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Fix documentation for System V semaphores. 

The formulas for SEMMNI and SEMMNS do not include the archiver
process, which was converted to an auxiliary process in v14, and
the WAL summarizer process, which was introduced in v17.  This
commit corrects these formulas and adds a missing reference to
max_wal_senders nearby.  Since this section of the documentation
tends to be incorrect quite often, we should likely give up on
documenting the exact formulas in favor of something less fragile,
but that is left as a future exercise.

Reported-by: Sami Imseih
Reviewed-by: Sami Imseih
Discussion: https://postgr.es/m/20240517164452.GA1914161%40nathanxps13
Backpatch-through: 12
This commit is contained in:
Nathan Bossart 2024-06-03 12:10:43 -05:00
parent 0795a35d13
commit d0c11ea298
1 changed files with 5 additions and 4 deletions
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9
doc/src/sgml/runtime.sgml
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@ -781,13 +781,13 @@ psql: error: connection to server on socket "/tmp/.s.PGSQL.5432" failed: No such
<row>
<entry><varname>SEMMNI</varname></entry>
<entry>Maximum number of semaphore identifiers (i.e., sets)</entry>
<entry>at least <literal>ceil((max_connections + autovacuum_max_workers + max_wal_senders + max_worker_processes + 5) / 16)</literal> plus room for other applications</entry>
<entry>at least <literal>ceil((max_connections + autovacuum_max_workers + max_wal_senders + max_worker_processes + 6) / 16)</literal> plus room for other applications</entry>
</row>
<row>
<entry><varname>SEMMNS</varname></entry>
<entry>Maximum number of semaphores system-wide</entry>
<entry><literal>ceil((max_connections + autovacuum_max_workers + max_wal_senders + max_worker_processes + 5) / 16) * 17</literal> plus room for other applications</entry>
<entry><literal>ceil((max_connections + autovacuum_max_workers + max_wal_senders + max_worker_processes + 6) / 16) * 17</literal> plus room for other applications</entry>
</row>
<row>
@ -838,7 +838,8 @@ psql: error: connection to server on socket "/tmp/.s.PGSQL.5432" failed: No such
When using System V semaphores,
<productname>PostgreSQL</productname> uses one semaphore per allowed connection
(<xref linkend="guc-max-connections"/>), allowed autovacuum worker process
(<xref linkend="guc-autovacuum-max-workers"/>) and allowed background
(<xref linkend="guc-autovacuum-max-workers"/>), allowed WAL sender process
(<xref linkend="guc-max-wal-senders"/>), and allowed background
process (<xref linkend="guc-max-worker-processes"/>), in sets of 16.
Each such set will
also contain a 17th semaphore which contains a <quote>magic
@ -852,7 +853,7 @@ psql: error: connection to server on socket "/tmp/.s.PGSQL.5432" failed: No such
linkend="sysvipc-parameters"/>). The parameter <varname>SEMMNI</varname>
determines the limit on the number of semaphore sets that can
exist on the system at one time. Hence this parameter must be at
least <literal>ceil((max_connections + autovacuum_max_workers + max_wal_senders + max_worker_processes + 5) / 16)</literal>.
least <literal>ceil((max_connections + autovacuum_max_workers + max_wal_senders + max_worker_processes + 6) / 16)</literal>.
Lowering the number
of allowed connections is a temporary workaround for failures,
which are usually confusingly worded <quote>No space