Virtual machine CPU ready

Resolving The Problem

What';s Happening

vCenter triggers a virtual machine CPU ready alarm.

Troubleshooting

CPU ready time is a vSphere metric that records the amount of time a VM is ready to use CPU but was unable to schedule physical CPU time because all the vSphere ESXi host CPU resources are busy. CPU ready time is dependent on the number of VMs on the host and their CPU loads. It is normal for a VM to average between 0–50 ms of CPU ready time; anything over 1000 ms is considered to lead to VM performance problems.

VMs that are configured with multiple vCPUs will suffer from an increased amount of ready time than compared to VMs with fewer vCPUs configured. Ready time increases as the VM needs more vCPU resources to be made available at the same time and, therefore, may have to wait for extended times for all the required vCPUS to be free to be made available to the VM with more vCPUs.

To be able to interpret ready times, it is essential to know the relationship between the percentage of esxtop and milli-seconds used in the performance graphs in vCenter, so refer to this VMware KB article for Converting between CPU summation and CPU % ready values (2002181)

Refer to the VMware documentation Performance monitoring utilities: resxtop and esxtop on the usage of esxtop. To check the CPU ready value, examine the RDY% in esxtop field for the percentage of time that the VM was ready but could not be scheduled to run on a physical CPU. Under normal operating conditions, this value should remain under 5%. If the ready time values are high on the VM that experience bad performance, then check for CPU limiting:

• Make sure the VM is not constrained by a CPU limit set by itself.
• Make sure that the VM is not constrained by its resource pool.
• If there is no CPU limiting, then the host could be too oversubscribed. Consider adding additional capacity to the cluster.
• Evaluate if the VMs need to have the number of configured vCPUs, and consider reducing the number of vCPUs.

Refer to the VMware Community article, Interpreting esxtop statistics for information on esxtop.