nova/doc/source/user/filter-scheduler.rst

Filter Scheduler
================

The **Filter Scheduler** supports `filtering` and `weighting` to make informed
decisions on where a new instance should be created. This Scheduler supports
working with Compute Nodes only.

Filtering
---------

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

During its work Filter Scheduler iterates over all found compute nodes,
evaluating each against a set of filters. The list of resulting hosts is
ordered by weighers. The Scheduler then chooses hosts for the requested
number of instances, choosing the most weighted hosts. For a specific
filter to succeed for a specific host, the filter matches the user
request against the state of the host plus some extra magic as defined
by each filter (described in more detail below).

If the Scheduler cannot find candidates for the next instance, it means that
there are no appropriate hosts where that instance can be scheduled.

The Filter Scheduler has to be quite flexible to support the required variety
of `filtering` and `weighting` strategies. If this flexibility is insufficient
you can implement `your own filtering algorithm`.

There are many standard filter classes which may be used
(:mod:`nova.scheduler.filters`):

* |AllHostsFilter| - does no filtering. It passes all the available hosts.
* |ImagePropertiesFilter| - filters hosts based on properties defined
  on the instance's image. It passes hosts that can support the properties
  specified on the image used by the instance.
* |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.
* |ComputeCapabilitiesFilter| - checks that the capabilities provided by the
  host compute service satisfy any extra specifications associated with the
  instance type. It passes hosts that can create the specified instance type.

  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, when a key doesn't contain
  a colon (:), the key's contents are important. If this key is an attribute of
  HostState object, like ``free_disk_mb``, the filter also treats the extra
  specs key as the key to be matched. If not, the filter will ignore the key.

  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")

* |AggregateInstanceExtraSpecsFilter| - checks that the aggregate metadata
  satisfies any extra specifications associated with the instance type (that
  have no scope or are scoped with ``aggregate_instance_extra_specs``).
  It passes hosts that can create the specified instance type.
  The extra specifications can have the same operators as
  |ComputeCapabilitiesFilter|. To specify multiple values for the same key
  use a comma. E.g., "value1,value2". All hosts are passed if no extra_specs
  are specified.
* |ComputeFilter| - passes all hosts that are operational and enabled.
* |IsolatedHostsFilter| - filter based on
  :oslo.config:option:`filter_scheduler.isolated_images`,
  :oslo.config:option:`filter_scheduler.isolated_hosts`
  and :oslo.config:option:`filter_scheduler.restrict_isolated_hosts_to_isolated_images`
  flags.
* |JsonFilter| - allows simple JSON-based grammar for selecting hosts.
* |NumInstancesFilter| - filters compute nodes by number of instances.
  Nodes with too many instances will be filtered. The host will be
  ignored by the scheduler if more than
  :oslo.config:option:`filter_scheduler.max_instances_per_host` already exist
  on the host.
* |AggregateNumInstancesFilter| - filters hosts by number of instances with
  per-aggregate :oslo.config:option:`filter_scheduler.max_instances_per_host`
  setting. If no per-aggregate value is found, it will fall back to the global
  default :oslo.config:option:`filter_scheduler.max_instances_per_host`.
  If more than one value is found for a host (meaning the host is in two or more
  different aggregates with different max instances per host settings),
  the minimum value will be used.
* |IoOpsFilter| - filters hosts by concurrent I/O operations on it.
  hosts with too many concurrent I/O operations will be filtered.
  :oslo.config:option:`filter_scheduler.max_io_ops_per_host` setting. Maximum
  number of I/O intensive instances allowed to run on this host, the host will
  be ignored by scheduler if more than
  :oslo.config:option:`filter_scheduler.max_io_ops_per_host`
  instances such as build/resize/snapshot etc are running on it.
* |AggregateIoOpsFilter| - filters hosts by I/O operations with per-aggregate
  :oslo.config:option:`filter_scheduler.max_io_ops_per_host` setting. If no
  per-aggregate value is found, it will fall back to the global default
  `:oslo.config:option:`filter_scheduler.max_io_ops_per_host`. If more than
  one value is found for a host (meaning the host is in two or more different
  aggregates with different max io operations settings), the minimum value
  will be used.
* |PciPassthroughFilter| - Filter that schedules instances on a host if the host
  has devices to meet the device requests in the 'extra_specs' for the flavor.
* |SimpleCIDRAffinityFilter| - allows a new instance on a host within
  the same IP block.
* |DifferentHostFilter| - allows the instance on a different host from a
  set of instances.
* |SameHostFilter| - puts the instance on the same host as another instance in
  a set of instances.
* |AggregateTypeAffinityFilter| - limits instance_type by aggregate.
   This filter passes hosts if no instance_type key is set or
   the instance_type aggregate metadata value contains the name of the
   instance_type requested. 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. e.g. 'm1.nano' or "m1.nano,m1.small"
* |ServerGroupAntiAffinityFilter| - This filter implements anti-affinity for a
  server group.  First you must create a server group with a policy of
  'anti-affinity' via the server groups API.  Then, when you boot a new server,
  provide a scheduler hint of 'group=<uuid>' where <uuid> is the UUID of the
  server group you created.  This will result in the server getting added to the
  group.  When the server gets scheduled, anti-affinity will be enforced among
  all servers in that group.
* |ServerGroupAffinityFilter| - This filter works the same way as
  ServerGroupAntiAffinityFilter. The difference is that when you create the server
  group, you should specify a policy of 'affinity'.
* |AggregateMultiTenancyIsolation| - isolate tenants in specific aggregates.
  To specify multiple tenants use a comma. Eg. "tenant1,tenant2"
* |AggregateImagePropertiesIsolation| - isolates hosts based on image
  properties and aggregate metadata. Use a comma to specify multiple values for the
  same property. The filter will then ensure at least one value matches.
* |MetricsFilter| - filters hosts based on metrics weight_setting. Only hosts with
  the available metrics are passed.
* |NUMATopologyFilter| - filters hosts based on the NUMA topology requested by the
  instance, if any.

Now we can focus on these standard filter classes in some detail. Some filters
such as |AllHostsFilter| and |NumInstancesFilter| are relatively simple and can be
understood from the code. For example, |NumInstancesFilter| has the following
implementation:

.. code-block:: python

    class NumInstancesFilter(filters.BaseHostFilter):
        """Filter out hosts with too many instances."""

        def _get_max_instances_per_host(self, host_state, spec_obj):
            return CONF.filter_scheduler.max_instances_per_host

        def host_passes(self, host_state, spec_obj):
            num_instances = host_state.num_instances
            max_instances = self._get_max_instances_per_host(host_state, spec_obj)
            passes = num_instances < max_instances
            return passes

Here :oslo.config:option:`filter_scheduler.max_instances_per_host` means the
maximum number of instances that can be on a host.

The |AvailabilityZoneFilter| looks at the availability zone of compute node
and availability zone from the properties of the request. Each compute service
has its own availability zone. So deployment engineers have an option to run
scheduler with availability zones support and can configure availability zones
on each compute host. This class's method ``host_passes`` returns ``True`` if
availability zone mentioned in request is the same on the current compute host.

The |ImagePropertiesFilter| filters hosts based on the architecture,
hypervisor type and virtual machine mode specified in the
instance. For example, an instance might require a host that supports the ARM
architecture on a qemu compute host. The |ImagePropertiesFilter| will only
pass hosts that can satisfy this request. These instance
properties are populated from properties defined on the instance's image.
E.g. an image can be decorated with these properties using
``glance image-update img-uuid --property architecture=arm --property
hypervisor_type=qemu``
Only hosts that satisfy these requirements will pass the
|ImagePropertiesFilter|.

|ComputeCapabilitiesFilter| checks if the host satisfies any ``extra_specs``
specified on the instance type.  The ``extra_specs`` can contain key/value pairs.
The key for the filter is either non-scope format (i.e. no ``:`` contained), or
scope format in capabilities scope (i.e. ``capabilities:xxx:yyy``). One example
of capabilities scope is ``capabilities:cpu_info:features``, which will match
host's cpu features capabilities. The |ComputeCapabilitiesFilter| will only
pass hosts whose capabilities satisfy the requested specifications.  All hosts
are passed if no ``extra_specs`` are specified.

|ComputeFilter| is quite simple and passes any host whose compute service is
enabled and operational.

Now we are going to |IsolatedHostsFilter|. There can be some special hosts
reserved for specific images. These hosts are called **isolated**. So the
images to run on the isolated hosts are also called isolated. The filter
checks if :oslo.config:option:`filter_scheduler.isolated_images` flag named
in instance specifications is the same as the host specified in
:oslo.config:option:`filter_scheduler.isolated_hosts`. Isolated
hosts can run non-isolated images if the flag
:oslo.config:option:`filter_scheduler.restrict_isolated_hosts_to_isolated_images`
is set to false.

|DifferentHostFilter| - method ``host_passes`` returns ``True`` if the host to
place an instance on is different from all the hosts used by a set of instances.

|SameHostFilter| does the opposite to what |DifferentHostFilter| does.
``host_passes`` returns ``True`` if the host we want to place an instance on is
one of the hosts used by a set of instances.

|SimpleCIDRAffinityFilter| looks at the subnet mask and investigates if
the network address of the current host is in the same sub network as it was
defined in the request.

|JsonFilter| - this filter provides the opportunity to write complicated
queries for the hosts capabilities filtering, based on simple JSON-like syntax.
There can be used the following operations for the host states properties:
``=``, ``<``, ``>``, ``in``, ``<=``, ``>=``, that can be combined with the following
logical operations: ``not``, ``or``, ``and``. For example, the following query can be
found in tests:

::

    ['and',
        ['>=', '$free_ram_mb', 1024],
        ['>=', '$free_disk_mb', 200 * 1024]
    ]

This query will filter all hosts with free RAM greater or equal than 1024 MB
and at the same time with free disk space greater or equal than 200 GB.

Many filters use data from ``scheduler_hints``, that is defined in the moment of
creation of the new server for the user. The only exception for this rule is
|JsonFilter|, that takes data from the schedulers ``HostState`` data structure
directly. Variable naming, such as the ``$free_ram_mb`` example above, should
be based on those attributes.

The |NUMATopologyFilter| considers 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 the related nova-spec document:

* https://opendev.org/openstack/nova-specs/src/branch/master/specs/juno/implemented/virt-driver-numa-placement.rst

and try to match it with the topology exposed by the host, accounting for the
:oslo.config:option:`ram_allocation_ratio` and
:oslo.config:option:`cpu_allocation_ratio` for over-subscription. The filtering
is done in the following manner:

* Filter will attempt to pack instance cells onto host cells.
* It will consider the standard over-subscription limits for each host NUMA cell,
  and provide limits to the compute host accordingly (as mentioned above).
* 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.

Configuring Filters
-------------------

To use filters you specify two settings:

* :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.

The default values for these settings in nova.conf are:

::

    --filter_scheduler.available_filters=nova.scheduler.filters.all_filters
    --filter_scheduler.enabled_filters=ComputeFilter,AvailabilityZoneFilter,ComputeCapabilitiesFilter,ImagePropertiesFilter,ServerGroupAntiAffinityFilter,ServerGroupAffinityFilter

With this configuration, all filters in ``nova.scheduler.filters``
would be available, and by default the |ComputeFilter|,
|AvailabilityZoneFilter|, |ComputeCapabilitiesFilter|,
|ImagePropertiesFilter|, |ServerGroupAntiAffinityFilter|,
and |ServerGroupAffinityFilter| would be used.

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.

.. _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:

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

.. 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, nova will use the ``FilterScheduler`` for the scheduler
driver. All of the standard nova filters and the custom ``AcmeFilter`` filter
are available to the ``FilterScheduler``, but just the ``ComputeFilter`` and
``AcmeFilter`` will be used on each request.

Weights
-------

Filter Scheduler uses the so-called **weights** during its work. A weigher is a
way to select the best suitable host from a group of valid hosts by giving
weights to all the hosts in the list.

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) + ...

A weigher should be a subclass of ``weights.BaseHostWeigher`` and they 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``.

The Filter Scheduler weighs hosts based on the config option
`filter_scheduler.weight_classes`, this defaults to
`nova.scheduler.weights.all_weighers`, which selects the following weighers:

* |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 in the configuration file as the followings::

    metrics_weight_setting = name1=1.0, name2=-1.0

  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 |PciPassthroughFilter| or
  |NUMATopologyFilter| filters must be enabled.

  :Configuration Option: ``[filter_scheduler] pci_weight_multiplier``. Only
    positive values are allowed for the multiplier as a negative value would
    force non-PCI instances away from non-PCI hosts, thus, causing future
    scheduling issues.

  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.
* |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| 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.

Filter Scheduler makes a local list of acceptable hosts by repeated filtering and
weighing. Each time it chooses a host, it virtually consumes resources on it,
so subsequent selections can adjust accordingly. It is useful if the customer
asks for a large block of instances, because weight is computed for
each instance requested.

.. image:: /_static/images/filtering-workflow-2.png

At the end Filter Scheduler sorts selected hosts by their weight and attempts
to provision instances on the chosen hosts.

P.S.: you can find more examples of using Filter Scheduler and standard filters
in :mod:`nova.tests.scheduler`.

