In a previous post on minimizing CPU latency with NUMA, we discussed performance degradation on multiprocessor VMs in which the number of vCPUs was greater than the number of vCPUs in a NUMA node.

6:1 or greater is often going to cause a problem.

3:1 to 5:1 may begin to cause performance degradation.

A general guide for performance of from the Best Practices recommendations is:

The exact amount of CPU overcommitment a VMware host can accommodate will depend on the VMs and the applications they are running.

Do not allocate more vCPUs than needed to a VM as this can unnecessarily limit resource availability for other VMs and increase CPU Ready wait time.

Start with one vCPU per VM and increase as needed.Considerations in Allocating vCPUs to VMs ► Best Practices recommendations This can increase efficiency if there is enough CPU Idle time to provide for scheduling two threads, but in practice performance increases are up to a maximum of 30% and are strongly application dependent. When one thread is idle or waiting, the other thread can execute instructions. Hyperthreading works by providing a second execution thread to a processor core. Please note that hyperthreading does not actually double the available pCPU. If the cores use hyperthreading, the number of logical cores is calculated as: (# pCPU) X (2 threads/physical processor) = # Virtual Processors (vCPU)įor example, if you have 2 processors with 6 cores each: (2 Processor Sockets) X (6 Cores/Processor) = 12 Physical Processors (pCPU) (12 pCPU) X (2 threads/physical processor) = 24 Virtual Processors (vCPU) The number of physical cores (pCPU) available on a host is calculated as: (# Processor Sockets) X (# Cores/Processor) = # Physical Processors (pCPU)

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