==================
Compute schedulers
==================

Compute uses the ``nova-scheduler`` service to determine how to dispatch
compute requests. For example, the ``nova-scheduler`` service determines on
which host or node a VM should launch. You can configure the scheduler through
a variety of options.

In the default configuration, this scheduler considers hosts that meet all the
following criteria:

* Are in the requested :term:`Availability Zone` (``AvailabilityZoneFilter``).

* Can service the request meaning the nova-compute service handling the target
  node is available and not disabled (``ComputeFilter``).

* Satisfy the extra specs associated with the instance type
  (``ComputeCapabilitiesFilter``).

* Satisfy any architecture, hypervisor type, or virtual machine mode properties
  specified on the instance's image properties (``ImagePropertiesFilter``).

* Are on a different host than other instances of a group (if requested)
  (``ServerGroupAntiAffinityFilter``).

* Are in a set of group hosts (if requested) (``ServerGroupAffinityFilter``).

The scheduler chooses a new host when an instance is migrated, resized,
evacuated or unshelved after being shelve offloaded.

When evacuating instances from a host, the scheduler service honors the target
host defined by the administrator on the :command:`nova evacuate` command.  If
a target is not defined by the administrator, the scheduler determines the
target host. For information about instance evacuation, see
:ref:`Evacuate instances <node-down-evacuate-instances>`.


.. _compute-scheduler-filters:

Prefilters
----------

As of the Rocky release, the scheduling process includes a prefilter step to
increase the efficiency of subsequent stages. These *prefilters* are largely
optional and serve to augment the request that is sent to placement to reduce
the set of candidate compute hosts based on attributes that placement is able
to answer for us ahead of time. In addition to the prefilters listed here, also
see :ref:`tenant-isolation-with-placement` and
:ref:`availability-zones-with-placement`.

Compute Image Type Support
~~~~~~~~~~~~~~~~~~~~~~~~~~~

.. versionadded:: 20.0.0 (Train)

Starting in the Train release, there is a prefilter available for
excluding compute nodes that do not support the ``disk_format`` of the
image used in a boot request. This behavior is enabled by setting
:oslo.config:option:`scheduler.query_placement_for_image_type_support` to
``True``. For example, the libvirt driver, when using ceph as an ephemeral
backend, does not support ``qcow2`` images (without an expensive conversion
step). In this case (and especially if you have a mix of ceph and
non-ceph backed computes), enabling this feature will ensure that the
scheduler does not send requests to boot a ``qcow2`` image to computes
backed by ceph.

Compute Disabled Status Support
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

.. versionadded:: 20.0.0 (Train)

Starting in the Train release, there is a mandatory `pre-filter
<https://specs.openstack.org/openstack/nova-specs/specs/train/approved/pre-filter-disabled-computes.html>`_
which will exclude disabled compute nodes similar to (but does not fully
replace) the `ComputeFilter`_. Compute node resource providers with the
``COMPUTE_STATUS_DISABLED`` trait will be excluded as scheduling candidates.
The trait is managed by the ``nova-compute`` service and should mirror the
``disabled`` status on the related compute service record in the
`os-services`_ API. For example, if a compute service's status is ``disabled``,
the related compute node resource provider(s) for that service should have the
``COMPUTE_STATUS_DISABLED`` trait. When the service status is ``enabled`` the
``COMPUTE_STATUS_DISABLED`` trait shall be removed.

If the compute service is down when the status is changed, the trait will be
synchronized by the compute service when it is restarted. Similarly, if an
error occurs when trying to add or remove the trait on a given resource
provider, the trait will be synchronized when the ``update_available_resource``
periodic task runs - which is controlled by the
:oslo.config:option:`update_resources_interval` configuration option.

.. _os-services: https://docs.openstack.org/api-ref/compute/#compute-services-os-services

Isolate Aggregates
~~~~~~~~~~~~~~~~~~

.. versionadded:: 20.0.0 (Train)

Starting in the Train release, there is an optional placement pre-request filter
:doc:`/reference/isolate-aggregates`
When enabled, the traits required in the server's flavor and image must be at
least those required in an aggregate's metadata in order for the server to be
eligible to boot on hosts in that aggregate.


The Filter Scheduler
--------------------

.. versionchanged:: 23.0.0 (Wallaby)

    Support for custom filters was removed. Only the filter scheduler is now
    supported by nova.

Nova's scheduler, known as the *filter scheduler*, supports filtering and
weighting to make informed decisions on where a new instance should be created.

When the scheduler receives a request for a resource, it first applies filters
to determine which hosts are eligible for consideration when dispatching a
resource. Filters are binary: either a host is accepted by the filter, or it is
rejected. Hosts that are accepted by the filter are then processed by a
different algorithm to decide which hosts to use for that request, described in
the :ref:`weights` section.

**Filtering**

.. figure:: /_static/images/filtering-workflow-1.png

The :oslo.config:option:`filter_scheduler.available_filters` config option
provides the Compute service with the list of the filters that are available
for use by the scheduler. The default setting specifies all of the filters that
are included with the Compute service. This configuration option can be
specified multiple times. For example, if you implemented your own custom
filter in Python called ``myfilter.MyFilter`` and you wanted to use both the
built-in filters and your custom filter, your :file:`nova.conf` file would
contain:

.. code-block:: ini

   [filter_scheduler]
   available_filters = nova.scheduler.filters.all_filters
   available_filters = myfilter.MyFilter

The :oslo.config:option:`filter_scheduler.enabled_filters` configuration option
in ``nova.conf`` defines the list of filters that are applied by the
``nova-scheduler`` service.


Filters
-------

The following sections describe the available compute filters.

Filters are configured using the following config options:

- :oslo.config:option:`filter_scheduler.available_filters` - Defines filter
  classes made available to the scheduler. This setting can be used multiple
  times.
- :oslo.config:option:`filter_scheduler.enabled_filters` - Of the available
  filters, defines those that the scheduler uses by default.

Each filter selects hosts in a different way and has different costs. The order
of :oslo.config:option:`filter_scheduler.enabled_filters` affects scheduling
performance. The general suggestion is to filter out invalid hosts as soon as
possible to avoid unnecessary costs. We can sort
:oslo.config:option:`filter_scheduler.enabled_filters`
items by their costs in reverse order. For example, ``ComputeFilter`` is better
before any resource calculating filters like ``NUMATopologyFilter``.

