==============
CPU topologies
==============

The NUMA topology and CPU pinning features in OpenStack provide high-level
control over how instances run on hypervisor CPUs and the topology of virtual
CPUs available to instances. These features help minimize latency and maximize
performance.

.. include:: /common/numa-live-migration-warning.txt

SMP, NUMA, and SMT
------------------

Symmetric multiprocessing (SMP)
  SMP is a design found in many modern multi-core systems. In an SMP system,
  there are two or more CPUs and these CPUs are connected by some interconnect.
  This provides CPUs with equal access to system resources like memory and
  input/output ports.

Non-uniform memory access (NUMA)
  NUMA is a derivative of the SMP design that is found in many multi-socket
  systems. In a NUMA system, system memory is divided into cells or nodes that
  are associated with particular CPUs. Requests for memory on other nodes are
  possible through an interconnect bus. However, bandwidth across this shared
  bus is limited. As a result, competition for this resource can incur
  performance penalties.

Simultaneous Multi-Threading (SMT)
  SMT is a design complementary to SMP. Whereas CPUs in SMP systems share a bus
  and some memory, CPUs in SMT systems share many more components. CPUs that
  share components are known as thread siblings.  All CPUs appear as usable
  CPUs on the system and can execute workloads in parallel. However, as with
  NUMA, threads compete for shared resources.

Non-Uniform I/O Access (NUMA I/O)
  In a NUMA system, I/O to a device mapped to a local memory region is more
  efficient than I/O to a remote device. A device connected to the same socket
  providing the CPU and memory offers lower latencies for I/O operations due to
  its physical proximity. This generally manifests itself in devices connected
  to the PCIe bus, such as NICs or vGPUs, but applies to any device support
  memory-mapped I/O.

In OpenStack, SMP CPUs are known as *cores*, NUMA cells or nodes are known as
*sockets*, and SMT CPUs are known as *threads*. For example, a quad-socket,
eight core system with Hyper-Threading would have four sockets, eight cores per
socket and two threads per core, for a total of 64 CPUs.

PCPU and VCPU
-------------

PCPU
  Resource class representing an amount of dedicated CPUs for a single guest.

VCPU
  Resource class representing a unit of CPU resources for a single guest
  approximating the processing power of a single physical processor.

Customizing instance NUMA placement policies
--------------------------------------------

.. important::

   The functionality described below is currently only supported by the
   libvirt/KVM and Hyper-V driver. The Hyper-V driver may require :ref:`some
   host configuration <configure-hyperv-numa>` for this to work.

When running workloads on NUMA hosts, it is important that the vCPUs executing
processes are on the same NUMA node as the memory used by these processes.
This ensures all memory accesses are local to the node and thus do not consume
the limited cross-node memory bandwidth, adding latency to memory accesses.
Similarly, large pages are assigned from memory and benefit from the same
performance improvements as memory allocated using standard pages. Thus, they
also should be local. Finally, PCI devices are directly associated with
specific NUMA nodes for the purposes of DMA. Instances that use PCI or SR-IOV
devices should be placed on the NUMA node associated with these devices.

NUMA topology can exist on both the physical hardware of the host and the
virtual hardware of the instance. In OpenStack, when booting a process, the
hypervisor driver looks at the NUMA topology field of both the instance and
the host it is being booted on, and uses that information to generate an
appropriate configuration.

By default, an instance floats across all NUMA nodes on a host. NUMA awareness
can be enabled implicitly through the use of huge pages or pinned CPUs or
explicitly through the use of flavor extra specs or image metadata. If the
instance has requested a specific NUMA topology, compute will try to pin the
vCPUs of different NUMA cells on the instance to the corresponding NUMA cells
on the host. It will also expose the NUMA topology of the instance to the
guest OS.

In all cases where NUMA awareness is used, the ``NUMATopologyFilter``
filter must be enabled. Details on this filter are provided in
:doc:`/admin/configuration/schedulers`.

.. caution::

   The NUMA node(s) used are normally chosen at random. However, if a PCI
   passthrough or SR-IOV device is attached to the instance, then the NUMA
   node that the device is associated with will be used. This can provide
   important performance improvements. However, booting a large number of
   similar instances can result in unbalanced NUMA node usage. Care should
   be taken to mitigate this issue. See this `discussion`_ for more details.

.. caution::

   Inadequate per-node resources will result in scheduling failures. Resources
   that are specific to a node include not only CPUs and memory, but also PCI
   and SR-IOV resources. It is not possible to use multiple resources from
   different nodes without requesting a multi-node layout. As such, it may be
   necessary to ensure PCI or SR-IOV resources are associated with the same
   NUMA node or force a multi-node layout.

When used, NUMA awareness allows the operating system of the instance to
intelligently schedule the workloads that it runs and minimize cross-node
memory bandwidth. To restrict an instance's vCPUs to a single host NUMA node,
run:

.. code-block:: console

   $ openstack flavor set [FLAVOR_ID] --property hw:numa_nodes=1

Some workloads have very demanding requirements for memory access latency or
bandwidth that exceed the memory bandwidth available from a single NUMA node.
For such workloads, it is beneficial to spread the instance across multiple
host NUMA nodes, even if the instance's RAM/vCPUs could theoretically fit on a
single NUMA node. To force an instance's vCPUs to spread across two host NUMA
nodes, run:

.. code-block:: console

   $ openstack flavor set [FLAVOR_ID] --property hw:numa_nodes=2

The allocation of instances vCPUs and memory from different host NUMA nodes can
be configured. This allows for asymmetric allocation of vCPUs and memory, which
can be important for some workloads. To spread the 6 vCPUs and 6 GB of memory
of an instance across two NUMA nodes and create an asymmetric 1:2 vCPU and
memory mapping between the two nodes, run:

.. code-block:: console

   $ openstack flavor set [FLAVOR_ID] --property hw:numa_nodes=2
   # configure guest node 0
   $ openstack flavor set [FLAVOR_ID] \
     --property hw:numa_cpus.0=0,1 \
     --property hw:numa_mem.0=2048
   # configure guest node 1
   $ openstack flavor set [FLAVOR_ID] \
     --property hw:numa_cpus.1=2,3,4,5 \
     --property hw:numa_mem.1=4096

.. note::

    Hyper-V does not support asymmetric NUMA topologies, and the Hyper-V
    driver will not spawn instances with such topologies.

For more information about the syntax for ``hw:numa_nodes``, ``hw:numa_cpus.N``
and ``hw:num_mem.N``, refer to the :ref:`NUMA
topology <extra-specs-numa-topology>` guide.

Customizing instance CPU pinning policies
-----------------------------------------

.. important::

   The functionality described below is currently only supported by the
   libvirt/KVM driver and requires :ref:`some host configuration
   <configure-libvirt-pinning>` for this to work. Hyper-V does not support CPU
   pinning.

.. note::

   There is no correlation required between the NUMA topology exposed in the
   instance and how the instance is actually pinned on the host. This is by
   design. See this `invalid bug
   <https://bugs.launchpad.net/nova/+bug/1466780>`_ for more information.

By default, instance vCPU processes are not assigned to any particular host
CPU, instead, they float across host CPUs like any other process. This allows
for features like overcommitting of CPUs. In heavily contended systems, this
provides optimal system performance at the expense of performance and latency
for individual instances.

Some workloads require real-time or near real-time behavior, which is not
possible with the latency introduced by the default CPU policy. For such
workloads, it is beneficial to control which host CPUs are bound to an
instance's vCPUs. This process is known as pinning. No instance with pinned
CPUs can use the CPUs of another pinned instance, thus preventing resource
contention between instances.

CPU pinning policies can be used to determine whether an instance should be
pinned or not. There are two policies: ``dedicated`` and ``shared`` (the
default). The ``dedicated`` CPU policy is used to specify that an instance
should use pinned CPUs. To configure a flavor to use the ``dedicated`` CPU
policy, run:

.. code-block:: console

   $ openstack flavor set [FLAVOR_ID] --property hw:cpu_policy=dedicated

This works by ensuring ``PCPU`` allocations are used instead of ``VCPU``
allocations. As such, it is also possible to request this resource type
explicitly. To configure this, run:

.. code-block:: console

   $ openstack flavor set [FLAVOR_ID] --property resources:PCPU=N

where ``N`` is the number of vCPUs defined in the flavor.

.. note::

   It is not currently possible to request ``PCPU`` and ``VCPU`` resources in
   the same instance.

The ``shared`` CPU policy is used to specify that an instance **should not**
use pinned CPUs. To configure a flavor to use the ``shared`` CPU policy, run:

.. code-block:: console

   $ openstack flavor set [FLAVOR_ID] --property hw:cpu_policy=shared

.. note::

   For more information about the syntax for ``hw:cpu_policy``, refer to the
   :doc:`/user/flavors` guide.

It is also possible to configure the CPU policy via image metadata. This can
be useful when packaging applications that require real-time or near real-time
behavior by ensuring instances created with a given image are always pinned
regardless of flavor. To configure an image to use the ``dedicated`` CPU
policy, run:

.. code-block:: console

   $ openstack image set [IMAGE_ID] --property hw_cpu_policy=dedicated

Likewise, to configure an image to use the ``shared`` CPU policy, run:

.. code-block:: console

   $ openstack image set [IMAGE_ID] --property hw_cpu_policy=shared

.. note::

   For more information about image metadata, refer to the `Image metadata`_
   guide.

.. important::

   Flavor-based policies take precedence over image-based policies. For
   example, if a flavor specifies a CPU policy of ``dedicated`` then that
   policy will be used. If the flavor specifies a CPU policy of
   ``shared`` and the image specifies no policy or a policy of ``shared`` then
   the ``shared`` policy will be used. However, the flavor specifies a CPU
   policy of ``shared`` and the image specifies a policy of ``dedicated``, or
   vice versa, an exception will be raised. This is by design. Image metadata
   is often configurable by non-admin users, while flavors are only
   configurable by admins. By setting a ``shared`` policy through flavor
   extra-specs, administrators can prevent users configuring CPU policies in
   images and impacting resource utilization.

Customizing instance CPU thread pinning policies
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

.. important::

   The functionality described below requires the use of pinned instances and
   is therefore currently only supported by the libvirt/KVM driver and requires
   :ref:`some host configuration <configure-libvirt-pinning>` for this to work.
   Hyper-V does not support CPU pinning.

When running pinned instances on SMT hosts, it may also be necessary to
consider the impact that thread siblings can have on the instance workload. The
presence of an SMT implementation like Intel Hyper-Threading can boost
performance `by up to 30%`__ for some workloads.  However, thread siblings
share a number of components and contention on these components can diminish
performance for other workloads. For this reason, it is also possible to
explicitly request hosts with or without SMT.

__ https://software.intel.com/en-us/articles/how-to-determine-the-effectiveness-of-hyper-threading-technology-with-an-application

To configure whether an instance should be placed on a host with SMT or not, a
CPU thread policy may be specified. For workloads where sharing benefits
performance, you can request hosts **with** SMT.  To configure this, run:

.. code-block:: console

   $ openstack flavor set [FLAVOR_ID] \
     --property hw:cpu_policy=dedicated \
     --property hw:cpu_thread_policy=require

This will ensure the instance gets scheduled to a host with SMT by requesting
hosts that report the ``HW_CPU_HYPERTHREADING`` trait. It is also possible to
request this trait explicitly. To configure this, run:

.. code-block:: console

   $ openstack flavor set [FLAVOR_ID] \
     --property resources:PCPU=N \
     --property trait:HW_CPU_HYPERTHREADING=required

For other workloads where performance is impacted by contention for resources,
you can request hosts **without** SMT. To configure this, run:

.. code-block:: console

   $ openstack flavor set [FLAVOR_ID] \
     --property hw:cpu_policy=dedicated \
     --property hw:cpu_thread_policy=isolate

This will ensure the instance gets scheduled to a host with SMT by requesting
hosts that **do not** report the ``HW_CPU_HYPERTHREADING`` trait. It is also
possible to request this trait explicitly. To configure this, run:

