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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 returns all the available hosts after its work.
  • |AvailabilityZoneFilter| - filters hosts by availability zone. It returns hosts with the same availability zone as the requested instance has in its properties.
  • |ComputeFilter| - checks that the capabilities provided by the compute service satisfy the extra specifications, associated with the instance type. It returns a list of hosts that can create instance type.
  • |CoreFilter| - filters based on CPU core utilization. It will approve host if it has 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. So, it returns only the hosts with enough available RAM.
  • |SimpleCIDRAffinityFilter| - allows to put a new instance on a host within the same IP block.

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.

|ComputeFilter| checks if host can create instance_type. Let's note that instance types describe the compute, memory and storage capacity of nova compute nodes, it is the list of characteristics such as number of vCPUs, amount RAM and so on. So |ComputeFilter| looks at hosts' capabilities (host without requested specifications can't be chosen for the creating of the instance), checks if the hosts service is up based on last heartbeat. Finally, this Scheduler can verify if host satisfies some extra specifications associated with the instance type (of course if there are no such extra specifications, every host suits them).

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.

|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 exeption for this rule is |JsonFilter|, that takes data in some strange difficult to understand way.

To use filters you specify next two settings:

  • scheduler_available_filters - points available filters.
  • scheduler_default_filters - points filters to be used by default from the list of available ones.

Host Manager sets up these flags in nova.conf by default on the next values:


These two lines mean, that all the filters in the nova.scheduler.filters would be available, and the default ones would be |RamFilter|, |ComputeFilter| and |AvailabilityZoneFilter|.

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.

So in the end file nova.conf should contain lines like these:


As you see, flag scheduler_driver is set up for the FilterSchedule, available filters can be specified more than once and description of the default filters should not contain full paths with class names you need, only class names.

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.


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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