7.3. Overcommitting Virtualized CPUs

The KVM hypervisor supports overcommitting virtualized CPUs (vCPUs). Virtualized CPUs can be overcommitted as far as load limits of guest virtual machines allow. Use caution when overcommitting vCPUs, as loads near 100% may cause dropped requests or unusable response times.

In Red Hat Enterprise Linux 7, it is possible to overcommit guests with more than one vCPU, known as symmetric multiprocessing (SMP) virtual machines. However, you may experience performance deterioration when running more cores on the virtual machine than are present on your physical CPU.

For example, a virtual machine with four vCPUs should not be run on a host machine with a dual core processor, but on a quad core host. Overcommitting SMP virtual machines beyond the physical number of processing cores causes significant performance degradation, due to programs getting less CPU time than required. In addition, it is not recommended to have more than 10 total allocated vCPUs per physical processor core.

With SMP guests, some processing overhead is inherent. CPU overcommitting can increase the SMP overhead, because using time-slicing to allocate resources to guests can make inter-CPU communication inside a guest slower. This overhead increases with guests that have a larger number of vCPUs, or a larger overcommit ratio.

Virtualized CPUs are overcommitted best when when a single host has multiple guests, and each guest has a small number of vCPUs, compared to the number of host CPUs. KVM should safely support guests with loads under 100% at a ratio of five vCPUs (on 5 virtual machines) to one physical CPU on one single host. The KVM hypervisor will switch between all of the virtual machines, making sure that the load is balanced.

For best performance, Red Hat recommends assigning guests only as many vCPUs as are required to run the programs that are inside each guest.