In addition to defining the number of vCPUs assigned to an instance, limits on the use of these CPU cycles can be further imposed. Nova relies on the underlying hypervisor for the specific implementation of the CPU limits, and thus the values available may vary. Our example is based on QEMU/KVM.
CPU limits are a special case of flavor attributes that you may encounter.
To add a CPU limit to a flavor, you will need the following:
- The
openstackcommand-line client - The
openrcfile containing appropriate credentials - The name of the flavor to change
- The values you would like to set the CPU limit to (share of time the allotted CPU is allowed to consume in milliseconds per cycle)
These values in our example are as follows:
cpu_quota= 5000 mscpu_period= 2500 ms
The following commands are used to add CPU limits to a flavor.
- First, view the current attributes of the flavor that we are changing, noting that we have no
propertiesset:openstack flavor show openstack.cookbookThis gives the following output:
- Add the CPU limit:
openstack flavor set --property quota:cpu_quota=5000 --property quota:cpu_period=2500 openstack.cookbook
- View the CPU limit by interrogating the flavor again, noting the
propertiesfield:openstack flavor show openstack.cookbookThis gives an output like the following:
In order to help mitigate the noisy neighbor issue, or to provide further definition of service levels, OpenStack supports CPU limits. In OpenStack, this is called the Instance Resource Quota. CPU limits are part of the flavor definition. Meaning, all instances of a given flavor will have the same CPU limits.
Imposing CPU limits are hypervisor specific. For KVM/libvirt environments, CPU limits are enforced using cgroups, using a combination of the following three values:
cpu_sharescpu_quotacpu_period
A full discussion of what these values are and how they interact is beyond the scope of this book. However, you can find a detailed explanation of them on the OpenStack wiki at the following: