Trying to understand strange VM CPU Usage reporting

[digahill]

I have a pretty simple question regarding how VM CPU Usage is reported on in vSphere. I've searched for the answer, but haven't found any discussions on the subject.

1. I booted my esxi host. No machines are set to auto run.
2. I booted one machine, host name PBX, that has a very consistent (and relatively light) workload.
3. I monitored the vSphere Web Client performance chart for the VM's CPU Usage in real time.
4. I tracked the CPU usage for about 30 minutes, noting that it was fairly stable around 4%.
5. I then booted 6 additional VMs on the same esxi host.
6. The actual work PBX is doing at this point does not change. However, the CPU Usage metric starts to increase significantly from the 4% baseline. Again, the PBX workload is the same as it has been for the last 30 minutes. No more demand on the CPU whatsoever. Why is my CPU Usage metric rising during the boot of other VMs? Why does the CPU Usage metric STAY higher than the previous baseline, even after the boot storm is over? Why is Demand rising?

I don't think this specifically relates to boot storms, but that was the best way to reproduce the scenario and highlight my question. Perhaps I have a fundamental misunderstanding of what the CPU Usage metric means. And CPU Demand. Please enlighten me if so.

Graph attached.

[daphnissov]

This can mean several things including the co-stop value is rising in your host as more VMs are brought online and contend for physical CPU cores. This can also be seen if the power policy of your ESXi host is non-optimal where the host is introducing sleep states in order to throttle the CPU. Ensure your VMs are properly sized and ensure your host is using a power policy that allows ESXi to control the power state.

[digahill]

daphnissov,

I appreciate your reply but I still don't understand why CPU Usage is rising. The VM is not doing any more work than it had previously. I fully understand why co-stop, ready, waits, etc. would rise. But the vmware definition of CPU Usage is, "Amount of actively used virtual CPU as a percentage of total available CPU."

Why does my VM suddenly start actively using more virtual CPU at 1:14PM, when there is no additional workload on that VM? The only thing I did was start up some other VMs. I understand that the Host's CPU Usage metric would rise. But why the VM's?