In medium/large environments having AvailabilityZoneFilter before any
capability or resource calculating filters can be useful.

.. _AggregateImagePropertiesIsolation:

``AggregateImagePropertiesIsolation``
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

.. versionchanged:: 12.0.0 (Liberty)

    Prior to 12.0.0 Liberty, it was possible to specify and use arbitrary
    metadata with this filter. Starting in Liberty, nova only parses
    :glance-doc:`standard metadata <admin/useful-image-properties.html>`. If
    you wish to use arbitrary metadata, consider using the
    :ref:`AggregateInstanceExtraSpecsFilter` filter instead.

Matches properties defined in an image's metadata against those of aggregates
to determine host matches:

* If a host belongs to an aggregate and the aggregate defines one or more
  metadata that matches an image's properties, that host is a candidate to boot
  the image's instance.

* If a host does not belong to any aggregate, it can boot instances from all
  images.

For example, the following aggregate ``myWinAgg`` has the Windows operating
system as metadata (named 'windows'):

.. code-block:: console

   $ openstack aggregate show myWinAgg
   +-------------------+----------------------------+
   | Field             | Value                      |
   +-------------------+----------------------------+
   | availability_zone | zone1                      |
   | created_at        | 2017-01-01T15:36:44.000000 |
   | deleted           | False                      |
   | deleted_at        | None                       |
   | hosts             | ['sf-devel']               |
   | id                | 1                          |
   | name              | myWinAgg                   |
   | properties        | os_distro='windows'        |
   | updated_at        | None                       |
   +-------------------+----------------------------+

In this example, because the following Win-2012 image has the ``windows``
property, it boots on the ``sf-devel`` host (all other filters being equal):

.. code-block:: console

   $ openstack image show Win-2012
   +------------------+------------------------------------------------------+
   | Field            | Value                                                |
   +------------------+------------------------------------------------------+
   | checksum         | ee1eca47dc88f4879d8a229cc70a07c6                     |
   | container_format | bare                                                 |
   | created_at       | 2016-12-13T09:30:30Z                                 |
   | disk_format      | qcow2                                                |
   | ...                                                                     |
   | name             | Win-2012                                             |
   | ...                                                                     |
   | properties       | os_distro='windows'                                  |
   | ...                                                                     |

You can configure the ``AggregateImagePropertiesIsolation`` filter by using the
following options in the ``nova.conf`` file:

- :oslo.config:option:`filter_scheduler.aggregate_image_properties_isolation_namespace`
- :oslo.config:option:`filter_scheduler.aggregate_image_properties_isolation_separator`

.. note::

   This filter has limitations as described in `bug 1677217
   <https://bugs.launchpad.net/nova/+bug/1677217>`_
   which are addressed in placement :doc:`/reference/isolate-aggregates`
   request filter.

Refer to :doc:`/admin/aggregates` for more information.


.. _AggregateInstanceExtraSpecsFilter:

``AggregateInstanceExtraSpecsFilter``
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Matches properties defined in extra specs for an instance type against
admin-defined properties on a host aggregate.  Works with specifications that
are scoped with ``aggregate_instance_extra_specs``.  Multiple values can be
given, as a comma-separated list. For backward compatibility, also works with
non-scoped specifications; this action is highly discouraged because it
conflicts with :ref:`ComputeCapabilitiesFilter` filter when you enable both
filters.

Refer to :doc:`/admin/aggregates` for more information.


.. _AggregateIoOpsFilter:

``AggregateIoOpsFilter``
~~~~~~~~~~~~~~~~~~~~~~~~

Filters host by disk allocation with a per-aggregate ``max_io_ops_per_host``
value. If the per-aggregate value is not found, the value falls back to the
global setting defined by the
`:oslo.config:option:`filter_scheduler.max_io_ops_per_host` config option.
If the host is in more than one aggregate and more than one value is found, the
minimum value will be used.

Refer to :doc:`/admin/aggregates` and :ref:`IoOpsFilter` for more information.


.. _AggregateMultiTenancyIsolation:

``AggregateMultiTenancyIsolation``
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Ensures hosts in tenant-isolated host aggregates will only be available to a
specified set of tenants. If a host is in an aggregate that has the
``filter_tenant_id`` metadata key, the host can build instances from only that
tenant or comma-separated list of tenants. A host can be in different
aggregates. If a host does not belong to an aggregate with the metadata key,
the host can build instances from all tenants. This does not restrict the
tenant from creating servers on hosts outside the tenant-isolated aggregate.

For example, consider there are two available hosts for scheduling, ``HostA``
and ``HostB``. ``HostB`` is in an aggregate isolated to tenant ``X``. A server
create request from tenant ``X`` will result in either ``HostA`` *or* ``HostB``
as candidates during scheduling. A server create request from another tenant
``Y`` will result in only ``HostA`` being a scheduling candidate since
``HostA`` is not part of the tenant-isolated aggregate.

.. note::

    There is a `known limitation
    <https://bugs.launchpad.net/nova/+bug/1802111>`_ with the number of tenants
    that can be isolated per aggregate using this filter. This limitation does
    not exist, however, for the :ref:`tenant-isolation-with-placement`
    filtering capability added in the 18.0.0 Rocky release.


.. _AggregateNumInstancesFilter:

``AggregateNumInstancesFilter``
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Filters host in an aggregate by number of instances with a per-aggregate
``max_instances_per_host`` value. If the per-aggregate value is not found, the
value falls back to the global setting defined by the
:oslo.config:option:`filter_scheduler.max_instances_per_host` config option.
If the host is in more than one aggregate and thus more than one value is
found, the minimum value will be used.

Refer to :doc:`/admin/aggregates` and :ref:`NumInstancesFilter` for more
information.


.. _AggregateTypeAffinityFilter:

``AggregateTypeAffinityFilter``
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Filters hosts in an aggregate if the name of the instance's flavor matches that
of the ``instance_type`` key set in the aggregate's metadata or if the
``instance_type`` key is not set.

The value of the ``instance_type`` metadata entry is a string that may contain
either a single ``instance_type`` name or a comma-separated list of
``instance_type`` names, such as ``m1.nano`` or ``m1.nano,m1.small``.

.. note::

    Instance types are a historical name for flavors.

Refer to :doc:`/admin/aggregates` for more information.


``AllHostsFilter``
~~~~~~~~~~~~~~~~~~

This is a no-op filter. It does not eliminate any of the available hosts.


