ESXi and VM CPU Performance Issues

The following is a description of some common ESXi and VM CPU Performance Issues:

High Ready Time

Ready Time above 10% could indicate CPU contention and might impact the Performance of CPU intensive application. However, some less CPU sensitive application and virtual machines can have much higher values of ready time and still perform satisfactorily.

High CoStop (CSTP) Time

CoStop time indicates that there are more vCPUs than necessary, and that the excess vCPUs make overhead that drags down the performance of the VM. The VM will likely run better with fewer vCPUs. The vCPU(s) with high CoStop is being kept from running while the other, more-idle vCPUs are catching up to the busy one.

CPU Limits

CPU Limits directly prevent a virtual machine from using more than a set amount of CPU resources. Any CPU limit might cause a CPU performance problem if the virtual machine needs resources beyond the limit.

Host CPU Saturation

Guest CPU (vCPU) Saturation is when the application inside the virtual machine is using 90% or more of the CPU resources assigned to the virtual machine. This may be an indicator that the application is being bottlenecked on vCPU resource. In these situations, adding additional vCPU resources to the virtual machine might improve performance.

Incorrect SMP Usage

Using large SMP virtual machines can cause extra overhead. Virtual machines should be correctly sized for the application that is intended to run in the virtual machine. Some applications may only support multithreading up to a certain number of threads.  Assignment of additional vCPU to the virtual machine may cause additional overhead.  If vCPU usage shows that a machine that is configured with multiple vCPUs is only using one of them, then it might be an indicator that the application inside the virtual machine is unable to take advantage of the additional vCPU capacity, or that the guest OS is not configured correctly.

Low Guest Usage

Low in-guest CPU utilization might be an indicator that the application is not configured correctly or that the application is starved on some other resource such as I/O or Memory and therefore cannot fully utilize the assigned vCPU resources.

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