CPU Ready Percentage with PowerCLI

An important statistic to always be aware of is CPU Ready Time.

CPU Ready Time is the amount of time a VM is ready with a workload to use CPU but has to wait to be assigned a slot on the actual hardware CPU. Having a High CPU time can mean you have a host that is under CPU Contention.

Looking at the pic above, that is the CPU Ready of a VM over the past week. On average, it seems this VM has to wait over 1 second(>1000ms) each time it wants to use CPU cycles. That kind of seems like a lot to me.

But we have to rememeber that the above is a sample taken over a period of time so all is not as it seems. A better way is to look at the CPU Ready Percentage….it involves math 🙂

Straight from a KB Article:

To calculate the CPU ready % from the CPU ready summation value, use this formula:

(CPU summation value / (<chart default update interval in seconds> * 1000)) * 100 = CPU ready %

Perfect sense right!!! Maybe not so let’s do it with our example

(1421/1800 seconds * 1000)*100= .08%(rounded)

So around .08% of time it has to wait for CPU resources. Said a different way 99.02% of the time it doesn’t have to wait.

This means that despite what we may have concluded just by looking at the summation value, this VM almost never has to wait on CPU resources, which is good!

So let’s look at the PowerCLI

param($vms="*", $interval)
 Switch ($interval)
 {
 "day" {$days=-1;$mins=5;$divider=3000}
 "week" {$days=-7;$mins=30;$divider=18000}
 "month" {$days=-30;$mins=120;$divider=72000}
 }

$groups=Get-Stat -Entity (get-vm $vms ) -Stat cpu.ready.summation -start (get-date).adddays($days) -finish (get-date) -interval $mins -instance "" -ea silentlycontinue|group entity

$output=@()

ForEach ($group in $groups)
 {
 $temp= ""|select-Object Name, "Ready%"
 $temp.name=$group.name

$temp."ready%"= “{0:n2}” -f (($group.group |measure-object value -ave).average/$divider)

$output+=$temp
 }

$output

It is pretty straightforward. You give it a vm/vms, or it will grab all of them. You give the script an interval: day, week,month. Then it is on its way. Be warned any time you are grabbing stats with PowerCLI it might take a while. This becomes even more true for the number of VMs you are grabbing stats for….

Hope this helps and here are two places you can get even more info!

CPU Ready Time in VMware and How to Interpret its Real Meaning

Converting between CPU summation and CPU % ready values