.. _AvailabilityZoneFilter:

``AvailabilityZoneFilter``
~~~~~~~~~~~~~~~~~~~~~~~~~~

Filters hosts by availability zone. It passes hosts matching the availability
zone specified in the instance properties.  Use a comma to specify multiple
zones. The filter will then ensure it matches any zone specified.

You must enable this filter for the scheduler to respect availability zones in
requests.

Refer to :doc:`/admin/availability-zones` for more information.

.. _ComputeCapabilitiesFilter:

``ComputeCapabilitiesFilter``
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Filters hosts by matching properties defined in flavor extra specs against compute
capabilities. If an extra specs key contains a colon (``:``), anything before
the colon is treated as a namespace and anything after the colon is treated as
the key to be matched.  If a namespace is present and is not ``capabilities``,
the filter ignores the namespace.
For example ``capabilities:cpu_info:features`` is a valid scope format.
For backward compatibility, the filter also treats the
extra specs key as the key to be matched if no namespace is present; this
action is highly discouraged because it conflicts with
:ref:`AggregateInstanceExtraSpecsFilter` filter when you enable both filters.

The extra specifications can have an operator at the beginning of the value
string of a key/value pair. If there is no operator specified, then a
default operator of ``s==`` is used. Valid operators are:

* ``=`` (equal to or greater than as a number; same as vcpus case)
* ``==`` (equal to as a number)
* ``!=`` (not equal to as a number)
* ``>=`` (greater than or equal to as a number)
* ``<=`` (less than or equal to as a number)
* ``s==`` (equal to as a string)
* ``s!=`` (not equal to as a string)
* ``s>=`` (greater than or equal to as a string)
* ``s>`` (greater than as a string)
* ``s<=`` (less than or equal to as a string)
* ``s<`` (less than as a string)
* ``<in>`` (substring)
* ``<all-in>`` (all elements contained in collection)
* ``<or>`` (find one of these)

Examples are: ``>= 5``, ``s== 2.1.0``, ``<in> gcc``, ``<all-in> aes mmx``, and
``<or> fpu <or> gpu``

Some of attributes that can be used as useful key and their values contains:

* ``free_ram_mb`` (compared with a number, values like ``>= 4096``)
* ``free_disk_mb`` (compared with a number, values like ``>= 10240``)
* ``host`` (compared with a string, values like ``<in> compute``, ``s== compute_01``)
* ``hypervisor_type`` (compared with a string, values like ``s== QEMU``, ``s== powervm``)
* ``hypervisor_version`` (compared with a number, values like ``>= 1005003``, ``== 2000000``)
* ``num_instances`` (compared with a number, values like ``<= 10``)
* ``num_io_ops`` (compared with a number, values like ``<= 5``)
* ``vcpus_total`` (compared with a number, values like ``= 48``, ``>=24``)
* ``vcpus_used`` (compared with a number, values like ``= 0``, ``<= 10``)

Some virt drivers support reporting CPU traits to the Placement service. With
that feature available, you should consider using traits in flavors instead of
``ComputeCapabilitiesFilter`` because traits provide consistent naming for CPU
features in some virt drivers and querying traits is efficient. For more
details, refer to :doc:`/user/support-matrix`,
:ref:`Required traits <extra-specs-required-traits>`,
:ref:`Forbidden traits <extra-specs-forbidden-traits>` and
`Report CPU features to the Placement service <https://specs.openstack.org/openstack/nova-specs/specs/rocky/approved/report-cpu-features-as-traits.html>`_.

Also refer to `Compute capabilities as traits`_.


.. _ComputeFilter:

``ComputeFilter``
-----------------

Passes all hosts that are operational and enabled.

In general, you should always enable this filter.


``DifferentHostFilter``
-----------------------

Schedules the instance on a different host from a set of instances.  To take
advantage of this filter, the requester must pass a scheduler hint, using
``different_host`` as the key and a list of instance UUIDs as the value. This
filter is the opposite of the ``SameHostFilter``.

For example, when using the :command:`openstack server create` command, use the
``--hint`` flag:

.. code-block:: console

   $ openstack server create \
     --image cedef40a-ed67-4d10-800e-17455edce175 --flavor 1 \
     --hint different_host=a0cf03a5-d921-4877-bb5c-86d26cf818e1 \
     --hint different_host=8c19174f-4220-44f0-824a-cd1eeef10287 \
     server-1

With the API, use the ``os:scheduler_hints`` key. For example:

.. code-block:: json

   {
       "server": {
           "name": "server-1",
           "imageRef": "cedef40a-ed67-4d10-800e-17455edce175",
           "flavorRef": "1"
       },
       "os:scheduler_hints": {
           "different_host": [
               "a0cf03a5-d921-4877-bb5c-86d26cf818e1",
               "8c19174f-4220-44f0-824a-cd1eeef10287"
           ]
       }
   }


.. _ImagePropertiesFilter:

``ImagePropertiesFilter``
~~~~~~~~~~~~~~~~~~~~~~~~~

Filters hosts based on properties defined on the instance's image.  It passes
hosts that can support the specified image properties contained in the
instance. Properties include the architecture, hypervisor type, hypervisor
version, and virtual machine mode.

For example, an instance might require a host that runs an ARM-based processor,
and QEMU as the hypervisor.  You can decorate an image with these properties by
using:

.. code-block:: console

   $ openstack image set --architecture arm --property img_hv_type=qemu \
     img-uuid

The image properties that the filter checks for are:

``hw_architecture``
  Describes the machine architecture required by the image.  Examples are
  ``i686``, ``x86_64``, ``arm``, and ``ppc64``.

  .. versionchanged:: 12.0.0 (Liberty)

      This was previously called ``architecture``.

``img_hv_type``
  Describes the hypervisor required by the image.  Examples are ``qemu``
  and ``hyperv``.

  .. note::

     ``qemu`` is used for both QEMU and KVM hypervisor types.

  .. versionchanged:: 12.0.0 (Liberty)

      This was previously called ``hypervisor_type``.

``img_hv_requested_version``
  Describes the hypervisor version required by the image.  The property is
  supported for HyperV hypervisor type only.  It can be used to enable support for
  multiple hypervisor versions, and to prevent instances with newer HyperV tools
  from being provisioned on an older version of a hypervisor. If available, the
  property value is compared to the hypervisor version of the compute host.

