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Lower default value of autovacuum_worker_slots in initdb as needed.

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Commit c758119e5b increased the default number of semaphores
required for autovacuum workers from 3 to 16.  Unfortunately, some
systems have very low default settings for SEMMNS, and this change
moved the minimum required for Postgres well beyond that limit (see
commit 38da053463 for more details).

With this commit, initdb will lower the default value for
autovacuum_worker_slots as needed, just like it already does for
parameters such as max_connections and shared_buffers.  We test
for (max_connections / 6) slots, which conveniently has the
following properties:

* For the initial max_connections default of 100, the default of
  autovacuum_worker_slots will be 16, which is its initial default
  value specified in the documentation and in guc_tables.c.

* For the lowest possible max_connections default of 25, the
  default of autovacuum_worker_slots will be 4, which means we only
  need one additional semaphore for autovacuum workers (as compared
  to before commit c758119e5b).  This leaves some wiggle room for
  new auxiliary workers, etc. on systems with low SEMMNS, and it
  ensures that the default number of slots will be greater than or
  equal to the default value of autovacuum_max_workers (3).

Reported-by: Tom Lane
Suggested-by: Andres Freund
Reviewed-by: Tom Lane
Discussion: https://postgr.es/m/1346002.1736198977%40sss.pgh.pa.us
This commit is contained in:
Nathan Bossart
2025-01-07 14:38:55 -06:00
parent 0e5b14410e
commit 6d01541960
2 changed files with 37 additions and 8 deletions
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doc/src/sgml/config.sgml
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@ -8639,8 +8639,9 @@ COPY postgres_log FROM '/full/path/to/logfile.csv' WITH csv;
<listitem>
<para>
Specifies the number of backend slots to reserve for autovacuum worker
processes. The default is 16. This parameter can only be set at server
start.
processes. The default is typically 16 slots, but might be less if
your kernel settings will not support it (as determined during initdb).
This parameter can only be set at server start.
</para>
<para>
When changing this value, consider also adjusting