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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.



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 least costed host and appends it to the list of selected costs).

If it turns up, that it can't find candidates for the next instance, it means that there are no more appropriate instances locally.

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 (:mod:`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.

  • |ComputeCapabilityFilter| - checks that the capabilities provided by the host compute service satisfy any extra specifications associated with the instance type (that have no scope, see |TrustedFilter| for details). It passes hosts that can create the specified instance type.

    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)
* <or> (find one of these)

Examples are: ">= 5", "s== 2.1.0", "<in> gcc", and "<or> fpu <or> gpu"
  • |AggregateInstanceExtraSpecsFilter| - checks that the aggregate metadata satisfies any extra specifications associated with the instance type (that have no scope). It passes hosts that can create the specified instance type. The extra specifications can have the same operators as |ComputeCapabilityFilter|.
  • |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.
  • |IsolatedHostsFilter| - filter based on "image_isolated" and "host_isolated" 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.
  • |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 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.

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|.

|ComputeCapabilitesFilter| checks if the host satisfies any 'extra specs' specified on the instance type. The 'extra specs' can contain key/value pairs, and 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.

|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:

    ['>=', '$free_ram_mb', 1024],
    ['>=', '$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 in some strange difficult to understand way.

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 is `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|.

To use filters you specify next two settings:

  • scheduler_available_filters - Defines filter classes made available to the

    scheduler. 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:


With this configuration, all filters in nova.scheduler.filters would be available, and by default the |RamFilter|, |ComputeFilter|, |AvailabilityZoneFilter|, |ComputeCapabilityFilter|, and |ImagePropertiesFilter| 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:


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.

Costs and weights

Filter Scheduler uses so-called weights and costs during its work.

Costs are the computed integers, expressing hosts measure of fitness to be chosen as a result of the request. Of course, costs are computed due to hosts characteristics compared with characteristics from the request. So trying to put instance on a not appropriate host (for example, trying to put really simple and plain instance on a high performance host) would have high cost, and putting instance on an appropriate host would have low.

So let's find out, how does all this computing work happen.

Before weighting Filter Scheduler creates the list of tuples containing weights and cost functions to use for weighing hosts. These functions can be got from cache, if this operation had been done before (this cache depends on topic of node, Filter Scheduler works with only the Compute Nodes, so the topic would be "compute" here). If there is no cost functions in cache associated with "compute", Filter Scheduler tries to get these cost functions from nova.conf. Weight in tuple means weight of cost function matching with it. It also can be got from nova.conf. After that Scheduler weights host, using selected cost functions. It does this using weighted_sum method, which parameters are:

  • weighted_fns - list of cost functions created with their weights;
  • host_states - hosts to be weighted;
  • weighing_properties - dictionary of values that can influence weights.

This method firstly creates a grid of function results (it just counts value of each function using host_state and weighing_properties) - scores, where it would be one row per host and one function per column. The next step is to multiply value from the each cell of the grid by the weight of appropriate cost function. And the final step is to sum values in the each row - it would be the weight of host, described in this line. This method returns the host with the lowest weight - the best one.

If we concentrate on cost functions, it would be important to say that we use compute_fill_first_cost_fn function by default, which simply returns hosts free RAM:

def compute_fill_first_cost_fn(host_state, weighing_properties):
    """More free ram = higher weight. So servers will less free ram will be
    return host_state.free_ram_mb

You can implement your own variant of cost function for the hosts capabilities you would like to mention. Using different cost functions (as you understand, there can be a lot of ones used in the same time) can make the chose of next host for the creating of the new instance flexible.

These cost functions should be set up in the nova.conf with the flag least_cost_functions (there can be more than one functions separated by commas). By default this line would look like this:


As for weights of cost functions, they also should be described in nova.conf. The line with this description looks the following way: function_name_weight.

As for default cost function, it would be: compute_fill_first_cost_fn_weight, and by default it is -1.0.


Negative function's weight means that the more free RAM Compute Node has, the better it is. Nova tries to spread instances as much as possible over the Compute Nodes. Positive weight here would mean that Nova would fill up a single Compute Node first.

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`.

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