  To filter the hosts by the hypervisor version, add the
  ``img_hv_requested_version`` property on the image as metadata and pass an
  operator and a required hypervisor version as its value:

  .. code-block:: console

     $ openstack image set --property hypervisor_type=hyperv --property \
       hypervisor_version_requires=">=6000" img-uuid

  .. versionchanged:: 12.0.0 (Liberty)

      This was previously called ``hypervisor_version_requires``.

``hw_vm_mode``
  describes the hypervisor application binary interface (ABI) required by the
  image. Examples are ``xen`` for Xen 3.0 paravirtual ABI, ``hvm`` for native
  ABI, and ``exe`` for container virt executable ABI.

  .. versionchanged:: 12.0.0 (Liberty)

      This was previously called ``vm_mode``.


``IsolatedHostsFilter``
~~~~~~~~~~~~~~~~~~~~~~~

Allows the admin to define a special (isolated) set of images and a special
(isolated) set of hosts, such that the isolated images can only run on the
isolated hosts, and the isolated hosts can only run isolated images.  The flag
``restrict_isolated_hosts_to_isolated_images`` can be used to force isolated
hosts to only run isolated images.

The logic within the filter depends on the
``restrict_isolated_hosts_to_isolated_images`` config option, which defaults
to True. When True, a volume-backed instance will not be put on an isolated
host. When False, a volume-backed instance can go on any host, isolated or
not.

The admin must specify the isolated set of images and hosts using the
:oslo.config:option:`filter_scheduler.isolated_hosts` and
:oslo.config:option:`filter_scheduler.isolated_images` config options.
For example:

.. code-block:: ini

   [filter_scheduler]
   isolated_hosts = server1, server2
   isolated_images = 342b492c-128f-4a42-8d3a-c5088cf27d13, ebd267a6-ca86-4d6c-9a0e-bd132d6b7d09

You can also specify that isolated host only be used for specific isolated
images using the
:oslo.config:option:`filter_scheduler.restrict_isolated_hosts_to_isolated_images`
config option.


.. _IoOpsFilter:

``IoOpsFilter``
~~~~~~~~~~~~~~~

Filters hosts by concurrent I/O operations on it. Hosts with too many
concurrent I/O operations will be filtered out. The
:oslo.config:option:`filter_scheduler.max_io_ops_per_host` option specifies the
maximum number of I/O intensive instances allowed to run on a host.
A host will be ignored by the scheduler if more than
:oslo.config:option:`filter_scheduler.max_io_ops_per_host` instances in build,
resize, snapshot, migrate, rescue or unshelve task states are running on it.


``JsonFilter``
~~~~~~~~~~~~~~~

.. warning::

    This filter is not enabled by default and not comprehensively
    tested, and thus could fail to work as expected in non-obvious ways.
    Furthermore, the filter variables are based on attributes of the
    `HostState`_ class which could change from release to release so usage
    of this filter is generally not recommended. Consider using other filters
    such as the :ref:`ImagePropertiesFilter` or
    :ref:`traits-based scheduling <extra-specs-required-traits>`.

Allows a user to construct a custom filter by passing a
scheduler hint in JSON format. The following operators are supported:

* ``=``
* ``<``
* ``>``
* ``in``
* ``<=``
* ``>=``
* ``not``
* ``or``
* ``and``

Unlike most other filters that rely on information provided via scheduler
hints, this filter filters on attributes in the `HostState`_ class such as the
following variables:

* ``$free_ram_mb``
* ``$free_disk_mb``
* ``$hypervisor_hostname``
* ``$total_usable_ram_mb``
* ``$vcpus_total``
* ``$vcpus_used``

Using the :command:`openstack server create` command, use the ``--hint`` flag:

.. code-block:: console

   $ openstack server create --image 827d564a-e636-4fc4-a376-d36f7ebe1747 \
     --flavor 1 --hint query='[">=","$free_ram_mb",1024]' server1

With the API, use the ``os:scheduler_hints`` key:

.. code-block:: json

   {
       "server": {
           "name": "server-1",
           "imageRef": "cedef40a-ed67-4d10-800e-17455edce175",
           "flavorRef": "1"
       },
       "os:scheduler_hints": {
           "query": "[\">=\",\"$free_ram_mb\",1024]"
       }
   }

.. _HostState: https://opendev.org/openstack/nova/src/branch/master/nova/scheduler/host_manager.py


``MetricsFilter``
~~~~~~~~~~~~~~~~~

Use in collaboration with the ``MetricsWeigher`` weigher.  Filters hosts that
do not report the metrics specified in
:oslo.config:option:`metrics.weight_setting`, thus ensuring the metrics
weigher will not fail due to these hosts.


.. _NUMATopologyFilter:

``NUMATopologyFilter``
~~~~~~~~~~~~~~~~~~~~~~

Filters hosts based on the NUMA topology that was specified for the instance
through the use of flavor ``extra_specs`` in combination with the image
properties, as described in detail in :doc:`/admin/cpu-topologies`. The filter
will try to match the exact NUMA cells of the instance to those of the host. It
will consider the standard over-subscription limits for each host NUMA cell,
and provide limits to the compute host accordingly.

This filter is essential if using instances with features that rely on NUMA,
such as instance NUMA topologies or CPU pinning.

.. note::

   If instance has no topology defined, it will be considered for any host.  If
   instance has a topology defined, it will be considered only for NUMA capable
   hosts.


.. _NumInstancesFilter:

``NumInstancesFilter``
~~~~~~~~~~~~~~~~~~~~~~

Filters hosts based on the number of instances running on them. Hosts that have
more instances running than specified by the
:oslo.config:option:`filter_scheduler.max_instances_per_host` config option are
filtered out.


.. _PciPassthroughFilter:

``PciPassthroughFilter``
~~~~~~~~~~~~~~~~~~~~~~~~

The filter schedules instances on a host if the host has devices that meet the
device requests in the ``extra_specs`` attribute for the flavor.

This filter is essential if using instances with PCI device requests or where
SR-IOV-based networking is in use on hosts.


``SameHostFilter``
~~~~~~~~~~~~~~~~~~

Schedules an instance on the same host as all other instances in a set of
instances. To take advantage of this filter, the requester must pass a
scheduler hint, using ``same_host`` as the key and a list of instance UUIDs as
the value.  This filter is the opposite of the ``DifferentHostFilter``.

