Reset the cell cache for database access in Service
We have had a gate bug for a long time where occasionally the scheduler
service gets into a state where many requests fail in it with
CellTimeout errors. Example:
Timed out waiting for response from cell <cell uuid>
Through the use of much DNM patch debug logging in oslo.db, it was
revealed that service child processes (workers) were sometimes starting
off with already locked internal oslo.db locks. This is a known issue
in python [1] where if a parent process forks a child process while a
lock is held, the child will inherit the held lock which can never be
acquired.
The python issue is not considered a bug and the recommended way to
handle it is by making use of the os.register_at_fork() in the oslo.db
to reinitialize its lock. The method is new in python 3.7, so as long
as we still support python 3.6, we must handle the situation outside of
oslo.db.
We can do this by clearing the cell cache that holds oslo.db database
transaction context manager objects during service start(). This way,
we get fresh oslo.db locks that are in an unlocked state when a child
process begins.
We can also take this opportunity to resolve part of a TODO to clear
the same cell cache during service reset() (SIGHUP) since it is another
case where we intended to clear it. The rest of the TODO related to
periodic clearing of the cache is removed after discussion on the
review, as such clearing would be unsynchronized among multiple
services and for periods of time each service might have a different
view of cached cells than another.
Reviewed: https:/ /review. opendev. org/717662 /git.openstack. org/cgit/ openstack/ nova/commit/ ?id=941559042f6 09ee43ff3160c0f 0d0c45187be17f
Committed: https:/
Submitter: Zuul
Branch: master
commit 941559042f609ee 43ff3160c0f0d0c 45187be17f
Author: melanie witt <email address hidden>
Date: Fri Apr 3 21:22:27 2020 +0000
Reset the cell cache for database access in Service
We have had a gate bug for a long time where occasionally the scheduler
service gets into a state where many requests fail in it with
CellTimeout errors. Example:
Timed out waiting for response from cell <cell uuid>
Through the use of much DNM patch debug logging in oslo.db, it was
revealed that service child processes (workers) were sometimes starting
off with already locked internal oslo.db locks. This is a known issue
in python [1] where if a parent process forks a child process while a
lock is held, the child will inherit the held lock which can never be
acquired.
The python issue is not considered a bug and the recommended way to at_fork( ) in the oslo.db
handle it is by making use of the os.register_
to reinitialize its lock. The method is new in python 3.7, so as long
as we still support python 3.6, we must handle the situation outside of
oslo.db.
We can do this by clearing the cell cache that holds oslo.db database
transaction context manager objects during service start(). This way,
we get fresh oslo.db locks that are in an unlocked state when a child
process begins.
We can also take this opportunity to resolve part of a TODO to clear
the same cell cache during service reset() (SIGHUP) since it is another
case where we intended to clear it. The rest of the TODO related to
periodic clearing of the cache is removed after discussion on the
review, as such clearing would be unsynchronized among multiple
services and for periods of time each service might have a different
view of cached cells than another.
Closes-Bug: #1844929
[1] https:/ /bugs.python. org/issue6721
Change-Id: Id233f673a57461 cc312e304873a41 442d732c051