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[Brian Haley <1766812@xxxxxxxxxxxxxxxxxx>]

The DHCP agent was changed to use GreenPool thread, and will dynamically
increase the number as of commit
7369b69e2ef5b1b3c30b237885c2648c63f1dffb. This was a similar change as
presented in the linked patch. For this reason I'll mark this bug fixed.

** Changed in: neutron
       Status: New => Fix Released

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https://bugs.launchpad.net/bugs/1766812

Title:
  the machine running dhcp agent will have very high cpu load when start
  dhcp agent after the agent down more than 150 seconds

Status in neutron:
  Fix Released

Bug description:
  This issue can be reproduced by following steps:

  openstack Ocata version, centos 7.2

  1. two dhcp agent nodes
  2. neutron-server side config allow_automatic_dhcp_failover is True and dhcp_agents_per_network is 2
  3. create a lot of networks and each one have one subnet, I created 200.The more networks, the higher cpu load of dhcp agent node, and the longer high cpu load duration
  4. stop one dhcp agent, and wait at least more than 150s (agent_down_time * 2). It is best to check the distribution of networks on two dhcp agent nodes. Neutron-server will remove the networks of the dead dhcp agent after 150s, it is better to wait until all the networks is removed from the dead dhcp agent in the db. So if have 200 networks, you can do the next step after more than 5 minites.
  5. start the dhcp agent above, and use top to check the cpu situation, after a while, you will see very high cpu load.

  If you have rabbitmq web UI, after do the 5 step, the dhcp agent will
  sync the networks and the dhcp agent consumer has not been created
  yet. Neutron-server find that the dhcp agent is active and re schedule
  network to the dhcp agent, you will find that the messages heap up in
  the dhcp agent side. After the dhcp agent finished syncing networks,
  the dhcp agent consumer is created and will consume the messages but
  not deal. When the dhcp agent queue consumes the heap messages and
  deal, the cpu load of dhcp agent node will become higher and higher.

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