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Reviewed:  https://review.opendev.org/c/openstack/neutron/+/829022
Committed: https://opendev.org/openstack/neutron/commit/b5b519a4faa52677d5b9d5553217ed40aac70483
Submitter: "Zuul (22348)"
Branch:    master

commit b5b519a4faa52677d5b9d5553217ed40aac70483
Author: Sahid Orentino Ferdjaoui <sahid-orentino.ferdjaoui-ext@xxxxxxxxxx>
Date:   Mon Feb 14 13:52:10 2022 +0100

    segments: fix scheduling duplicate segments
    
    It has been reported by bug #1959750 that for a given network the
    agent is scheduling all segments reported by agent.
    
    This commit is fixing the issue by ensuring scheduling segments per
    network that they belong to only.
    A test has been updated to demonstract the fact that for a given
    network we only schedule the related segments.
    
    Closes-bug: #1959750
    Signed-off-by: Sahid Orentino Ferdjaoui <sahid.ferdjaoui@xxxxxxxxxxxxxxxxxxxxxxxx>
    Change-Id: Icd57570004055903c6817a75d814ed65db3fa53c


** Changed in: neutron
       Status: In Progress => Fix Released

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https://bugs.launchpad.net/bugs/1959750

Title:
  potential performance issue when scheduling network segments

Status in neutron:
  Fix Released

Bug description:
  During some investigations regarding segments we may have noticed
  performance issues related to the current algorithm that schedules
  network segments on hosts.

  When an agent is reporting a change in segment, the process goes to
  the function `auto_schedule_new_network_segments` with the list of the
  segments that this host handles.

  This function is retrieving from the segments the related networks,
  then we can notice that the algorithm is running a double for loop.
  That one iterates through network and per segments to schedule network
  segments on all hosts.

          for network_id in network_ids:
              for segment in segments:
                  self._schedule_network(
                      payload.context, network_id, dhcp_notifier,
                          candidate_hosts=segment['hosts'])

  Depending on the design chosen, in a setup that has hundred segments
  per host with hundred networks and potentially segments that share the
  same list of hosts, we will endup by calling _schedule_network 10000
  times with duplication.

  To avoid such duplication and unnecessary calls of _schedule_network
  for the same hosts we may want to provide a datastructure that is
  storing for each network the hosts already scheduled.

       for network_id in network_ids:
           for segment in segments:
               if not _already_scheduled(network_id, segment['hosts']):
                   self._schedule_network(
                       payload.context, network_id, dhcp_notifier,
                       candidate_hosts=segment['hosts'])

  With this same scenario, and by using such algorithm we may reduce the
  number of call per the number of networks, 100.

  Thanks,
  s.

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