.. |AllHostsFilter| replace:: :class:`AllHostsFilter <nova.scheduler.filters.all_hosts_filter.AllHostsFilter>`
.. |ImagePropertiesFilter| replace:: :class:`ImagePropertiesFilter <nova.scheduler.filters.image_props_filter.ImagePropertiesFilter>`
.. |AvailabilityZoneFilter| replace:: :class:`AvailabilityZoneFilter <nova.scheduler.filters.availability_zone_filter.AvailabilityZoneFilter>`
.. |BaseHostFilter| replace:: :class:`BaseHostFilter <nova.scheduler.filters.BaseHostFilter>`
.. |ComputeCapabilitiesFilter| replace:: :class:`ComputeCapabilitiesFilter <nova.scheduler.filters.compute_capabilities_filter.ComputeCapabilitiesFilter>`
.. |ComputeFilter| replace:: :class:`ComputeFilter <nova.scheduler.filters.compute_filter.ComputeFilter>`
.. |IsolatedHostsFilter| replace:: :class:`IsolatedHostsFilter <nova.scheduler.filters.isolated_hosts_filter>`
.. |JsonFilter| replace:: :class:`JsonFilter <nova.scheduler.filters.json_filter.JsonFilter>`
.. |NumInstancesFilter| replace:: :class:`NumInstancesFilter <nova.scheduler.filters.num_instances_filter.NumInstancesFilter>`
.. |AggregateNumInstancesFilter| replace:: :class:`AggregateNumInstancesFilter <nova.scheduler.filters.num_instances_filter.AggregateNumInstancesFilter>`
.. |IoOpsFilter| replace:: :class:`IoOpsFilter <nova.scheduler.filters.io_ops_filter.IoOpsFilter>`
.. |AggregateIoOpsFilter| replace:: :class:`AggregateIoOpsFilter <nova.scheduler.filters.io_ops_filter.AggregateIoOpsFilter>`
.. |PciPassthroughFilter| replace:: :class:`PciPassthroughFilter <nova.scheduler.filters.pci_passthrough_filter.PciPassthroughFilter>`
.. |SimpleCIDRAffinityFilter| replace:: :class:`SimpleCIDRAffinityFilter <nova.scheduler.filters.affinity_filter.SimpleCIDRAffinityFilter>`
.. |DifferentHostFilter| replace:: :class:`DifferentHostFilter <nova.scheduler.filters.affinity_filter.DifferentHostFilter>`
.. |SameHostFilter| replace:: :class:`SameHostFilter <nova.scheduler.filters.affinity_filter.SameHostFilter>`
.. |AggregateTypeAffinityFilter| replace:: :class:`AggregateTypeAffinityFilter <nova.scheduler.filters.type_filter.AggregateTypeAffinityFilter>`
.. |ServerGroupAntiAffinityFilter| replace:: :class:`ServerGroupAntiAffinityFilter <nova.scheduler.filters.affinity_filter.ServerGroupAntiAffinityFilter>`
.. |ServerGroupAffinityFilter| replace:: :class:`ServerGroupAffinityFilter <nova.scheduler.filters.affinity_filter.ServerGroupAffinityFilter>`
.. |AggregateInstanceExtraSpecsFilter| replace:: :class:`AggregateInstanceExtraSpecsFilter <nova.scheduler.filters.aggregate_instance_extra_specs.AggregateInstanceExtraSpecsFilter>`
.. |AggregateMultiTenancyIsolation| replace:: :class:`AggregateMultiTenancyIsolation <nova.scheduler.filters.aggregate_multitenancy_isolation.AggregateMultiTenancyIsolation>`
.. |NUMATopologyFilter| replace:: :class:`NUMATopologyFilter <nova.scheduler.filters.numa_topology_filter.NUMATopologyFilter>`
.. |RAMWeigher| replace:: :class:`RAMWeigher <nova.scheduler.weights.ram.RAMWeigher>`
.. |CPUWeigher| replace:: :class:`CPUWeigher <nova.scheduler.weights.cpu.CPUWeigher>`
.. |AggregateImagePropertiesIsolation| replace:: :class:`AggregateImagePropertiesIsolation <nova.scheduler.filters.aggregate_image_properties_isolation.AggregateImagePropertiesIsolation>`
.. |MetricsFilter| replace:: :class:`MetricsFilter <nova.scheduler.filters.metrics_filter.MetricsFilter>`
.. |MetricsWeigher| replace:: :class:`MetricsWeigher <nova.scheduler.weights.metrics.MetricsWeigher>`
.. |IoOpsWeigher| replace:: :class:`IoOpsWeigher <nova.scheduler.weights.io_ops.IoOpsWeigher>`
.. |PCIWeigher| replace:: :class:`PCIWeigher <nova.scheduler.weights.pci.PCIWeigher>`
.. |ServerGroupSoftAffinityWeigher| replace:: :class:`ServerGroupSoftAffinityWeigher <nova.scheduler.weights.affinity.ServerGroupSoftAffinityWeigher>`
.. |ServerGroupSoftAntiAffinityWeigher| replace:: :class:`ServerGroupSoftAntiAffinityWeigher <nova.scheduler.weights.affinity.ServerGroupSoftAntiAffinityWeigher>`
.. |DiskWeigher| replace:: :class:`DiskWeigher <nova.scheduler.weights.disk.DiskWeigher>`
.. |BuildFailureWeigher| replace:: :class:`BuildFailureWeigher <nova.scheduler.weights.compute.BuildFailureWeigher>`
.. |CrossCellWeigher| replace:: :class:`CrossCellWeigher <nova.scheduler.weights.cross_cell.CrossCellWeigher>`