For example, when using the :command:`openstack server create` command, use the
``--hint`` flag:

.. code-block:: console

   $ openstack server create \
     --image cedef40a-ed67-4d10-800e-17455edce175 --flavor 1 \
     --hint same_host=a0cf03a5-d921-4877-bb5c-86d26cf818e1 \
     --hint same_host=8c19174f-4220-44f0-824a-cd1eeef10287 \
     server-1

With the API, use the ``os:scheduler_hints`` key:

.. code-block:: json

   {
       "server": {
           "name": "server-1",
           "imageRef": "cedef40a-ed67-4d10-800e-17455edce175",
           "flavorRef": "1"
       },
       "os:scheduler_hints": {
           "same_host": [
               "a0cf03a5-d921-4877-bb5c-86d26cf818e1",
               "8c19174f-4220-44f0-824a-cd1eeef10287"
           ]
       }
   }


.. _ServerGroupAffinityFilter:

``ServerGroupAffinityFilter``
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Restricts instances belonging to a server group to the same host(s).  To take
advantage of this filter, the requester must create a server group with an
``affinity`` policy, and pass a scheduler hint, using ``group`` as the key and
the server group UUID as the value.

For example, when using the :command:`openstack server create` command, use the
``--hint`` flag:

.. code-block:: console

   $ openstack server group create --policy affinity group-1
   $ openstack server create --image IMAGE_ID --flavor 1 \
     --hint group=SERVER_GROUP_UUID server-1


.. _ServerGroupAntiAffinityFilter:

``ServerGroupAntiAffinityFilter``
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Restricts instances belonging to a server group to separate hosts.
To take advantage of this filter, the requester must create a
server group with an ``anti-affinity`` policy, and pass a scheduler hint, using
``group`` as the key and the server group UUID as the value.

For example, when using the :command:`openstack server create` command, use the
``--hint`` flag:

.. code-block:: console

   $ openstack server group create --policy anti-affinity group-1
   $ openstack server create --image IMAGE_ID --flavor 1 \
     --hint group=SERVER_GROUP_UUID server-1


``SimpleCIDRAffinityFilter``
~~~~~~~~~~~~~~~~~~~~~~~~~~~~

.. todo::

    Does this filter still work with neutron?

Schedules the instance based on host IP subnet range.  To take advantage of
this filter, the requester must specify a range of valid IP address in CIDR
format, by passing two scheduler hints:

``build_near_host_ip``
  The first IP address in the subnet (for example, ``192.168.1.1``)

``cidr``
  The CIDR that corresponds to the subnet (for example, ``/24``)

When using the :command:`openstack server create` command, use the ``--hint``
flag. For example, to specify the IP subnet ``192.168.1.1/24``:

.. code-block:: console

   $ openstack server create \
     --image cedef40a-ed67-4d10-800e-17455edce175 --flavor 1 \
     --hint build_near_host_ip=192.168.1.1 --hint cidr=/24 \
     server-1

With the API, use the ``os:scheduler_hints`` key:

.. code-block:: json

   {
       "server": {
           "name": "server-1",
           "imageRef": "cedef40a-ed67-4d10-800e-17455edce175",
           "flavorRef": "1"
       },
       "os:scheduler_hints": {
           "build_near_host_ip": "192.168.1.1",
           "cidr": "24"
       }
   }


.. _weights:

Weights
-------

.. figure:: /_static/images/nova-weighting-hosts.png

When resourcing instances, the filter scheduler filters and weights each host
in the list of acceptable hosts. Each time the scheduler selects a host, it
virtually consumes resources on it and subsequent selections are adjusted
accordingly. This process is useful when the customer asks for the same large
amount of instances because a weight is computed for each requested instance.

In order to prioritize one weigher against another, all the weighers have to
define a multiplier that will be applied before computing the weight for a node.
All the weights are normalized beforehand so that the multiplier can be applied
easily.Therefore the final weight for the object will be::

    weight = w1_multiplier * norm(w1) + w2_multiplier * norm(w2) + ...

Hosts are weighted based on the following config options:

- :oslo.config:option:`filter_scheduler.host_subset_size`
- :oslo.config:option:`filter_scheduler.weight_classes`


``RAMWeigher``
~~~~~~~~~~~~~~

Compute weight based on available RAM on the compute node.
Sort with the largest weight winning. If the multiplier,
:oslo.config:option:`filter_scheduler.ram_weight_multiplier`, is negative, the
host with least RAM available will win (useful for stacking hosts, instead
of spreading).

Starting with the Stein release, if per-aggregate value with the key
``ram_weight_multiplier`` is found, this
value would be chosen as the ram weight multiplier. Otherwise, it will fall
back to the :oslo.config:option:`filter_scheduler.ram_weight_multiplier`.
If more than one value is found for a host in aggregate metadata, the minimum
value will be used.


``CPUWeigher``
~~~~~~~~~~~~~~

Compute weight based on available vCPUs on the compute node.
Sort with the largest weight winning. If the multiplier,
:oslo.config:option:`filter_scheduler.cpu_weight_multiplier`, is negative, the
host with least CPUs available will win (useful for stacking hosts, instead
of spreading).

Starting with the Stein release, if per-aggregate value with the key
``cpu_weight_multiplier`` is found, this
value would be chosen as the cpu weight multiplier. Otherwise, it will fall
back to the :oslo.config:option:`filter_scheduler.cpu_weight_multiplier`. If
more than one value is found for a host in aggregate metadata, the minimum
value will be used.


``DiskWeigher``
~~~~~~~~~~~~~~~

Hosts are weighted and sorted by free disk space with the
largest weight winning.  If the multiplier is negative, the host with less disk
space available will win (useful for stacking hosts, instead of spreading).

Starting with the Stein release, if per-aggregate value with the key
``disk_weight_multiplier`` is found, this
value would be chosen as the disk weight multiplier. Otherwise, it will fall
back to the :oslo.config:option:`filter_scheduler.disk_weight_multiplier`. If
more than one value is found for a host in aggregate metadata, the minimum value
will be used.


