Re: autovac issue with large number of tables

On Mon, 27 Jul 2020 at 06:43, Nasby, Jim <nasbyj@amazon.com> wrote:
>
> A database with a very large number of  tables eligible for autovacuum can result in autovacuum workers “stuck” in a
tightloop of table_recheck_autovac() constantly reporting nothing to do on the table. This is because a database with a
verylarge number of tables means it takes a while to search the statistics hash to verify that the table still needs to
beprocessed[1]. If a worker spends some time processing a table, when it’s done it can spend a significant amount of
timerechecking each table that it identified at launch (I’ve seen a worker in this state for over an hour). A simple
work-aroundin this scenario is to kill the worker; the launcher will quickly fire up a new worker on the same database,
andthat worker will build a new list of tables. 
>
>
>
> That’s not a complete solution though… if the database contains a large number of very small tables you can end up in
astate where 1 or 2 workers is busy chugging through those small tables so quickly than any additional workers spend
alltheir time in table_recheck_autovac(), because that takes long enough that the additional workers are never able to
“leapfrog”the workers that are doing useful work. 
>

As another solution, I've been considering adding a queue having table
OIDs that need to vacuumed/analyzed on the shared memory (i.g. on
DSA). Since all autovacuum workers running on the same database can
see a consistent queue, the issue explained above won't happen and
probably it makes the implementation of prioritization of tables being
vacuumed easier which is sometimes discussed on pgsql-hackers. I guess
it might be worth to discuss including this idea.

Regards,

--
Masahiko Sawada            http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services