Re: autovacuum locking question

Mike Schanne <mschanne@kns.com> writes:
> I am investigating a performance problem in our application and am seeing something unexpected in the postgres logs
regardingthe autovacuum. 

> 2019-12-01 13:05:39.029 UTC,"wb","postgres",6966,"127.0.0.1:53976",5ddbd990.1b36,17099,"INSERT waiting",2019-11-25
13:39:28UTC,12/1884256,12615023,LOG,00000,"process 6966 still waiting for RowExclusiveLock on relation 32938 of
database32768 after 1000.085 ms","Process holding the lock: 6045. Wait queue: 6966.",,,,,"INSERT INTO myschema.mytable
(...)VALUES (...) RETURNING process.mytable.mytable_id",13,,"" 
> 2019-12-01 13:05:39.458 UTC,,,6045,,5de3b800.179d,1,,2019-12-01 12:54:24 UTC,10/417900,0,ERROR,57014,"canceling
autovacuumtask",,,,,"automatic vacuum of table ""postgres.myschema.mytable""",,,,"" 

> My understanding from reading the documentation was that a vacuum can run concurrently with table inserts/updates,
butfrom reading the logs it appears they are conflicting over a row lock.  This particular table gets very frequent
inserts/updates(10-100 inserts / sec) so I am concerned that if the autovacuum is constantly canceled, then the table
nevergets cleaned and its performance will continue to degrade over time.  Is it expected for the vacuum to be canceled
byan insert in this way? 

The main part of an autovacuum operation should go through OK.  The only
part that would get canceled in response to somebody taking a
non-exclusive lock is the last step, which is truncation of unused blocks
at the end of the table; that requires an exclusive lock.  Normally,
skipping that step isn't terribly problematic.

> We are using postgres 9.6.10.

IIRC, we've made improvements in this area since 9.6, to allow a
partial truncation to be done if someone wants the lock, rather
than just failing entirely.

            regards, tom lane