Re: autovac issue with large number of tables

On Mon, 27 Jul 2020 at 06:43, Nasby, Jim <nasbyj(at)amazon(dot)com> wrote:
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> A database with a very large number of tables eligible for autovacuum can result in autovacuum workers “stuck” in a tight loop of table_recheck_autovac() constantly reporting nothing to do on the table. This is because a database with a very large number of tables means it takes a while to search the statistics hash to verify that the table still needs to be processed[1]. If a worker spends some time processing a table, when it’s done it can spend a significant amount of time rechecking each table that it identified at launch (I’ve seen a worker in this state for over an hour). A simple work-around in this scenario is to kill the worker; the launcher will quickly fire up a new worker on the same database, and that worker will build a new list of tables.
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> That’s not a complete solution though… if the database contains a large number of very small tables you can end up in a state where 1 or 2 workers is busy chugging through those small tables so quickly than any additional workers spend all their 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.
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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