Wednesday, July 11, 2018

List of most common CPU performance issues


High Ready Time: A CPU is in the Ready state when the virtual machine is ready to run but unable to run because the vSphere scheduler is unable to find physical host CPU resources to run the virtual machine on. 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 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: When the Physical CPUs of a vSphere host are being consistently utilized at 85% or more then the vSphere host may be saturated. When a vSphere host is saturated, it is more difficult for the scheduler to find free physical CPU resources in order to run virtual machines.

Guest 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.

Oversizing VM vCPUs: Using large SMP (Symmetric Multi-Processing) virtual machines can cause unnecessary 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, which is configured with multiple vCPUs and 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 incorrectly configured.

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 of some other resource such as I/O or Memory and therefore cannot fully utilize the assigned vCPU resources.

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