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[Amir Nikpour <2089726@xxxxxxxxxxxxxxxxxx>]

Public bug reported:

In the OVN Agent (e.g., metadata agent) within the Neutron project, event handling for RowEvent instances assumes parallel execution of their run methods. However, due to the design of the ovsdbapp library's RowEventHandler class, these methods are executed sequentially [0]. This sequential execution creates a performance bottleneck, especially in high-traffic environments.
For example, consider the PortBindingEvent class in the OVN Agent, where the run method uses locks to safely handle concurrent execution [1]:

def run(self, event, row, old):
    resync = False

    with _SYNC_STATE_LOCK.read_lock():
        self.log_row(row)
        try:
            self.agent.provision_datapath(row)
        except ConfigException:
            # We're now in the reader lock mode, we need to exit the context and then use writer lock
            resync = True
    if resync:
        self.agent.resync()


This method relies on _SYNC_STATE_LOCK.read_lock() for thread-safe execution, which is only meaningful if run methods can execute concurrently. However, since RowEventHandler in ovsdbapp processes these events sequentially, the expected concurrency does not occur, resulting in underutilized resources and increased latency.

Suggested Solution:
Parallelize run Method Execution: Modify RowEventHandler to use concurrency mechanisms, such as a thread pool, to execute the run methods for different events in parallel instead of sequentially.


[0] https://github.com/openstack/ovsdbapp/blob/stable/2024.2/ovsdbapp/event.py#L177

[1]
https://github.com/openstack/neutron/blob/stable/2024.2/neutron/agent/ovn/metadata/agent.py#L135

** Affects: neutron
     Importance: Undecided
     Assignee: Amir Nikpour (amniik)
         Status: New

** Changed in: neutron
     Assignee: (unassigned) => Amir Nikpour (amniik)

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https://bugs.launchpad.net/bugs/2089726

Title:
  Sequential Execution of run Methods in OVN Agent

Status in neutron:
  New

Bug description:
  In the OVN Agent (e.g., metadata agent) within the Neutron project, event handling for RowEvent instances assumes parallel execution of their run methods. However, due to the design of the ovsdbapp library's RowEventHandler class, these methods are executed sequentially [0]. This sequential execution creates a performance bottleneck, especially in high-traffic environments.
  For example, consider the PortBindingEvent class in the OVN Agent, where the run method uses locks to safely handle concurrent execution [1]:

  def run(self, event, row, old):
      resync = False

      with _SYNC_STATE_LOCK.read_lock():
          self.log_row(row)
          try:
              self.agent.provision_datapath(row)
          except ConfigException:
              # We're now in the reader lock mode, we need to exit the context and then use writer lock
              resync = True
      if resync:
          self.agent.resync()

  
  This method relies on _SYNC_STATE_LOCK.read_lock() for thread-safe execution, which is only meaningful if run methods can execute concurrently. However, since RowEventHandler in ovsdbapp processes these events sequentially, the expected concurrency does not occur, resulting in underutilized resources and increased latency.

  Suggested Solution:
  Parallelize run Method Execution: Modify RowEventHandler to use concurrency mechanisms, such as a thread pool, to execute the run methods for different events in parallel instead of sequentially.

  
  [0] https://github.com/openstack/ovsdbapp/blob/stable/2024.2/ovsdbapp/event.py#L177

  [1]
  https://github.com/openstack/neutron/blob/stable/2024.2/neutron/agent/ovn/metadata/agent.py#L135

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