.. code-block:: console

   $ openstack flavor set [FLAVOR_ID] \
     --property resources:PCPU=N \
     --property trait:HW_CPU_HYPERTHREADING=forbidden

Finally, for workloads where performance is minimally impacted, you may use
thread siblings if available and fallback to not using them if necessary. This
is the default, but it can be set explicitly:

.. code-block:: console

   $ openstack flavor set [FLAVOR_ID] \
     --property hw:cpu_policy=dedicated \
     --property hw:cpu_thread_policy=prefer

This does not utilize traits and, as such, there is no trait-based equivalent.

.. note::

   For more information about the syntax for ``hw:cpu_thread_policy``, refer to
   the :doc:`/user/flavors` guide.

As with CPU policies, it also possible to configure the CPU thread policy via
image metadata. This can be useful when packaging applications that require
real-time or near real-time behavior by ensuring instances created with a given
image are always pinned regardless of flavor. To configure an image to use the
``require`` CPU policy, run:

.. code-block:: console

   $ openstack image set [IMAGE_ID] \
     --property hw_cpu_policy=dedicated \
     --property hw_cpu_thread_policy=require

Likewise, to configure an image to use the ``isolate`` CPU thread policy, run:

.. code-block:: console

   $ openstack image set [IMAGE_ID] \
     --property hw_cpu_policy=dedicated \
     --property hw_cpu_thread_policy=isolate

Finally, to configure an image to use the ``prefer`` CPU thread policy, run:

.. code-block:: console

   $ openstack image set [IMAGE_ID] \
     --property hw_cpu_policy=dedicated \
     --property hw_cpu_thread_policy=prefer

If the flavor does not specify a CPU thread policy then the CPU thread policy
specified by the image (if any) will be used. If both the flavor and image
specify a CPU thread policy then they must specify the same policy, otherwise
an exception will be raised.

.. note::

   For more information about image metadata, refer to the `Image metadata`_
   guide.

Customizing instance emulator thread pinning policies
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

.. important::

   The functionality described below requires the use of pinned instances and
   is therefore currently only supported by the libvirt/KVM driver and requires
   :ref:`some host configuration <configure-libvirt-pinning>` for this to work.
   Hyper-V does not support CPU pinning.

In addition to the work of the guest OS and applications running in an
instance, there is a small amount of overhead associated with the underlying
hypervisor. By default, these overhead tasks - known collectively as emulator
threads - run on the same host CPUs as the instance itself and will result in a
minor performance penalty for the instance. This is not usually an issue,
however, for things like real-time instances, it may not be acceptable for
emulator thread to steal time from instance CPUs.

Emulator thread policies can be used to ensure emulator threads are run on
cores separate from those used by the instance. There are two policies:
``isolate`` and ``share``. The default is to run the emulator threads on the
same core. The ``isolate`` emulator thread policy is used to specify that
emulator threads for a given instance should be run on their own unique core,
chosen from one of the host cores listed in
:oslo.config:option:`compute.cpu_dedicated_set`. To configure a flavor to use
the ``isolate`` emulator thread policy, run:

.. code-block:: console

   $ openstack flavor set [FLAVOR_ID] \
     --property hw:cpu_policy=dedicated \
     --property hw:emulator_threads_policy=isolate

The ``share`` policy is used to specify that emulator threads from a given
instance should be run on the pool of host cores listed in
:oslo.config:option:`compute.cpu_shared_set`. To configure a flavor to use the
``share`` emulator thread policy, run:

.. code-block:: console

   $ openstack flavor set [FLAVOR_ID] \
     --property hw:cpu_policy=dedicated \
     --property hw:emulator_threads_policy=share

.. note::

   For more information about the syntax for ``hw:emulator_threads_policy``,
   refer to the :doc:`/user/flavors` guide.

Customizing instance CPU topologies
-----------------------------------

.. important::

   The functionality described below is currently only supported by the
   libvirt/KVM driver.