``MetricsWeigher``
~~~~~~~~~~~~~~~~~~

This weigher can compute the weight based on the compute node
host's various metrics. The to-be weighed metrics and their weighing ratio
are specified using the :oslo.config:option:`metrics.weight_setting` config
option. For example:

.. code-block:: ini

    [metrics]
    weight_setting = name1=1.0, name2=-1.0

You can specify the metrics that are required, along with the weight of those
that are not and are not available using the
:oslo.config:option:`metrics.required` and
:oslo.config:option:`metrics.weight_of_unavailable` config options,
respectively.

Starting with the Stein release, if per-aggregate value with the key
`metrics_weight_multiplier` is found, this value would be chosen as the
metrics weight multiplier. Otherwise, it will fall back to the
:oslo.config:option:`metrics.weight_multiplier`. If more than
one value is found for a host in aggregate metadata, the minimum value will
be used.


``IoOpsWeigher``
~~~~~~~~~~~~~~~~

The weigher can compute the weight based on the compute node
host's workload. The default is to preferably choose light workload compute
hosts. If the multiplier is positive, the weigher prefer choosing heavy
workload compute hosts, the weighing has the opposite effect of the default.

Starting with the Stein release, if per-aggregate value with the key
``io_ops_weight_multiplier`` is found, this
value would be chosen as the IO ops weight multiplier. Otherwise, it will fall
back to the :oslo.config:option:`filter_scheduler.io_ops_weight_multiplier`.
If more than one value is found for a host in aggregate metadata, the minimum
value will be used.

``PCIWeigher``
~~~~~~~~~~~~~~

Compute a weighting based on the number of PCI devices on the
host and the number of PCI devices requested by the instance. For example,
given three hosts - one with a single PCI device, one with many PCI devices,
and one with no PCI devices - nova should prioritise these differently based
on the demands of the instance. If the instance requests a single PCI device,
then the first of the hosts should be preferred. Similarly, if the instance
requests multiple PCI devices, then the second of these hosts would be
preferred. Finally, if the instance does not request a PCI device, then the
last of these hosts should be preferred.

For this to be of any value, at least one of the :ref:`PciPassthroughFilter` or
:ref:`NUMATopologyFilter` filters must be enabled.

Starting with the Stein release, if per-aggregate value with the key
``pci_weight_multiplier`` is found, this
value would be chosen as the pci weight multiplier. Otherwise, it will fall
back to the :oslo.config:option:`filter_scheduler.pci_weight_multiplier`.
If more than one value is found for a host in aggregate metadata, the
minimum value will be used.

.. important::

    Only positive values are allowed for the multiplier of this weigher as a
    negative value would force non-PCI instances away from non-PCI hosts, thus,
    causing future scheduling issues.

``ServerGroupSoftAffinityWeigher``
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The weigher can compute the weight based
on the number of instances that run on the same server group. The largest
weight defines the preferred host for the new instance. For the multiplier
only a positive value is allowed for the calculation.

Starting with the Stein release, if per-aggregate value with the key
``soft_affinity_weight_multiplier`` is
found, this value would be chosen as the soft affinity weight multiplier.
Otherwise, it will fall back to the
:oslo.config:option:`filter_scheduler.soft_affinity_weight_multiplier`.
If more than one value is found for a host in aggregate metadata, the
minimum value will be used.

``ServerGroupSoftAntiAffinityWeigher``
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The weigher can compute the weight based on the number of instances that run on
the same server group as a negative value. The largest weight defines the
preferred host for the new instance.  For the multiplier only a positive value
is allowed for the calculation.

Starting with the Stein release, if per-aggregate value with the key
``soft_anti_affinity_weight_multiplier`` is found, this value would be chosen
as the soft anti-affinity weight multiplier. Otherwise, it will fall back to
the
:oslo.config:option:`filter_scheduler.soft_anti_affinity_weight_multiplier`.
If more than one value is found for a host in aggregate metadata, the minimum
value will be used.

``BuildFailureWeigher``
~~~~~~~~~~~~~~~~~~~~~~~

Weigh hosts by the number of recent failed boot attempts.
It considers the build failure counter and can negatively weigh hosts with
recent failures. This avoids taking computes fully out of rotation.

Starting with the Stein release, if per-aggregate value with the key
``build_failure_weight_multiplier`` is found, this value would be chosen as the
build failure weight multiplier. Otherwise, it will fall back to the
:oslo.config:option:`filter_scheduler.build_failure_weight_multiplier`.  If
more than one value is found for a host in aggregate metadata, the minimum
value will be used.

.. _cross-cell-weigher:

``CrossCellWeigher``
~~~~~~~~~~~~~~~~~~~~

.. versionadded:: 21.0.0 (Ussuri)

Weighs hosts based on which cell they are in. "Local" cells are preferred when
moving an instance. Use configuration option
:oslo.config:option:`filter_scheduler.cross_cell_move_weight_multiplier` to
control the weight. If per-aggregate value with the key
`cross_cell_move_weight_multiplier` is found, this value would be chosen as the
cross-cell move weight multiplier. Otherwise, it will fall back to the
:oslo.config:option:`filter_scheduler.cross_cell_move_weight_multiplier`.  If
more than one value is found for a host in aggregate metadata, the minimum
value will be used.


Utilization-aware scheduling
----------------------------

.. warning::

    This feature is poorly tested and may not work as expected. It may be
    removed in a future release. Use at your own risk.

It is possible to schedule instances using advanced scheduling decisions. These
decisions are made based on enhanced usage statistics encompassing data like
memory cache utilization, memory bandwidth utilization, or network bandwidth
utilization. This is disabled by default.  The administrator can configure how
the metrics are weighted in the configuration file by using the
:oslo.config:option:`metrics.weight_setting` config option.  For example to
configure ``metric1`` with ``ratio1`` and ``metric2`` with ``ratio2``:

.. code-block:: ini

   [metrics]
   weight_setting = "metric1=ratio1, metric2=ratio2"


Allocation ratios
-----------------

Allocation ratios allow for the overcommit of host resources.
The following configuration options exist to control allocation ratios
per compute node to support this overcommit of resources:

* :oslo.config:option:`cpu_allocation_ratio` allows overriding the ``VCPU``
  inventory allocation ratio for a compute node
* :oslo.config:option:`ram_allocation_ratio` allows overriding the ``MEMORY_MB``
  inventory allocation ratio for a compute node
* :oslo.config:option:`disk_allocation_ratio` allows overriding the ``DISK_GB``
  inventory allocation ratio for a compute node

Prior to the 19.0.0 Stein release, if left unset, the ``cpu_allocation_ratio``
defaults to 16.0, the ``ram_allocation_ratio`` defaults to 1.5, and the
``disk_allocation_ratio`` defaults to 1.0.

Starting with the 19.0.0 Stein release, the following configuration options
control the initial allocation ratio values for a compute node:

* :oslo.config:option:`initial_cpu_allocation_ratio` the initial VCPU
  inventory allocation ratio for a new compute node record, defaults to 16.0
* :oslo.config:option:`initial_ram_allocation_ratio` the initial MEMORY_MB
  inventory allocation ratio for a new compute node record, defaults to 1.5
* :oslo.config:option:`initial_disk_allocation_ratio` the initial DISK_GB
  inventory allocation ratio for a new compute node record, defaults to 1.0

Scheduling considerations
~~~~~~~~~~~~~~~~~~~~~~~~~

