<row>

<entry><varname>SEMMNI</></>

<entry>Maximum number of semaphore identifiers (i.e., sets)</>

<entry>at least <literal>ceil((max_connections + autovacuum_max_workers +4) / 16)</literal></>

<entry>at least <literal>ceil((max_connections + autovacuum_max_workers +max_worker_processes + 5) / 16)</literal></>

</row>

<row>

<entry><varname>SEMMNS</></>

<entry>Maximum number of semaphores system-wide</>

<entry><literal>ceil((max_connections + autovacuum_max_workers +4) / 16) * 17</literal> plus room for other applications</>

<entry><literal>ceil((max_connections + autovacuum_max_workers +max_worker_processes + 5) / 16) * 17</literal> plus room for other applications</>

</row>

<row>

<para>

<productname>PostgreSQL</> uses one semaphore per allowed connection

(<xref linkend="guc-max-connections">) and allowed autovacuum worker

process (<xref linkend="guc-autovacuum-max-workers">), in sets of 16.

(<xref linkend="guc-max-connections">), allowed autovacuum worker process

(<xref linkend="guc-autovacuum-max-workers">) 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

number</quote>, to detect collision with semaphore sets used by

other applications. The maximum number of semaphores in the system

is set by <varname>SEMMNS</>, which consequently must be at least

as high as <varname>max_connections</> plus

<varname>autovacuum_max_workers</>, plus one extra for each 16

<varname>autovacuum_max_workers</> plus <varname>max_worker_processes</>,

plus one extra for each 16

allowed connections plus workers (see the formula in <xref

linkend="sysvipc-parameters">). The parameter <varname>SEMMNI</>

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 +4) / 16)</>.

least <literal>ceil((max_connections + autovacuum_max_workers +max_worker_processes + 5) / 16)</>.

Lowering the number

of allowed connections is a temporary workaround for failures,

which are usually confusingly worded <quote>No space