Hardware Over Commiting
OpenStack Compute uses the following ratios by default:
- CPU allocation ratio: 16:1
- RAM allocation ratio: 1.5:1
The default CPU allocation ratio of 16:1 means that the scheduler allocates up to 16 virtual cores per physical core. For example, if a physical node has 12 cores, the scheduler sees 192 available virtual cores. With typical flavor definitions of 4 virtual cores per instance, this ratio would provide 48 instances on a physical node.