HINT: This isn't a clean backport, as we keep the subnet in-use field.
We can't backport the db update that would remove the field.
in [1] a lock was introduced with the goal of preventing subnets from
being deleted while ports are being created in them in parallel.
This was acheived by aquiring an exclusive lock on the row of the
subnet in the Subnet table when adding/modifying a port or deleting
the subnet.
However as this was a exclusive lock it also prevented concurrent port
modifications on the same subnet from happening. This can cause
performance issues on environment with large shared subnets (e.g. a
large external subnet).
To reduce the lock contention for this case we split the lock in two
parts:
* For normal port operations we will aquire a shared lock on the
row of the subnet. This allows multiple such operations to happen in
parallel.
* For deleting a subnet we will aquire an exclusive lock on the row of
the subnet. This lock can not be aquired when there is any shared
lock currently on the row.
With this we maintain the same locking level as before, but reduce the
amount of lock contention happening and thereby improve throughput.
The performance improvement can be measured using rally test [2].
(improving from 21 to 18 seconds).
Alternatively it can be tested using 250 parallel curl calls to create a
port in the same network. This improves from 113s to 42s.
[1]: https://review.opendev.org/c/openstack/neutron/+/713045
[2]: https://github.com/openstack/rally-openstack/blob/master/samples/tasks/scenarios/neutron/create-and-delete-ports.json
Closes-Bug: #2009055
Change-Id: I31b1a9c2f986f59fee0da265acebbd88d2f8e4f8
(cherry picked from commit c0af5b3b5e)
d25c129ec2