nova/doc/source/devref/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 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 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 (nova.scheduler.filters):

• AllHostsFilter <nova.scheduler.filters.all_hosts_filter.AllHostsFilter> - frankly speaking, this filter does no operation. It passes all the available hosts.

• ImagePropertiesFilter <nova.scheduler.filters.image_props_filter.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 <nova.scheduler.filters.availability_zone_filter.AvailabilityZoneFilter> - filters hosts by availability zone. It passes hosts matching the availability zone specified in the instance properties.

• ComputeCapabilitiesFilter <nova.scheduler.filters.compute_capabilities_filter.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 like "capabilities: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)
* <or> (find one of these)

Examples are: ">= 5", "s== 2.1.0", "<in> gcc", and "<or> fpu <or> gpu"

• AggregateInstanceExtraSpecsFilter <nova.scheduler.filters.aggregate_instance_extra_specs.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 ComputeCapabilitiesFilter <nova.scheduler.filters.compute_capabilities_filter.ComputeCapabilitiesFilter>.
• ComputeFilter <nova.scheduler.filters.compute_filter.ComputeFilter> - passes all hosts that are operational and enabled.
• CoreFilter <nova.scheduler.filters.core_filter.CoreFilter> - filters based on CPU core utilization. It passes hosts with sufficient number of CPU cores.
• IsolatedHostsFilter <nova.scheduler.filters.isolated_hosts_filter> - filter based on "image_isolated" and "host_isolated" flags.
• JsonFilter <nova.scheduler.filters.json_filter.JsonFilter> - allows simple JSON-based grammar for selecting hosts.
• RamFilter <nova.scheduler.filters.ram_filter.RamFilter> - filters hosts by their RAM. Only hosts with sufficient RAM to host the instance are passed.
• SimpleCIDRAffinityFilter <nova.scheduler.filters.affinity_filter.SimpleCIDRAffinityFilter> - allows to put a new instance on a host within the same IP block.
• DifferentHostFilter <nova.scheduler.filters.affinity_filter.DifferentHostFilter> - allows to put the instance on a different host from a set of instances.
• SameHostFilter <nova.scheduler.filters.affinity_filter.SameHostFilter> - puts the instance on the same host as another instance in a set of of instances.
• RetryFilter <nova.scheduler.filters.retry_filter.RetryFilter> - filters hosts that have been attempted for scheduling. Only passes hosts that have not been previously attempted.
• TrustedFilter <nova.scheduler.filters.trusted_filter.TrustedFilter> - filters hosts based on their trust. Only passes hosts that meet the trust requirements specified in the instance properties.
• TypeAffinityFilter <nova.scheduler.filters.type_filter.TypeAffinityFilter> - Only passes hosts that are not already running an instance of the requested type.
• AggregateTypeAffinityFilter <nova.scheduler.filters.type_filter.AggregateTypeAffinityFilter> - limits instance_type by aggregate.
• GroupAntiAffinityFilter <nova.scheduler.filters.affinity_filter.GroupAntiAffinityFilter> - ensures that each instance in group is on a different host.
• AggregateMultiTenancyIsolation <nova.scheduler.filters.aggregate_multitenancy_isolation.AggregateMultiTenancyIsolation> - isolate tenants in specific aggregates.

Now we can focus on these standard filter classes in details. I will pass the simplest ones, such as AllHostsFilter <nova.scheduler.filters.all_hosts_filter.AllHostsFilter>, CoreFilter <nova.scheduler.filters.core_filter.CoreFilter> and RamFilter <nova.scheduler.filters.ram_filter.RamFilter> are, because their functionality is quite simple and can be understood just from the code. For example class RamFilter <nova.scheduler.filters.ram_filter.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 <nova.scheduler.filters.availability_zone_filter.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 <nova.scheduler.filters.image_props_filter.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 <nova.scheduler.filters.image_props_filter.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 <nova.scheduler.filters.image_props_filter.ImagePropertiesFilter>.

ComputeCapabilitiesFilter <nova.scheduler.filters.compute_capabilities_filter.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 <nova.scheduler.filters.compute_capabilities_filter.ComputeCapabilitiesFilter> will only pass hosts whose capabilities satisfy the requested specifications. All hosts are passed if no 'extra specs' are specified.

ComputeFilter <nova.scheduler.filters.compute_filter.ComputeFilter> is quite simple and passes any host whose compute service is enabled and operational.

Now we are going to IsolatedHostsFilter <nova.scheduler.filters.isolated_hosts_filter>. 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 <nova.scheduler.filters.affinity_filter.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 <nova.scheduler.filters.affinity_filter.SameHostFilter> does the opposite to what DifferentHostFilter <nova.scheduler.filters.affinity_filter.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 <nova.scheduler.filters.affinity_filter.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.

GroupAntiAffinityFilter <nova.scheduler.filters.affinity_filter.GroupAntiAffinityFilter> its method host_passes returns True if host to place the instance on is not in a group of hosts. The group of hosts is maintained by a group name. The scheduler hint contains the group name.

JsonFilter <nova.scheduler.filters.json_filter.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 <nova.scheduler.filters.json_filter.JsonFilter>, that takes data in some strange difficult to understand way.

The RetryFilter <nova.scheduler.filters.retry_filter.RetryFilter> filters hosts that have already been attempted for scheduling. It only passes hosts that have not been previously attempted.

The TrustedFilter <nova.scheduler.filters.trusted_filter.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 <nova.scheduler.filters.trusted_filter.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:

--scheduler_available_filters=nova.scheduler.filters.standard_filters
--scheduler_default_filters=RamFilter,ComputeFilter,AvailabilityZoneFilter,ComputeCapabilitiesFilter,ImagePropertiesFilter

With this configuration, all filters in nova.scheduler.filters would be available, and by default the RamFilter <nova.scheduler.filters.ram_filter.RamFilter>, ComputeFilter <nova.scheduler.filters.compute_filter.ComputeFilter>, AvailabilityZoneFilter <nova.scheduler.filters.availability_zone_filter.AvailabilityZoneFilter>, ComputeCapabilitiesFilter <nova.scheduler.filters.compute_capabilities_filter.ComputeCapabilitiesFilter>, and ImagePropertiesFilter <nova.scheduler.filters.image_props_filter.ImagePropertiesFilter> would be used.

If you want to create your own filter you just need to inherit from BaseHostFilter <nova.scheduler.filters.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.

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.

The Filter Scheduler weights hosts based on the config option scheduler_weight_classes, this defaults to nova.scheduler.weights.all_weighers, which selects the only weigher available -- the RamWeigher. Hosts are then weighted and sorted with the largest weight winning.

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 nova.tests.scheduler.