milliseconds so if you want to see it as percent you might want to use esxtop, resxtop (Remote CLI), a third party alternate product or you can calculate the values found there into percent yourself. In vSphere 4.0 the ready values are showing up under the performance grahs as percent like you expect.

We have seen that the cpu ready figures have dropped quite a bit when using hosts with newer cpus that support RVI or EPT. Another thing that seems to have a major effect on ready figures is to enable paravirtualization (VMI) for Linux VMs.

Here are some stats from the VMware Remote CLI 3.5U2 appliance as seen from different places. This appliance is based on debian 3.1 and is running with healthy performance and show very low ready values.

Virtual Infrastructure client:



Below we can see a VM running a Tomcat (java) web application. The ready figures was a bit high (but not critical) and the performance was not 100% optimal on the older generation cpu. After moving the VM with VMotion from the older 2.6GHz AMD (Santa Rosa) Opteron to a 2.5GHz (Barcelona) Opteron we could see that the ready figures dropped instantly while the cpu load remained the same:

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