Merge "Add spec for VM-scoped SR-IOV NUMA affinity"
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specs/ussuri/approved/vm-scoped-sriov-numa-affinity.rst
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specs/ussuri/approved/vm-scoped-sriov-numa-affinity.rst
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This work is licensed under a Creative Commons Attribution 3.0 Unported
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License.
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http://creativecommons.org/licenses/by/3.0/legalcode
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=======================================
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VM Scoped SR-IOV NUMA Affinity Policies
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=======================================
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https://blueprints.launchpad.net/nova/+spec/vm-scoped-sriov-numa-affinity
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In the Queens release [1]_ support was added to allow PCI NUMA affinity
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policies to be specified via PCI aliases. This work builds on a previous
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feature introduced in the Juno release [2]_ that introduced strict NUMA
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affinity for PCI devices; however, the Queens feature did not address
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the NUMA affinity of neutron SR-IOV interfaces which were also enforced by
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the original Juno enhancement. This spec seeks to provide a per-VM mechanism
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to set a VM-wide NUMA afinity policy for all PCI passthrough devices,
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including but not limited to neutron SR-IOV interfaces
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(vnic_type=direct,direct-phyical,macvtap,virtio-forwarder)
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Problem description
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===================
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In some environments the server form factor is restricted, preventing PCI
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devices from being physically installed across all NUMA nodes on a server,
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e.g. high density blade/multi server systems or non standard form factor
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equipment. In such an environment the default legacy policy which is applied
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to all neutron SR-IOV interfaces prevents VMs from using SR-IOV on a non local
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NUMA node if the VM has a NUMA topology (uses cpu pinning, vPMEM, hugepages or
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requests a NUMA topology explicitly).
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To use a remote SR-IOV device via neutron ports in such an environment the
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operator is forced to either configure the guest to have multiple NUMA nodes
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or disable NUMA reporting on the host server. Both options pessimize the
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performance of both the guest and host in different ways. While a VM with
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multiple virtual NUMA nodes can outperform a VM with the same resources and a
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single NUMA node in a memory bound workload, that is only true if the workload
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is NUMA-aware. A two-node NUMA topology, if enforced on a workload that is not
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NUMA-aware, can result in increased cross-NUMA traffic and result in a lower
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throughput. Similarly while disabling NUMA reporting at the hardware level
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is beneficial in some HPC workloads due to the increased memory bandwidth, it
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comes at the cost of increased memory latency, making it unsuitable for
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realtime workloads such as VOIP.
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Use Cases
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---------
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As an operator deploying openstack on high density or restricted form factor
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hardware, I wish to specify a per-VM NUMA affinity policy for SR-IOV devices
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via standard flavor extra specs.
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As a tenant or VNF vendor, I want to be able to customize the affinity of my
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VMs via image properties so I can express the NUMA affinity requirements of
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my workloads.
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Proposed change
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===============
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This spec proposes extending the PCI NUMA affinity polices introduced
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by [1]_ to all PCI and SR-IOV devices including neutron ports by adding a
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new flavor extra spec ``hw:pci_numa_affinity_policy`` and
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``hw_pci_numa_affinity_policy`` image metadata property.
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The new properties will accept one of three values: ``required``, ``preferred``
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and ``legacy`` as defined in [1]_. If a PCI device is requested using a flavor
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alias, the NUMA affinity policy specified in the flavor or image will
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take precedence over any policy set in the host PCI alias. If no
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PCI NUMA affinity policy is specified in the flavor or image, alias based
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PCI pass-through will fall back to the policy set in the alias. If no policy
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is set in the flavor or image and no policy is set in the alias the legacy
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policy will continue to be used. For neutron SR-IOV interfaces if no policy
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is set in the flavor or image the legacy policy will be used.
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.. NOTE::
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The Queens spec [1]_ originally contained both of the proposed flavor
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and image properties but were removed during implementation as the original
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neutron port usecase that motivated the feature was not captured in the spec.
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As a result, while the Queens feature addressed NUMA affinity for
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flavor-based PCI pass-through, no mechanism is available to specify the policy
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for neutron SR-IOV interfaces.
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Alternatives
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------------
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We could change the default policy to ``preferred`` if no policy is specified.
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This would optimize for cases where people do not care about NUMA affinity
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at the expense of requiring those who do to specify a policy.
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As this would be a change in behavior on upgrade it is not proposed that we
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take this approach.
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We could enable per-interface NUMA affinity polices. This is not mutually
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exclusive with this proposal and will be proposed separately as an additional
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feature. The flavor- and image-based approach covers 80% of the use cases
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enabled by per-interface NUMA affinity polices without requiring neutron api
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changes.
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Data model impact
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-----------------
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The image metadata object and related notification objects will be updated
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to contain the new PCI NUMA affinity field. As the PCI request spec object
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already has a NUMA affinity policy field for alias-based pass-through, no
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other data model changes are required.
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REST API impact
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---------------
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There will be no direct changes to any existing API. However,
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a new flavor extra spec will be introduced.
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Security impact
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---------------
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None
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Notifications impact
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--------------------
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The image metadata properties payload will be extended with the
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new property field. No other impact is expected.
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Other end user impact
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---------------------
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To utilize this feature operators and tenants will need to modify their
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images and flavors to add the ``hw:pci_numa_affinity_policy`` and
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``hw_pci_numa_affinity_policy`` properties.
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Performance Impact
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------------------
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None
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As the scheduler was already asserting legacy PCI affinity, passing
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a policy to assert instead should not affect the overall scheduling time.
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Depending on the policy selected the performance of the guest may improve
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or be reduced inline with the guarantees expressed by that policy.
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Other deployer impact
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---------------------
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As was previously required to enable NUMA affinity to be enforced for
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SR-IOV/PCI devices, the PCI pass-through and NUMA topology filters must be
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enabled.
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Developer impact
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----------------
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None
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Upgrade impact
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--------------
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None
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Implementation
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==============
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Assignee(s)
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-----------
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Primary assignee:
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sean-k-mooney
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Feature Liaison
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---------------
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Feature liaison:
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sean-k-mooney
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Work Items
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----------
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None
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Dependencies
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============
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None
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Testing
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=======
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As this feature relates to SR-IOV it cannot be tested in the upstream gate
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via tempest. Unit tests will be provided to assert that the policy
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is correctly conveyed to the existing PCI assignment code and the existing
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functional test can be extended as required.
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As this feature simply provides another way to specify the PCI affinity policy
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the code change is minimal and can leverage much of the existing test coverage.
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Documentation Impact
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====================
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A release note and updates to the existing user flavor docs will be provided,
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and the glance metadefs should be updated to reflect the new image property.
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References
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==========
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.. [1] https://specs.openstack.org/openstack/nova-specs/specs/queens/implemented/share-pci-between-numa-nodes.html
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.. [2] https://specs.openstack.org/openstack/nova-specs/specs/juno/approved/input-output-based-numa-scheduling.html
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History
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=======
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.. list-table:: Revisions
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:header-rows: 1
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* - Release Name
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- Description
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* - Ussuri
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- Introduced
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