.. note::
   Currently it also works with libvirt/QEMU driver but we don't recommend
   it in production use cases. This is because vCPUs are actually running
   in one thread on host in qemu TCG (Tiny Code Generator), which is the
   backend for libvirt/QEMU driver. Work to enable full multi-threading
   support for TCG (a.k.a. MTTCG) is on going in QEMU community. Please see
   this `MTTCG project`_ page for detail.

In addition to configuring how an instance is scheduled on host CPUs, it is
possible to configure how CPUs are represented in the instance itself. By
default, when instance NUMA placement is not specified, a topology of N
sockets, each with one core and one thread, is used for an instance, where N
corresponds to the number of instance vCPUs requested. When instance NUMA
placement is specified, the number of sockets is fixed to the number of host
NUMA nodes to use and the total number of instance CPUs is split over these
sockets.

Some workloads benefit from a custom topology. For example, in some operating
systems, a different license may be needed depending on the number of CPU
sockets. To configure a flavor to use a maximum of two sockets, run:

.. code-block:: console

   $ openstack flavor set [FLAVOR_ID] --property hw:cpu_sockets=2

Similarly, to configure a flavor to use one core and one thread, run:

.. code-block:: console

   $ openstack flavor set [FLAVOR_ID] \
     --property hw:cpu_cores=1 \
     --property hw:cpu_threads=1

.. caution::

   If specifying all values, the product of sockets multiplied by cores
   multiplied by threads must equal the number of instance vCPUs. If specifying
   any one of these values or the multiple of two values, the values must be a
   factor of the number of instance vCPUs to prevent an exception. For example,
   specifying ``hw:cpu_sockets=2`` on a host with an odd number of cores fails.
   Similarly, specifying ``hw:cpu_cores=2`` and ``hw:cpu_threads=4`` on a host
   with ten cores fails.

For more information about the syntax for ``hw:cpu_sockets``, ``hw:cpu_cores``
and ``hw:cpu_threads``, refer to the :doc:`/user/flavors` guide.

It is also possible to set upper limits on the number of sockets, cores, and
threads used. Unlike the hard values above, it is not necessary for this exact
number to used because it only provides a limit. This can be used to provide
some flexibility in scheduling, while ensuring certain limits are not
exceeded. For example, to ensure no more than two sockets are defined in the
instance topology, run:

.. code-block:: console

   $ openstack flavor set [FLAVOR_ID] --property hw:cpu_max_sockets=2

For more information about the syntax for ``hw:cpu_max_sockets``,
``hw:cpu_max_cores``, and ``hw:cpu_max_threads``, refer to the
:doc:`/user/flavors` guide.

Applications are frequently packaged as images. For applications that prefer
certain CPU topologies, configure image metadata to hint that created instances
should have a given topology regardless of flavor. To configure an image to
request a two-socket, four-core per socket topology, run:

.. code-block:: console

   $ openstack image set [IMAGE_ID] \
     --property hw_cpu_sockets=2 \
     --property hw_cpu_cores=4

To constrain instances to a given limit of sockets, cores or threads, use the
``max_`` variants. To configure an image to have a maximum of two sockets and a
maximum of one thread, run:

.. code-block:: console

   $ openstack image set [IMAGE_ID] \
     --property hw_cpu_max_sockets=2 \
     --property hw_cpu_max_threads=1

The value specified in the flavor is treated as the absolute limit.  The image
limits are not permitted to exceed the flavor limits, they can only be equal
to or lower than what the flavor defines. By setting a ``max`` value for
sockets, cores, or threads, administrators can prevent users configuring
topologies that might, for example, incur an additional licensing fees.

For more information about image metadata, refer to the `Image metadata`_
guide.

.. _configure-libvirt-pinning:

Configuring libvirt compute nodes for CPU pinning
-------------------------------------------------

.. versionchanged:: 20.0.0

   Prior to 20.0.0 (Train), it was not necessary to explicitly configure hosts
   for pinned instances. However, it was not possible to place pinned instances
   on the same host as unpinned CPUs, which typically meant hosts had to be
   grouped into host aggregates. If this was not done, unpinned instances would
   continue floating across all enabled host CPUs, even those that some
   instance CPUs were pinned to. Starting in 20.0.0, it is necessary to
   explicitly identify the host cores that should be used for pinned instances.

