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[Matt Riedemann <mriedem.os@xxxxxxxxx>]

Public bug reported:

The InstanceGroup.get_hosts() method is pretty inefficient when pulling
instances out of the database here:

https://github.com/openstack/nova/blob/c7e9e667426a6d88d396a59cb40d30763a3265f9/nova/objects/instance_group.py#L500

because that by default is going to join on the following tables:

https://github.com/openstack/nova/blob/c7e9e667426a6d88d396a59cb40d30763a3265f9/nova/db/sqlalchemy/api.py#L2098

    if columns_to_join is None:
        columns_to_join_new = ['info_cache', 'security_groups']
        manual_joins = ['metadata', 'system_metadata']

And then just turn around and only use the instance.host value:

https://github.com/openstack/nova/blob/c7e9e667426a6d88d396a59cb40d30763a3265f9/nova/objects/instance_group.py#L502

return list(set([instance.host for instance in instances
                 if instance.host]))

We should be:

1. Avoiding those unnecessary joins by passing expected_attrs=[] which
is a simple backportable fix.

2. Write a new DB API method which would get the set of distinct
instances.host values for the list of instance uuids where the
instances.host value is not None, so I think:

SELECT host FROM instances WHERE host IS NOT NULL AND deleted == 0 AND
uuid IN ($instance_uuids) GROUP BY instances.host;

That way we let the DB query do the work and we don't have to load up
all of the additional instances fields we don't care about.

The DB API optimization would likely need to be a remotable method
because InstanceGroup.get_hosts() is calling in the late affinity check
ComputeManager._validate_instance_group_policy() method in the nova-
compute service (and because InstanceGroup.get_hosts() is remotable
itself).

** Affects: nova
     Importance: Medium
     Assignee: Matt Riedemann (mriedem)
         Status: Triaged


** Tags: db performance

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https://bugs.launchpad.net/bugs/1830234

Title:
  InstanceGroup.get_hosts() uses inefficient DB queries

Status in OpenStack Compute (nova):
  Triaged

Bug description:
  The InstanceGroup.get_hosts() method is pretty inefficient when
  pulling instances out of the database here:

  https://github.com/openstack/nova/blob/c7e9e667426a6d88d396a59cb40d30763a3265f9/nova/objects/instance_group.py#L500

  because that by default is going to join on the following tables:

  https://github.com/openstack/nova/blob/c7e9e667426a6d88d396a59cb40d30763a3265f9/nova/db/sqlalchemy/api.py#L2098

      if columns_to_join is None:
          columns_to_join_new = ['info_cache', 'security_groups']
          manual_joins = ['metadata', 'system_metadata']

  And then just turn around and only use the instance.host value:

  https://github.com/openstack/nova/blob/c7e9e667426a6d88d396a59cb40d30763a3265f9/nova/objects/instance_group.py#L502

  return list(set([instance.host for instance in instances
                   if instance.host]))

  We should be:

  1. Avoiding those unnecessary joins by passing expected_attrs=[] which
  is a simple backportable fix.

  2. Write a new DB API method which would get the set of distinct
  instances.host values for the list of instance uuids where the
  instances.host value is not None, so I think:

  SELECT host FROM instances WHERE host IS NOT NULL AND deleted == 0 AND
  uuid IN ($instance_uuids) GROUP BY instances.host;

  That way we let the DB query do the work and we don't have to load up
  all of the additional instances fields we don't care about.

  The DB API optimization would likely need to be a remotable method
  because InstanceGroup.get_hosts() is calling in the late affinity
  check ComputeManager._validate_instance_group_policy() method in the
  nova-compute service (and because InstanceGroup.get_hosts() is
  remotable itself).

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