Use PCPU and VCPU in One Instance

The spec CPU resource tracking splits host CPUs into PCPU and VCPU resources, making it possible to run instances of dedicated CPU allocation policy and instances of shared CPU allocation policy in the same host. This spec aims to create such kind of instance that some of the vCPUs are dedicated (PCPU) CPUs and the remaining vCPUs are shared (VCPU) vCPUs and expose this information via the metadata API.

Problem description

The current CPU allocation policy, dedicated or shared, is applied to all vCPUs of an instance. However, with the introduction of CPU resource tracking, it is possible to propose a more fine-grained CPU allocation policy, which is based on the control over individual instance vCPU, and specifying the dedicated or shared CPU allocation policy to each instance vCPU.

Use Cases

As an operator, I would like to have an instance with some realtime CPUs for high performance, and at the same time, in order to increase instance density, I wish to make the remaining CPUs, which do not demand high performance, shared with other instances because I only care about the performance of realtime CPUs. One example is deploying the NFV task that is enhanced with DPDK framework in the instance, in which the data plane threads could be processed with the realtime CPUs and the control-plane tasks are scheduled on CPUs that may be shared with other instances.

As a Kubernetes administrator, I wish to run a multi-tier or auto-scaling application in Kubernetes, which is running in single OpenStack VM, with the expectation that using dedicated high-performance CPUs for application itself and deploying the containers on shared cores.

Proposed change

Introduce a new CPU allocation policy mixed

dedicated and shared are the existing instance CPU allocation policies that determine how instance CPU is scheduled on host CPU. This specification proposes a new CPU allocation policy, with the name mixed, to create a CPU mixed instance in such way that some instance vCPUs are allocated from computing node’s PCPU resource, and the rest of instance vCPUs are allocated from the VCPU resources. The CPU allocated from PCPU resource will be pinned on particular host CPUs which are defined in CONF.compute.dedicated_cpu_set, and the CPU from VCPU resource will be floating on the host CPUs which are defined in CONF.compute.shared_cpu_set. In this proposal, we call these two kinds of vCPUs as dedicated vCPU and shared vCPU respectively.

Instance CPU policy matrix

Nova operator may set the instance CPU allocation policy through the hw:cpu_policy and hw_cpu_policy interfaces, which may raise conflict. The CPU policy conflict is proposed to be solved with the following policy matrix:




























For example, if a dedicated CPU policy is specified in instance flavor hw:cpu_policy, then the instance CPU policy is dedicated, regardless of the setting specified in image property hw_cpu_policy. If shared is explicitly set in hw:cpu_policy, then a mixed policy specified in hw_cpu_policy is conflict, which will throw an exception, the instance booting request will be rejected.

If there is no explicit instance CPU policy specified in flavor or image property, the flavor matrix result would be ‘undefined’, and the final instance policy is further determined and resolved by resources:PCPU and resources:VCPU specified in flavor extra specs. Refer to section and the spec CPU resource tracking.

Affect over real-time vCPUs

Real-time vCPU also occupies the host CPU exclusively and does not share CPU with other instances, all real-time vCPUs are dedicated vCPUs. For a mixed instance with some real-time vCPUs, with this proposal, the vCPUs not in the instance real-time vCPU list are shared vCPUs.

Affect over emulator thread policy

If emulator thread policy is ISOLATE, the mixed instance will look for a dedicated host CPU for instance emulator thread, which is very similar to the case introduced by dedicated policy instance.

If the emulator thread policy is SHARE, then the instance emulator thread will float over the host CPUs defined in configuration CONF.compute.cpu_shared_set.

Set dedicated CPU bit-mask in hw:cpu_dedicated_mask for mixed instance

As an interface to create the mixed policy instance through legacy flavor extra specs or image properties, the flavor extra spec hw:cpu_dedicated_mask is introduced. If the extra spec hw:cpu_dedicated_mask is found in the instance flavor, then the information of the dedicated CPU could be found through parsing hw:cpu_dedicated_mask.

Here is the example to create an instance with mixed policy:

$ openstack flavor set <flavor_id> \
    --property hw:cpu_policy=mixed \
    --property hw:cpu_dedicated_mask=0-3,7

And, following is the proposing command to create a mixed instance which consists of multiple NUMA nodes by setting the dedicated vCPUs in hw:cpu_dedicated_mask:

$ openstack flavor set <flavor_id> \
    --property hw:cpu_policy=mixed \
    --property hw:cpu_dedicated_mask=2,7 \
    --property hw:numa_nodes=2 \
    --property hw:numa_cpus.0=0-2 \
    --property hw:numa_cpus.1=3-7 \
    --property hw:numa_mem.0=1024 \
    --property hw:numa_mem.1=2048


Please be aware that there is no equivalent setting in image properties for flavor extra spec hw:cpu_dedicated_mask. It will not be supported to create mixed instance through image properties.


The dedicated vCPU list of a mixed instance could be specified through the newly introduced dedicated CPU mask or the cpu-time CPU mask, the hw:cpu_realtime_mask or hw_cpu_realtime_mask, you cannot set it by setting dedicated CPU mask extra spec and real-time CPU mask at the same time.

Create mixed instance via resources:PCPU and resources:VCPU

CPU resource tracking introduced a way to create an instance with dedicated or shared CPU allocation policy through resources:PCPU and resources:VCPU interfaces, but did not allow requesting both PCPU resource and VCPU resource for one instance.