Nova treats host CPUs used for unpinned instances differently from those used
by pinned instances. The former are tracked in placement using the ``VCPU``
resource type and can be overallocated, while the latter are tracked using the
``PCPU`` resource type. By default, nova will report all host CPUs as ``VCPU``
inventory, however, this can be configured using the
:oslo.config:option:`compute.cpu_shared_set` config option, to specify which
host CPUs should be used for ``VCPU`` inventory, and the
:oslo.config:option:`compute.cpu_dedicated_set` config option, to specify which
host CPUs should be used for ``PCPU`` inventory.

Consider a compute node with a total of 24 host physical CPU cores with
hyperthreading enabled. The operator wishes to reserve 1 physical CPU core and
its thread sibling for host processing (not for guest instance use).
Furthermore, the operator wishes to use 8 host physical CPU cores and their
thread siblings for dedicated guest CPU resources. The remaining 15 host
physical CPU cores and their thread siblings will be used for shared guest vCPU
usage, with an 8:1 allocation ratio for those physical processors used for
shared guest CPU resources.

The operator could configure ``nova.conf`` like so::

    [DEFAULT]
    cpu_allocation_ratio=8.0

    [compute]
    cpu_dedicated_set=2-17
    cpu_shared_set=18-47

The virt driver will construct a provider tree containing a single resource
provider representing the compute node and report inventory of ``PCPU`` and
``VCPU`` for this single provider accordingly::

    COMPUTE NODE provider
        PCPU:
            total: 16
            reserved: 0
            min_unit: 1
            max_unit: 16
            step_size: 1
            allocation_ratio: 1.0
        VCPU:
            total: 30
            reserved: 0
            min_unit: 1
            max_unit: 30
            step_size: 1
            allocation_ratio: 8.0

Instances using the ``dedicated`` CPU policy or an explicit ``PCPU`` resource
request, ``PCPU`` inventory will be consumed. Instances using the ``shared``
CPU policy, meanwhile, will consume ``VCPU`` inventory.

.. note::

   ``PCPU`` and ``VCPU`` allocations are currently combined to calculate the
   value for the ``cores`` quota class.

.. _configure-hyperv-numa:

Configuring Hyper-V compute nodes for instance NUMA policies
------------------------------------------------------------

Hyper-V is configured by default to allow instances to span multiple NUMA
nodes, regardless if the instances have been configured to only span N NUMA
nodes. This behaviour allows Hyper-V instances to have up to 64 vCPUs and 1 TB
of memory.

Checking NUMA spanning can easily be done by running this following PowerShell
command:

.. code-block:: console

   (Get-VMHost).NumaSpanningEnabled

In order to disable this behaviour, the host will have to be configured to
disable NUMA spanning. This can be done by executing these following
PowerShell commands:

.. code-block:: console

   Set-VMHost -NumaSpanningEnabled $false
   Restart-Service vmms

In order to restore this behaviour, execute these PowerShell commands:

.. code-block:: console

   Set-VMHost -NumaSpanningEnabled $true
   Restart-Service vmms

The *Virtual Machine Management Service* (*vmms*) is responsible for managing
the Hyper-V VMs. The VMs will still run while the service is down or
restarting, but they will not be manageable by the ``nova-compute`` service. In
order for the effects of the host NUMA spanning configuration to take effect,
the VMs will have to be restarted.

Hyper-V does not allow instances with a NUMA topology to have dynamic
memory allocation turned on. The Hyper-V driver will ignore the configured
``dynamic_memory_ratio`` from the given ``nova.conf`` file when spawning
instances with a NUMA topology.

.. Links
.. _`Image metadata`: https://docs.openstack.org/image-guide/introduction.html#image-metadata
.. _`discussion`: http://lists.openstack.org/pipermail/openstack-dev/2016-March/090367.html
.. _`MTTCG project`: http://wiki.qemu.org/Features/tcg-multithread
