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[Chuck Short <chuck.short@xxxxxxxxxxxxx>]

** Changed in: nova
       Status: New => Opinion

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https://bugs.launchpad.net/bugs/1171019

Title:
  Speed up scheduler a little

Status in OpenStack Compute (Nova):
  Opinion

Bug description:
  This is a strategy to speed up the scheduler:

  These scheduler filters will have predictable results for certain
  instance_types:

      aggregate_instance_extra_specs, compute_capabalities_filter, core_filter,
      trusted_filter

  I propose that the filter be able to return a tuple like:

   ('instance_type', True)

  meaning that for this instance type, I'm always going to return True.

  Then the filter_scheduler can cache these results, and before it runs
  through the list of filters, it can check the instance_type and
  decrease the list of filters to actually call.

  ---------

  Secondly, these scheduler filters will have predictable results if a
  certain CONF setting is set:

      isolated_hosts_filter, trusted_filter if it had no
  'attestation_server'.

  So, basically if they found that they were badly configured, they
  could log it and return:

  ('always', True)

  To mean that they are always (until the program is restarted) going to
  allow all hosts.

  This assumes that the config will not change while the process is
  running.

  The scheduler can parse these tuples, and remember not to bother
  calling these filters in future runs.

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