The allocation ratio configuration is used both during reporting of compute
node `resource provider inventory`_ to the placement service and during
scheduling.

.. _resource provider inventory: https://docs.openstack.org/api-ref/placement/?expanded=#resource-provider-inventories

Usage scenarios
~~~~~~~~~~~~~~~

Since allocation ratios can be set via nova configuration, host aggregate
metadata and the placement API, it can be confusing to know which should be
used. This really depends on your scenario. A few common scenarios are detailed
here.

1. When the deployer wants to **always** set an override value for a resource
   on a compute node, the deployer should ensure that the
   :oslo.config:option:`DEFAULT.cpu_allocation_ratio`,
   :oslo.config:option:`DEFAULT.ram_allocation_ratio` and
   :oslo.config:option:`DEFAULT.disk_allocation_ratio` configuration options
   are set to a non-None value.
   This will make the ``nova-compute`` service overwrite any externally-set
   allocation ratio values set via the placement REST API.

2. When the deployer wants to set an **initial** value for a compute node
   allocation ratio but wants to allow an admin to adjust this afterwards
   without making any configuration file changes, the deployer should set the
   :oslo.config:option:`DEFAULT.initial_cpu_allocation_ratio`,
   :oslo.config:option:`DEFAULT.initial_ram_allocation_ratio` and
   :oslo.config:option:`DEFAULT.initial_disk_allocation_ratio` configuration
   options and then manage the allocation ratios using the placement REST API
   (or `osc-placement`_ command line interface).
   For example:

   .. code-block:: console

     $ openstack resource provider inventory set \
         --resource VCPU:allocation_ratio=1.0 \
         --amend 815a5634-86fb-4e1e-8824-8a631fee3e06

3. When the deployer wants to **always** use the placement API to set
   allocation ratios, then the deployer should ensure that the
   :oslo.config:option:`DEFAULT.cpu_allocation_ratio`,
   :oslo.config:option:`DEFAULT.ram_allocation_ratio` and
   :oslo.config:option:`DEFAULT.disk_allocation_ratio` configuration options
   are set to a None and then manage the allocation ratios using the placement
   REST API (or `osc-placement`_ command line interface).

   This scenario is the workaround for
   `bug 1804125 <https://bugs.launchpad.net/nova/+bug/1804125>`_.

.. versionchanged:: 19.0.0 (Stein)

    The :oslo.config:option:`DEFAULT.initial_cpu_allocation_ratio`,
    :oslo.config:option:`DEFAULT.initial_ram_allocation_ratio` and
    :oslo.config:option:`DEFAULT.initial_disk_allocation_ratio` configuration
    options were introduced in Stein. Prior to this release, setting any of
    :oslo.config:option:`DEFAULT.cpu_allocation_ratio`,
    :oslo.config:option:`DEFAULT.ram_allocation_ratio` or
    :oslo.config:option:`DEFAULT.disk_allocation_ratio` to a non-null value
    would ensure the user-configured value was always overriden.

.. _osc-placement: https://docs.openstack.org/osc-placement/latest/index.html

.. _hypervisor-specific-considerations:

Hypervisor-specific considerations
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Nova provides three configuration options that can be used to set aside some
number of resources that will not be consumed by an instance, whether these
resources are overcommitted or not:

- :oslo.config:option:`reserved_host_cpus`,
- :oslo.config:option:`reserved_host_memory_mb`
- :oslo.config:option:`reserved_host_disk_mb`

Some virt drivers may benefit from the use of these options to account for
hypervisor-specific overhead.

HyperV
    Hyper-V creates a VM memory file on the local disk when an instance starts.
    The size of this file corresponds to the amount of RAM allocated to the
    instance.

    You should configure the
    :oslo.config:option:`reserved_host_disk_mb` config option to
    account for this overhead, based on the amount of memory available
    to instances.


Cells considerations
--------------------

By default cells are enabled for scheduling new instances but they can be
disabled (new schedules to the cell are blocked). This may be useful for
users while performing cell maintenance, failures or other interventions. It is
to be noted that creating pre-disabled cells and enabling/disabling existing
cells should either be followed by a restart or SIGHUP of the nova-scheduler
service for the changes to take effect.

Command-line interface
~~~~~~~~~~~~~~~~~~~~~~

The :command:`nova-manage` command-line client supports the cell-disable
related commands. To enable or disable a cell, use
:command:`nova-manage cell_v2 update_cell` and to create pre-disabled cells,
use :command:`nova-manage cell_v2 create_cell`. See the
:ref:`man-page-cells-v2` man page for details on command usage.


.. _compute-capabilities-as-traits:

Compute capabilities as traits
------------------------------

.. versionadded:: 19.0.0 (Stein)

The ``nova-compute`` service will report certain ``COMPUTE_*`` traits based on
its compute driver capabilities to the placement service. The traits will be
associated with the resource provider for that compute service. These traits
can be used during scheduling by configuring flavors with
:ref:`Required traits <extra-specs-required-traits>` or
:ref:`Forbidden traits <extra-specs-forbidden-traits>`. For example, if you
have a host aggregate with a set of compute nodes that support multi-attach
volumes, you can restrict a flavor to that aggregate by adding the
``trait:COMPUTE_VOLUME_MULTI_ATTACH=required`` extra spec to the flavor and
then restrict the flavor to the aggregate
:ref:`as normal <config-sch-for-aggs>`.

Here is an example of a libvirt compute node resource provider that is
exposing some CPU features as traits, driver capabilities as traits, and a
custom trait denoted by the ``CUSTOM_`` prefix:

.. code-block:: console

  $ openstack --os-placement-api-version 1.6 resource provider trait list \
  > d9b3dbc4-50e2-42dd-be98-522f6edaab3f --sort-column name
  +---------------------------------------+
  | name                                  |
  +---------------------------------------+
  | COMPUTE_DEVICE_TAGGING                |
  | COMPUTE_NET_ATTACH_INTERFACE          |
  | COMPUTE_NET_ATTACH_INTERFACE_WITH_TAG |
  | COMPUTE_TRUSTED_CERTS                 |
  | COMPUTE_VOLUME_ATTACH_WITH_TAG        |
  | COMPUTE_VOLUME_EXTEND                 |
  | COMPUTE_VOLUME_MULTI_ATTACH           |
  | CUSTOM_IMAGE_TYPE_RBD                 |
  | HW_CPU_X86_MMX                        |
  | HW_CPU_X86_SSE                        |
  | HW_CPU_X86_SSE2                       |
  | HW_CPU_X86_SVM                        |
  +---------------------------------------+

**Rules**

There are some rules associated with capability-defined traits.

1. The compute service "owns" these traits and will add/remove them when the
   ``nova-compute`` service starts and when the ``update_available_resource``
   periodic task runs, with run intervals controlled by config option
   :oslo.config:option:`update_resources_interval`.

2. The compute service will not remove any custom traits set on the resource
   provider externally, such as the ``CUSTOM_IMAGE_TYPE_RBD`` trait in the
   example above.

3. If compute-owned traits are removed from the resource provider externally,
   for example by running ``openstack resource provider trait delete <rp_uuid>``,
   the compute service will add its traits again on restart or SIGHUP.

4. If a compute trait is set on the resource provider externally which is not
   supported by the driver, for example by adding the ``COMPUTE_VOLUME_EXTEND``
   trait when the driver does not support that capability, the compute service
   will automatically remove the unsupported trait on restart or SIGHUP.

5. Compute capability traits are standard traits defined in the `os-traits`_
   library.

.. _os-traits: https://opendev.org/openstack/os-traits/src/branch/master/os_traits/compute

:ref:`Further information on capabilities and traits
<taxonomy_of_traits_and_capabilities>` can be found in the
:doc:`Technical Reference Deep Dives section </reference/index>`.


.. _custom-scheduler-filters:

Writing Your Own Filter
-----------------------

To create **your own filter**, you must inherit from |BaseHostFilter| and
implement one method: ``host_passes``. This method should return ``True`` if a
host passes the filter and return ``False`` elsewhere. It takes two parameters:

* the ``HostState`` object allows to get attributes of the host
* the ``RequestSpec`` object describes the user request, including the flavor,
  the image and the scheduler hints

For further details about each of those objects and their corresponding
attributes, refer to the codebase (at least by looking at the other filters
code) or ask for help in the ``#openstack-nova`` IRC channel.

In addition, if your custom filter uses non-standard extra specs, you must
register validators for these extra specs. Examples of validators can be found
in the ``nova.api.validation.extra_specs`` module. These should be registered
via the ``nova.api.extra_spec_validator`` `entrypoint`__.

The module containing your custom filter(s) must be packaged and available in
the same environment(s) that the nova controllers, or specifically the
:program:`nova-scheduler` and :program:`nova-api` services, are available in.
As an example, consider the following sample package, which is the `minimal
structure`__ for a standard, setuptools-based Python package:

.. code-block:: none

    acmefilter/
        acmefilter/
            __init__.py
            validators.py
        setup.py

Where ``__init__.py`` contains:

.. code-block:: python

    from oslo_log import log as logging
    from nova.scheduler import filters

    LOG = logging.getLogger(__name__)

    class AcmeFilter(filters.BaseHostFilter):

        def host_passes(self, host_state, spec_obj):
            extra_spec = spec_obj.flavor.extra_specs.get('acme:foo')
            LOG.info("Extra spec value was '%s'", extra_spec)

            # do meaningful stuff here...

            return True

``validators.py`` contains:

.. code-block:: python

    from nova.api.validation.extra_specs import base

    def register():
        validators = [
            base.ExtraSpecValidator(
                name='acme:foo',
                description='My custom extra spec.'
                value={
                    'type': str,
                    'enum': [
                        'bar',
                        'baz',
                    ],
                },
            ),
        ]

        return validators

``setup.py`` contains:

.. code-block:: python

    from setuptools import setup

    setup(
        name='acmefilter',
        version='0.1',
        description='My custom filter',
        packages=[
            'acmefilter'
        ],
        entry_points={
            'nova.api.extra_spec_validators': [
                'acme = acmefilter.validators',
            ],
        },
    )

To enable this, you would set the following in :file:`nova.conf`:

.. code-block:: ini

    [filter_scheduler]
    available_filters = nova.scheduler.filters.all_filters
    available_filters = acmefilter.AcmeFilter
    enabled_filters = ComputeFilter,AcmeFilter

.. note::

    You **must** add custom filters to the list of available filters using the
    :oslo.config:option:`filter_scheduler.available_filters` config option in
    addition to enabling them via the
    :oslo.config:option:`filter_scheduler.enabled_filters` config option. The
    default ``nova.scheduler.filters.all_filters`` value for the former only
    includes the filters shipped with nova.

With these settings, all of the standard nova filters and the custom
``AcmeFilter`` filter are available to the scheduler, but just the
``ComputeFilter`` and ``AcmeFilter`` will be used on each request.

__ https://packaging.python.org/specifications/entry-points/
__ https://python-packaging.readthedocs.io/en/latest/minimal.html

Writing your own weigher
------------------------

To create your own weigher, you must inherit from |BaseHostWeigher|
A weigher can implement both the ``weight_multiplier`` and ``_weight_object``
methods or just implement the ``weight_objects`` method. ``weight_objects``
method is overridden only if you need access to all objects in order to
calculate weights, and it just return a list of weights, and not modify the
weight of the object directly, since final weights are normalized and computed
by ``weight.BaseWeightHandler``.


.. |BaseHostFilter| replace:: :class:`BaseHostFilter <nova.scheduler.filters.BaseHostFilter>`
.. |BaseHostWeigher| replace:: :class:`BaseHostFilter <nova.scheduler.weights.BaseHostWeigher>`
