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[Thierry Carrez <thierry.carrez+lp@xxxxxxxxx>]

** Changed in: nova
       Status: Fix Committed => Fix Released

** Changed in: nova
    Milestone: None => liberty-rc1

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https://bugs.launchpad.net/bugs/1385489

Title:
  ResourceTracker._update_usage_from_migrations() is inefficient due to
  multiple Instance.get_by_uuid() lookups

Status in OpenStack Compute (nova):
  Fix Released

Bug description:
  Here is our ResourceTracker._update_usage_from_migration() code:

      def _update_usage_from_migrations(self, context, resources,
  migrations):

          self.tracked_migrations.clear()

          filtered = {}

          # do some defensive filtering against bad migrations records in the
          # database:
          for migration in migrations:

              instance = migration['instance']

              if not instance:
                  # migration referencing deleted instance
                  continue

              uuid = instance['uuid']

              # skip migration if instance isn't in a resize state:
              if not self._instance_in_resize_state(instance):
                  LOG.warn(_("Instance not resizing, skipping migration."),
                           instance_uuid=uuid)
                  continue

              # filter to most recently updated migration for each instance:
              m = filtered.get(uuid, None)
              if not m or migration['updated_at'] >= m['updated_at']:
                  filtered[uuid] = migration

          for migration in filtered.values():
              instance = migration['instance']
              try:
                  self._update_usage_from_migration(context, instance, None,
                                                    resources, migration)
              except exception.FlavorNotFound:
                  LOG.warn(_("Flavor could not be found, skipping "
                             "migration."), instance_uuid=uuid)
                  continue

  Unfortunately, when the migration object's 'instance' attribute is
  accessed, a call across RPC and DB occurs:

  https://github.com/openstack/nova/blob/stable/icehouse/nova/objects/migration.py#L77-L80

      @property
      def instance(self):
          return instance_obj.Instance.get_by_uuid(self._context,
                                                   self.instance_uuid)

  For some very strange reason, the code in
  _update_usage_from_migration() builds a "filtered"dictionary with the
  migration objects that need to be accounted for in the resource
  usages, and then once it builds that filtered dictionary, it goes
  through the values and calls _update_usage_from_migration(), passing
  the migration object's instance object.

  There's no reason to do this at all. The filtered variable can go away
  and the call to _update_usage_from_migration() can occur in the main
  for loop, using the same instance variable from the original line:

   instance = migration['instance']

  That way, for each migration, we don't need to do two lookup by UUID
  calls through the conductor to get the migration's instance object...

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