Configuring Resource Overcommitment

OpenStack supports overcommitting of CPU and memory resources on compute nodes. Overcommitting is a technique of allocating more virtualized CPUs and/or memory than there are physical resources.


Overcommitting increases the amount of instances you are able to run, but reduces instance performance.
CPU and memory overcommit settings are represented as a ratio. OpenStack uses the following ratios by default:
  • Default CPU overcommit ratio - 16
  • Default memory overcommit ratio - 1.5
These default settings have the following implications:
  • The default CPU overcommit ratio of 16 means that up to 16 virtual cores can be assigned to a node for each physical core.
  • The default memory overcommit ratio of 1.5 means that instances can be assigned to a physical node if the total instance memory usage is less than 1.5 times the amount of physical memory available.
Use the cpu_allocation_ratio and ram_allocation_ratio directives in /etc/nova/nova.conf to change these default settings.