Optimizing vCPUs can impact the resource efficiency of any VMs. One way to optimize is to adjust how many vCPUs are assigned to a KVM instance. Hot plugging or hot unplugging vCPUs is when you configure vCPU count on a running KVM.

You can change the number of vCPUs that are active in a guest KVM using the virsh setvcpus command. By default, virsh setvcpus works on running guest KVMs. To change the number of vCPUs for a stopped KVM, add the --config option.

For example:

virsh setvcpus domain-name, id, or uuid count-value {--config | --live | --current} --guest

You can use the --config and --live options together if permitted by the hypervisor. If you don't specify --config, --live, or --current, the --live option is assumed. If you don't select an option and the guest KVM isn't running, the command fails. Furthermore, if no options are specified, it's up to the hypervisor whether the --config option is also assumed; and the hypervisor determines whether the XML configuration is adjusted to make the change persistent.

To improve the performance of a KVM, you can assign additional host RAM to a KVM instance. You can also decrease the amount of allocated memory to free up the resource for other KVMs or tasks. Hot plugging or hot unplugging memory is when you configure memory size on a running KVM.

You use the virsh setmem command to change the available memory for a KVM. To change the maximum memory that can be allocated, use the virsh setmaxmem command.

To change a KVM's memory allocation, run:

virsh setmem domain-name, id, or uuid --kilobytes size

You must specify the size as a scaled integer in kibibytes and the new value can't exceed the amount you specified for the KVM. Values lower than 64 MB are unlikely to work with most KVM guest operating systems. A higher maximum memory value doesn't affect active KVMs. If the new value is lower than the available memory, it shrinks memory usage possibly causing the KVM to crash.

Use following command options to allocate memory to a KVM instance:

• domain

  A string value representing the KVM name, ID, or UUID.

• size

  A number value representing the new memory size, as a scaled integer. The default unit is KiB, but you can select from other valid memory units:

  • b or bytes for bytes

  • KB for kilobytes (103 or blocks of 1,000 bytes)

  • k or KiB for kibibytes (210 or blocks of 1024 bytes)

  • MB for megabytes (106 or blocks of 1,000,000 bytes)

  • M or MiB for mebibytes (220 or blocks of 1,048,576 bytes)

  • GB for gigabytes (109 or blocks of 1,000,000,000 bytes)

  • G or GiB for gibibytes (230 or blocks of 1,073,741,824 bytes)

  • TB for terabytes (1012 or blocks of 1,000,000,000,000 bytes)

  • T or TiB for tebibytes (240 or blocks of 1,099,511,627,776 bytes)

• --config

  Changes the stored XML configuration for the guest KVM and takes effect when the guest is started.

• --live

  The guest KVM must be running and the change takes place immediately, thus hot plugging memory.

• --current

  Affects the memory on the current guest KVM.

To set the maximum memory that can be allocated to a KVM, run:

virsh setmaxmem domain-name_id_or_uuid size --current

You must specify the size as a scaled integer in kibibytes unless you also specify a supported memory unit, which are the same as for the virsh setmem command.

All other options for virsh setmaxmem are the same as for virsh setmem with one caveat. If you specify the --live option be aware that not all hypervisors support live changes to the maximum memory limit.