Did you know?
Assigning more vCPU’s to a VM may not necessarily increase CPU performance of a VM. Here are some best practice recommendations.
- Start with one vCPU per VM and increase as needed.
- Do not assign more vCPUs than needed to a VM as this can unnecessarily limit resource availability for other VMs and increase CPU Ready wait time.
- Performance degradation may occur on multiprocessor VMs in which the number of vCPUs was greater than the number of vCPUs in a NUMA node. Generally, try to keep multiprocessor VMs sized so that they fit within a NUMA node. In PvDC this would be (1, 2, 4, 6, and up o 8vCPU).
CPU Ready is a measure of the time a VM has to wait for CPU resources from the host. The larger the number of vCPU, the longer a VM may have to wait for resources from a host. This may result in performance degradation. A VM with lower vCPU will perform better than one with high vCPU.
Co-stop - VMware schedules all the vCPUs in a VM at the same time. If all the allocated vCPUs are not available at the same time, then the VM will be in a state of "co-stop" until the host can co-schedule all vCPUs. In its simplest form, co-stop indicates the amount time after the first vCPU is available until the remaining vCPUs are available for the VM to run. Sizing VMs to use the least number of vCPU’s possible minimizes the time needed for co-stop waits.
Analogy
You are a party of 4 at a restaurant and there are only 16 places to sit in the whole restaurant. You come in and tell the host/hostess that there are 4 of you and you want to sit together. The hostess promptly informs you that there are not 4 seats available and you’ll have to wait. While you are waiting, 3 people get up. Your party can’t sit down because there are only 3 seats, but the couple that just came in behind you can take 2 of those seats since that is all they need. This continues to happen: 2 get up, 1 sits down, 3 get up, 2 sit down.
Throughout the whole time you are waiting for those 4 seats to open up while watching everyone else eat and enjoy their food.
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