This specification proposes to let an instance request PCPU resource along with VCPU, and effectively applying for the mixed CPU allocation policy if the cpu_policy is not explicitly specified in the flavor list. So an instance with such flavors potentially creates a mixed policy instance:

$ openstack flavor set \
    --property "resources:PCPU"="<dedicated CPU number>" \
    --property "resources:VCPU"="<shared CPU number>" \

For mixed instance created in such way, both <shared CPU number> and <dedicated CPU number> must be greater than zero. Otherwise, it effectively creates the dedicated or shared policy instance, that all vCPUs in the instance is in a same allocation policy.

The resources:PCPU and resources::VCPU interfaces only put the request toward Placement service for how many PCPU and VCPU resources are required to fulfill the instance vCPU thread and emulator thread requirement. The PCPU and VCPU distribution on the instance, especially on the instance with multiple NUMA nodes, will be spread across the NUMA nodes in the round-robin way, and VCPU will be put ahead of PCPU. Here is one example and the instance is created with flavor below:


Instance emulator thread policy is not specified in the flavor, so it does not occupy any dedicated PCPU resource for it, all PCPU and VCPU resources will be used by vCPU threads, and the expected distribution on NUMA nodes is:



The demanding instance CPU number is the number of vCPU, specified by flavor.vcpus, plus the number of CPU that is special for emulator thread, and if the emulator thread policy is ISOLATE, the instance requests flavor.vcpus + 1 vCPUs, if the policy is not ISOLATE, the instance just requests flavor.vcpus vCPU.


Creating CPU mixed instance by extending the dedicated policy

Instead of adding a special instance CPU allocation policy, the CPU mixed instance is supported by extending the existing dedicated policy and specifying the vCPUs that are pinned to the host CPUs chosen from PCPU resource.

Following extra spec and the image property are defined to keep the dedicated vCPUs of a mixed policy instance:

hw:cpu_dedicated_mask=<cpu set string>
hw_cpu_dedicated_mask=<cpu set string>

The <cpu set string> shares the same definition defined above.

This was rejected at it overloads the dedicated policy to mean two things, depending on the value of another configuration option.

Creating mixed instance with hw:cpu_policy and resources:(P|V)CPU

Following commands was proposed as an example to create a mixed instance by an explicit request of PCPU resources, and infer the VCPU count by flavor::vcpus and PCPU count:

$ openstack flavor create mixed_vmf --vcpus 4 --ram 512 --disk 1
$ openstack flavor set mixed_vmf \
    --property hw:cpu_policy=mixed \
    --property resources:PCPU=2

This was rejected due to the mixing use of hw:cpu_policy and resources:PCPU. It is not recommended to mix placement style syntax with traditional extra specs.

Data model impact

Add the pcpuset field in InstanceNUMACell object to track the dedicated vCPUs of the instance NUMA cell, and the original InstanceNUMACell.cpuset is special for shared vCPU then.

This change will introduce some database migration work for the existing instance in a dedicated CPU allocation policy, since all vCPUs in such an instance are dedicated vCPUs which should be kept in pcpuset field, but they are stored in cpuset historically.

REST API impact

The metadata API will be extended with the dedicated vCPU info and a new OpenStack metadata version will be added to indicate this is a new metadata API.

The new field will be added to the meta_data.json:

dedicated_cpus=<cpu set string>

The <cpu set string> lists the dedicated vCPU set of the instance, which might be the content of hw:cpu_dedicated_mask or hw:cpu_realtime_mask or hw_cpu_realtime_mask or the CPU list generated with the round-robin policy as described in section.

The new cpu policy mixed is added to extra spec hw:cpu_policy.

Security impact


Notifications impact


Other end user impact

If the end user wants to create an instance with a mixed CPU allocation policy, the user is required to set corresponding flavor extra specs or image properties.

Performance Impact

This proposal affects the selection of instance CPU allocation policy, but the performance impact is trivial.

Other deployer impact


Developer impact


Upgrade impact

The mixed cpu policy is only available when the whole cluster upgrade finished. A service version will be bumped for detecting the upgrade.

The InstanceNUMACell.pcpuset is introduced for dedicated vCPUs and the InstanceNUMACell.cpuset is special for shared vCPUs, all existing instances in a dedicated CPU allocation policy should be updated by moving content in InstanceNUMACell.cpuset filed to InstanceNUMACell.pcpuset field. The underlying database keeping the InstanceNUACell object also need be updated to reflect this change.



Primary assignee:

Wang, Huaqiang <>

Feature Liaison

Feature liaison:

Stephen Finucane <>

Work Items

  • Add a new field, the pcpuset, for InstanceNUMACell for dedicated vCPUs.

  • Add new instance CPU allocation policy mixed property and resolve conflicts

  • Bump nova service version to indicate the new CPU policy in nova-compute

  • Add flavor extra spec hw:cpu_dedicated_mask and create mixed instance

  • Translate dedicated and shared CPU request to placement PCPU and VCPU resources request.

  • Change libvirt driver to create PCPU mapping and VCPU mapping

  • Add nova metadata service by offering final pCPU layout in dedicated_cpus field

  • Validate real-time CPU mask for mixed instance.




Functional and unit tests are required to cover:

  • Ensure to solve the conflicts between the CPU policy matrix

  • Ensure only dedicated vCPUs are possible to be real-time vCPUs

  • Ensure creating mixed policy instance properly either by flavor settings or by resources::PCPU=xx and resources::VCPU=xx settings.

  • Ensure shared vCPUs is placed before the dedicated vCPUs

  • Ensure the emulator CPU is properly scheduled according to its policy.

Documentation Impact

The documents should be changed to introduce the usage of new mixed CPU allocation policy and the new flavor extra specs.

Metadata service will be updated accordingly.




Release Name



Introduced, abandoned