How to do it…

To spot CPU overcommitment, there are a few CPU resource parameters that you should monitor closely. They are:

1. Log in to the ESXi Server using an SSH client (Putty).
2. Type esxtop and hit Enter.

1. Monitor the preceding values to understand CPU overcommitment.

This example uses esxtop to detect CPU overcommitment. Looking at the pCPU line near the top of the screen, you can determine that this host's two CPUs are 100 percent utilized. Four active VMs are shown, Res-Hungry-1 to Res-Hungry-4. These VMs are active because they have relatively high values in the %USED column. The values in the %USED column alone do not necessarily indicate that the CPUs are overcommitted. In the %RDY column, you see that the three active VMs have relatively high values. High %RDY values plus high %USED values are a sure indicator that your CPU resources are overcommitted.

From the CPU view, navigate to a VM and press the E key to expand the view. It will give a detailed vCPU view for the VM. This is important because, at a quick level, CPU that is ready as a metric is best referenced when looking at performance concerns more broadly than a specific VM. If there is high ready percentage noted, contention could be an issue, particularly if other VMs show high utilization when more vCPUs than physical cores are present. In that case, other VMs could lead to high ready time on a low idle VM. So, long story short, if the CPU ready time is high on VMs on a host, it's time to verify that no other VMs are seeing performance issues.

You can also use the vCenter performance chart to spot CPU overcommitment, as follows:

1. Log in to the vCenter Server using vSphere Web Client.
2. On the home screen, navigate to Hosts and Clusters.
3. Expand the left-hand navigation list.
4. Navigate to one of the ESXi hosts.
5. Navigate to the Monitor tab.
6. Navigate to the Performance tab.
7. Navigate to the Advanced view.
8. Click on Chart Options.
9. Navigate to CPU from Chart metrics.
10. Select only Used and Ready in the Counters section and click on OK:

Now you will see the ready time and the used time in the graph and you can spot the overcommitment. The following screenshot is an example output:

The following example shows that the host has high ready time: