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

During its work Filter Scheduler firstly makes dictionary of unfiltered hosts, then filters them using filter properties and finally chooses hosts for the requested number of instances (each time it chooses the most weighed host and appends it to the list of selected hosts).
If it turns up, that it can’t find candidates for the next instance, it means that there are no more appropriate hosts where the instance could be scheduled.
If we speak about filtering and weighting, their work is quite flexible in the Filter Scheduler. There are a lot of filtering strategies for the Scheduler to support. Also you can even implement your own algorithm of filtering.
There are some standard filter classes to use (nova.scheduler.filters
):
AllHostsFilter
- frankly speaking, this filter does no operation. 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 specified image properties contained in 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.The extra specifications can have a scope at the beginning of the key string of a key/value pair. The scope format is
scope:key
and can be nested, i.e.key_string := scope:key_string
. Example likecapabilities:cpu_info: features
is valid scope format. A key string without any:
is non-scope format. Each filter defines it’s valid scope, and not all filters accept non-scope format.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"
AggregateInstanceExtraSpecsFilter
- checks that the aggregate metadata satisfies any extra specifications associated with the instance type (that have no scope or are scoped withaggregate_instance_extra_specs
). It passes hosts that can create the specified instance type. The extra specifications can have the same operators asComputeCapabilitiesFilter
. To specify multiple values for the same key use a comma. E.g., “value1,value2”ComputeFilter
- passes all hosts that are operational and enabled.CoreFilter
- filters based on CPU core utilization. It passes hosts with sufficient number of CPU cores.AggregateCoreFilter
- filters hosts by CPU core number with per-aggregatecpu_allocation_ratio
setting. If no per-aggregate value is found, it will fall back to the global defaultcpu_allocation_ratio
. If more than one value is found for a host (meaning the host is in two different aggregate with different ratio settings), the minimum value will be used.IsolatedHostsFilter
- filter based onimage_isolated
,host_isolated
andrestrict_isolated_hosts_to_isolated_images
flags.JsonFilter
- allows simple JSON-based grammar for selecting hosts.RamFilter
- filters hosts by their RAM. Only hosts with sufficient RAM to host the instance are passed.AggregateRamFilter
- filters hosts by RAM with per-aggregateram_allocation_ratio
setting. If no per-aggregate value is found, it will fall back to the global defaultram_allocation_ratio
. If more than one value is found for a host (meaning the host is in two different aggregate with different ratio settings), the minimum value will be used.DiskFilter
- filters hosts by their disk allocation. Only hosts with sufficient disk space to host the instance are passed.disk_allocation_ratio
setting. It’s virtual disk to physical disk allocation ratio and it’s 1.0 by default. The total allow allocated disk size will be physical disk multiplied this ratio.AggregateDiskFilter
- filters hosts by disk allocation with per-aggregatedisk_allocation_ratio
setting. If no per-aggregate value is found, it will fall back to the global defaultdisk_allocation_ratio
. If more than one value is found for a host (meaning the host is in two or more different aggregates with different ratio settings), the minimum value will be used.NumInstancesFilter
- filters hosts by number of running instances on it. hosts with too many instances will be filtered.max_instances_per_host
setting. Maximum number of instances allowed to run on this host, the host will be ignored by scheduler if more thanmax_instances_per_host
are already existing on the host.AggregateNumInstancesFilter
- filters hosts by number of instances with per-aggregatemax_instances_per_host
setting. If no per-aggregate value is found, it will fall back to the global defaultmax_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.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 thanmax_io_ops_per_host
instances such as build/resize/snapshot etc are running on it.AggregateIoOpsFilter
- filters hosts by I/O operations with per-aggregatemax_io_ops_per_host
setting. If no per-aggregate value is found, it will fall back to the global defaultmax_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 to put a new instance on a host within the same IP block.DifferentHostFilter
- allows to put 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.RetryFilter
- filters hosts that have been attempted for scheduling. Only passes hosts that have not been previously attempted.TrustedFilter
- filters hosts based on their trust. Only passes hosts that meet the trust requirements specified in the instance properties.TypeAffinityFilter
- Only passes hosts that are not already running an instance of the requested type.AggregateTypeAffinityFilter
- limits instance_type by aggregate.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 details. I will pass the
simplest ones, such as AllHostsFilter
, CoreFilter
and RamFilter
are,
because their functionality is quite simple and can be understood just from the
code. For example class RamFilter
has the next realization:
class RamFilter(filters.BaseHostFilter):
"""Ram Filter with over subscription flag"""
def host_passes(self, host_state, filter_properties):
"""Only return hosts with sufficient available RAM."""
