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Filter Scheduler

The Scheduler is akin to a Dating Service. Requests for the creation of new instances come in and the most applicable Compute nodes are selected from a large pool of potential candidates. In a small deployment we may be happy with the currently available Chance Scheduler which randomly selects a Host from the available pool. Or if you need something a little more fancy you may want to use the Filter Scheduler, which selects Compute hosts from a logical partitioning of available hosts.


The Filter Scheduler supports filtering and weighing to make informed decisions on where a new instance should be created.

So, how does this all work?

Costs & Weights

When deciding where to place an Instance, we compare a Weighted Cost for each Host. The Weighting, currently, is just the sum of each Cost. Costs are nothing more than integers from 0 - max_int. Costs are computed by looking at the various Capabilities of the Host relative to the specs of the Instance being asked for. Trying to put a plain vanilla instance on a high performance host should have a very high cost. But putting a vanilla instance on a vanilla Host should have a low cost.

Some Costs are more esoteric. Consider a rule that says we should prefer Hosts that don't already have an instance on it that is owned by the user requesting it (to mitigate against machine failures). Here we have to look at all the other Instances on the host to compute our cost.

An example of some other costs might include selecting:
  • a GPU-based host over a standard CPU
  • a host with fast ethernet over a 10mbps line
  • a host that can run Windows instances
  • a host in the EU vs North America
  • etc

This Weight is computed for each Instance requested. If the customer asked for 1000 instances, the consumed resources on each Host are "virtually" depleted so the Cost can change accordingly.


Filtering and Weighing

The filtering (excluding compute nodes incapable of fulfilling the request) and weighing (computing the relative "fitness" of a compute node to fulfill the request) rules used are very subjective operations ... Service Providers will probably have a very different set of filtering and weighing rules than private cloud administrators. The filtering and weighing aspects of the FilterScheduler are flexible and extensible.


Host Filter

As we mentioned earlier, filtering hosts is a very deployment-specific process. Service Providers may have a different set of criteria for filtering Compute nodes than a University. To facilitate this, the FilterScheduler supports a variety of filtering strategies as well as an easy means for plugging in your own algorithms. Specifying filters involves 2 settings. One makes filters available for use. The second specifies which filters to use by default (out of the filters available). The reason for this second option is that there may be support to allow end-users to specify specific filters during a build at some point in the future.

Making filters available:

Filters are made available to the scheduler via the --scheduler_available_filters setting. This setting can be specified more than once and should contain lists of filter class names (with full paths) to make available. Specifying 'nova.scheduler.filters.standard_filters' will cause all standard filters under 'nova.scheduler.filters' to be made available. That is the default setting. Additionally, you can specify your own classes to be made available. For example, 'myfilter.MyFilterClass' can be specified. Now that you've configured which filters are available, you should set which ones you actually want to use by default.

Setting the default filtering classes:

The default filters to use are set via the --scheduler_default_filters setting. This setting should contain a list of class names. You should not specify the full paths with these class names. By default this flag is set to ['AvailabilityZoneFilter', 'RamFilter', 'ComputeFilter']. Below is a list of standard filter classes:

  • AllHostsFilter includes all hosts (essentially is a No-op filter)
  • AvailabilityZoneFilter provides host filtering based on availability_zone
  • ComputeFilter provides host filtering based on InstanceType extra_specs, comparing against host capability announcements
  • CoreFilter provides host filtering based on number of cpu cores
  • DifferentHostFilter provides host filtering based on scheduler_hint's 'different_host' value. With the scheduler_hints extension, this allows one to put a new instance on a different host from another instance
  • IsolatedHostsFilter provides host filtering based on the 'isolated_hosts' and 'isolated_images' flags/settings.
  • JSONFilter filters hosts based on simple JSON expression grammar. Using a LISP-like JSON structure the caller can request instances based on criteria well beyond what ComputeFilter specifies. See nova.tests.scheduler.test_host_filters for examples.
  • RamFilter provides host filtering based on the memory needed vs memory free
  • SameHostFilter provides host filtering based on scheduler_hint's 'same_host' value. With the scheduler_hints extension, this allows one to put a new instance on the same host as another instance
  • SimpleCIDRAffinityFilter provides host filtering based on scheduler_hint's 'build_near_host_ip' value. With the scheduler_hints extension, this allows one to put a new instance on a host within the same IP block.

To create your own HostFilter the user simply has to derive from nova.scheduler.filters.BaseHostFilter and implement one method: host_passes. This method accepts a HostState instance describing a host as well as a filter_properties dictionary. Host capabilities can be found in HostState.capabilities and other properites can be found in filter_properties like instance_type, etc. Your method should return True if it passes the filter.


Here are some of the main flags you should set in your nova.conf file:

# --scheduler_available_filters=myfilter.MyOwnFilter

scheduler_driver is the real workhorse of the operation. For Filter Scheduler, you need to specify a class derived from nova.scheduler.filter_scheduler.FilterScheduler. scheduler_default_filters are the host filters to be used for filtering candidate Compute nodes.

Some optional flags which are handy for debugging are:


Using the Fake virtualization driver is handy when you're setting this stuff up so you're not dealing with a million possible issues at once. When things seem to working correctly, switch back to whatever hypervisor your deployment uses.

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