What is a VMware vCPU?

Written by:
Daniele Carotenuto
Date Posted:
22 July 2011

A vCPU is a virtual central processing unit that runs on VMware, the market leader in virtualization software.

What is vCPU ?
A vCPU stands for virtual central processing unit. One or more vCPUs are assigned to every Virtual Machine (VM) within a cloud environment. Each vCPU is seen as a single physical CPU core by the VM’s operating system. If the host machine has multiple CPU cores at its disposal, then the vCPU is actually made up of a number of time slots across all of the available cores, thereby allowing multiple VMs to be hosted on a smaller number of physical cores.

How many vCPU’s can I use on my Virtual Machine (VM)?
A VM can use up to 4 vCPUs under VMware’s standard ESX host license, and up to 8 vCPUs on the Enterprise edition. The number of vCPUs assigned to your server depends on the amount of load the VM will be under.

What is the equivalent of 1vCPU in physical CPU terms?
A general estimation is that 1 vCPU = 1 Physical CPU Core. However, this is not entirely correct, as the vCPU is made up of time slots across all available physical cores, so in general 1vCPU is actually more powerful than a single core, especially if the physical CPUs have 8 cores.

VMware is designed to allow each VM to use a certain proportion of a core’s clock time using complex sharing algorithms in its “CPU Scheduler”. With an entreprise-grade cloud, if there is not enough CPU resources to go around (e.g. in the unlikely occurrence that the VMs are running vCPUs at 100%), the VMs can be automatically migrated using DRS and vMotion onto another host server which has spare CPU resources.

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