instance_type = filter_properties.get('instance_type')
requested_ram = instance_type['memory_mb']
free_ram_mb = host_state.free_ram_mb
total_usable_ram_mb = host_state.total_usable_ram_mb
used_ram_mb = total_usable_ram_mb - free_ram_mb
return total_usable_ram_mb * FLAGS.ram_allocation_ratio - used_ram_mb >= requested_ram
Here ram_allocation_ratio
means the virtual RAM to physical RAM allocation
ratio (it is 1.5
by default). Really, nice and simple.
Next standard filter to describe is AvailabilityZoneFilter
and it isn’t
difficult too. This filter just 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 classes 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. E.g., 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 define 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. This Scheduler
checks if image_isolated
flag named in instance specifications is the same
that the host has. Isolated hosts can run non isolated images if the flag
restrict_isolated_hosts_to_isolated_images
is set to false.
DifferentHostFilter
- its method host_passes
returns True
if host to
place instance on is different from all the hosts used by set of instances.
SameHostFilter
does the opposite to what DifferentHostFilter
does. So its
host_passes
returns True
if the host we want to place instance on is
one of the set of instances uses.
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, there is the query you can
find 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 RetryFilter
filters hosts that have already been attempted for scheduling.
It only passes hosts that have not been previously attempted.
The TrustedFilter
filters hosts based on their trust. Only passes hosts
that match the trust requested in the extra_specs
for the flavor. The key
for this filter must be scope format as trust:trusted_host
, where trust
is the scope of the key and trusted_host
is the actual key value.
The value of this pair (trusted
/untrusted
) must match the
integrity of a host (obtained from the Attestation service) before it is
passed by the TrustedFilter
.
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:
and try to match it with the topology exposed by the host, accounting for the
ram_allocation_ratio
and 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.
To use filters you specify next two settings:
scheduler_available_filters
- Defines filter classes made available to thescheduler. This setting can be used multiple times.
scheduler_default_filters
- Of the available filters, defines those that the scheduler uses by default.
The default values for these settings in nova.conf are:
--scheduler_available_filters=nova.scheduler.filters.standard_filters
--scheduler_default_filters=RamFilter,ComputeFilter,AvailabilityZoneFilter,ComputeCapabilitiesFilter,ImagePropertiesFilter,ServerGroupAntiAffinityFilter,ServerGroupAffinityFilter'
With this configuration, all filters in nova.scheduler.filters
would be available, and by default the RamFilter
, ComputeFilter
,
AvailabilityZoneFilter
, ComputeCapabilitiesFilter
,
ImagePropertiesFilter
, ServerGroupAntiAffinityFilter
,
and ServerGroupAffinityFilter
would be used.
If you want to create your own filter you just need to inherit from
BaseHostFilter
and implement one method:
host_passes
. This method should return True
if host passes the filter. It
takes host_state
(describes host) and filter_properties
dictionary as the
parameters.
As an example, nova.conf could contain the following scheduler-related settings:
--scheduler_driver=nova.scheduler.FilterScheduler
--scheduler_available_filters=nova.scheduler.filters.standard_filters
--scheduler_available_filters=myfilter.MyFilter
--scheduler_default_filters=RamFilter,ComputeFilter,MyFilter
With these settings, nova will use the FilterScheduler
for the scheduler
driver. The standard nova filters and MyFilter are available to the
FilterScheduler. The RamFilter, ComputeFilter, and MyFilter are used by
default when no filters are specified in the 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 must
implement the weight_multiplier
and weight_object
methods. If the
weight_objects
method is overridden 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 scheduler_weight_classes, this defaults to nova.scheduler.weights.all_weighers, which selects the following weighers:
RAMWeigher
Hosts are then weighted and sorted with the largest weight winning. If the multiplier is negative, the host with less RAM available will win (useful for stacking hosts, instead of spreading).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
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.
Filter Scheduler finds 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 the some large amount of instances, because weight is computed for each instance requested.

In the end Filter Scheduler sorts selected hosts by their weight and provisions instances on them.
P.S.: you can find more examples of using Filter Scheduler and standard filters
in :mod:nova.tests.scheduler
.