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.
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
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:featuresis 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”
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-aggregate
cpu_allocation_ratiosetting. If no per-aggregate value is found, it will fall back to the global default
cpu_allocation_ratio. If more than one value is found for a host (meaning the host is in two different aggregates with different ratio settings), the minimum value will be used.
IsolatedHostsFilter- filter based on
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-aggregate
ram_allocation_ratiosetting. If no per-aggregate value is found, it will fall back to the global default
ram_allocation_ratio. If more than one value is found for a host (meaning the host is in two different aggregates 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_ratiosetting. The virtual disk to physical disk allocation ratio, 1.0 by default. The total allowed allocated disk size will be physical disk multiplied this ratio.
AggregateDiskFilter- filters hosts by disk allocation with per-aggregate
disk_allocation_ratiosetting. If no per-aggregate value is found, it will fall back to the global default
disk_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 compute nodes by number of running instances. Nodes with too many instances will be filtered.
max_instances_per_hostsetting. Maximum number of instances allowed to run on this host. The host will be ignored by the scheduler if more than
max_instances_per_hostalready exist on the host.
AggregateNumInstancesFilter- filters hosts by number of instances with per-aggregate
max_instances_per_hostsetting. If no per-aggregate value is found, it will fall back to the global default
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.
max_io_ops_per_hostsetting. Maximum number of I/O intensive instances allowed to run on this host, the host will be ignored by scheduler if more than
max_io_ops_per_hostinstances such as build/resize/snapshot etc are running on it.
AggregateIoOpsFilter- filters hosts by I/O operations with per-aggregate
max_io_ops_per_hostsetting. If no per-aggregate value is found, it will fall back to the global default
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.
RetryFilter- filters hosts that have been attempted for scheduling. Only passes hosts that have not been previously attempted.
TrustedFilter(EXPERIMENTAL) - 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.
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. We’ll skip the
simplest ones, such as
because their functionality is relatively simple and can be understood 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
ram_allocation_ratio means the virtual RAM to physical RAM allocation
ratio (it is
1.5 by default).
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
availability zone mentioned in request is the same on the current compute host.
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
Only hosts that satisfy these requirements will pass the
ComputeCapabilitiesFilter checks if the host satisfies any
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
image_isolated flag named in instance specifications is the same
as the host. Isolated hosts can run non isolated images if the flag
restrict_isolated_hosts_to_isolated_images is set to false.
DifferentHostFilter - method
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
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:
>=, that can be combined with the following
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.
RetryFilter filters hosts that have already been attempted for
scheduling. It only passes hosts that have not been previously attempted. If a
compute node is raising an exception when spawning an instance, then the
compute manager will reschedule it by adding the failing host to a retry
dictionary so that the RetryFilter will not accept it as a possible
destination. That means that if all of your compute nodes are failing, then the
RetryFilter will return 0 hosts and the scheduler will raise a NoValidHost
exception even if the problem is related to 1:N compute nodes. If you see that
case in the scheduler logs, then your problem is most likely related to a
compute problem and you should check the compute logs.
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
is the scope of the key and
trusted_host is the actual key value.
The value of this pair (
untrusted) must match the
integrity of a host (obtained from the Attestation service) before it is
passed by 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
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 two settings:
filter_scheduler.available_filters- Defines filter classes made available to the scheduler. This setting can be used multiple times.
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:
With this configuration, all filters in
would be available, and by default the
ServerGroupAffinityFilter would be used.
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. It
host_state (describing the host) and
filter_properties dictionary as the
As an example, nova.conf could contain the following scheduler-related settings:
--scheduler.driver=nova.scheduler.FilterScheduler --filter_scheduler.available_filters=nova.scheduler.filters.all_filters --filter_scheduler.available_filters=myfilter.MyFilter --filter_scheduler.enabled_filters=RamFilter,ComputeFilter,MyFilter
When writing your own filter, be sure to add it to the list of available filters and enable it in the default filters. The “all_filters” setting only includes the filters shipped with nova.
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.
Each filter selects hosts in a different way and has different costs. The order
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
filter_scheduler.enabled_filters items by
their costs in reverse order. For example, ComputeFilter is better before any
resource calculating filters like RamFilter, CoreFilter.
In medium/large environments having AvailabilityZoneFilter before any capability or resource calculating filters can be useful.
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
_weight_object methods or just implement the
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
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:
RAMWeigherCompute weight based on available RAM on the compute node. Sort with the largest weight winning. If the multiplier is negative, the host with least RAM available will win (useful for stacking hosts, instead of spreading).
DiskWeigherHosts 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).
MetricsWeigherThis 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
IoOpsWeigherThe 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.
ServerGroupSoftAffinityWeigherThe 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 meaningful for the calculation as a negative value would mean that the affinity weigher would prefer non collocating placement.
ServerGroupSoftAntiAffinityWeigherThe 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 meaningful for the calculation as a negative value would mean that the anti-affinity weigher would prefer collocating placement.
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.
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