6.2. Overcommitting Virtualized CPUs
The KVM hypervisor supports overcommitting virtualized CPUs. 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.
Virtualized CPUs (vCPUs) are overcommitted best when a single host physical machine has multiple guest virtual machines that do not share the same vCPU. KVM should safely support guest virtual machines with loads under 100% at a ratio of five VCPUs (on 5 virtual machines) to one physical CPU on one single host physical machine. KVM will switch between all of the machines, making sure that the load is balanced.
Do not overcommit guest virtual machines on more than the physical number of processing cores. For example a guest virtual machine with four vCPUs should not be run on a host physical machine with a dual core processor, but on a quad core host. In addition, it is not recommended to have more than 10 total allocated vCPUs per physical processor core.
Do not overcommit CPUs in a production environment without extensive testing. Applications which use 100% of processing resources may become unstable in overcommitted environments. Test before deploying.
For more information on how to get the best performance out of your virtual machine, refer to the Red Hat Enterprise Linux 6 Virtualization Tuning and Optimization Guide.