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3.3. CPU Performance Options

Several CPU related options are available to your guest virtual machines. Configured correctly, these options can have a large impact on performance. The following image shows the CPU options available to your guests. The remainder of this section shows and explains the impact of these options.
CPU Performance Options

Figure 3.3. CPU Performance Options

3.3.1. Option: Available CPUs

Use this option to adjust the amount of virtual CPUs (vCPUs) available to the guest. It is also possible to change the number of vCPUs while the guest is running, which is referred to as hot plugging and hot unplugging.


The hot unplugging feature is only available as a Technology Preview. Therefore, it is not supported and not recommended for use in high-value deployments.
If you allocate more than is available on the host (known as overcommitting), a warning is displayed, as shown in the following image:
CPU overcommit

Figure 3.4. CPU overcommit

CPUs are overcommitted when the sum of vCPUs for all guests on the system is greater than the number of host CPUs on the system. You can overcommit CPUs with one or multiple guests if the total number of vCPUs is greater than the number of host CPUs.


As with memory overcommitting, CPU overcommitting can have a negative impact on performance, for example in situations with a heavy or unpredictable guest workload. Refer to the Red Hat Enterprise Linux 6 Virtualization Administration Guide for more details on overcommitting.