database/postgres
1
0
Fork 0
mirror of https://github.com/postgres/postgres.git synced 2025-06-26 12:21:12 +03:00
Code Activity

Introduce num_os_semaphores GUC.

Browse Source 
The documentation for System V IPC parameters provides complicated
formulas to determine the appropriate values for SEMMNI and SEMMNS.
Furthermore, these formulas have often been wrong because folks
forget to update them (e.g., when adding a new auxiliary process).

This commit introduces a new runtime-computed GUC named
num_os_semaphores that reports the number of semaphores needed for
the configured number of allowed connections, worker processes,
etc.  This new GUC allows us to simplify the formulas in the
documentation, and it should help prevent future inaccuracies.
Like the other runtime-computed GUCs, users can view it with
"postgres -C" before starting the server, which is useful for
preconfiguring the necessary operating system resources.

Reviewed-by: Tom Lane, Sami Imseih, Andres Freund, Robert Haas
Discussion: https://postgr.es/m/20240517164452.GA1914161%40nathanxps13
This commit is contained in:
Nathan Bossart
2024-07-26 15:28:55 -05:00
parent 8a53539bd6
commit 0dcaea5690
4 changed files with 54 additions and 13 deletions
Download Patch File Download Diff File Expand all files Collapse all files

18
doc/src/sgml/config.sgml
View File

@ -11145,6 +11145,24 @@ dynamic_library_path = 'C:\tools\postgresql;H:\my_project\lib;$libdir'
</listitem>
</varlistentry>
<varlistentry id="guc-num-os-semaphores" xreflabel="num_os_semaphores">
<term><varname>num_os_semaphores</varname> (<type>integer</type>)
<indexterm>
<primary><varname>num_os_semaphores</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Reports the number of semaphores that are needed for the server based
on the configured number of allowed connections
(<xref linkend="guc-max-connections"/>), allowed autovacuum worker
processes (<xref linkend="guc-autovacuum-max-workers"/>), allowed WAL
sender processes (<xref linkend="guc-max-wal-senders"/>), allowed
background processes (<xref linkend="guc-max-worker-processes"/>), etc.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-segment-size" xreflabel="segment_size">
<term><varname>segment_size</varname> (<type>integer</type>)
<indexterm>

31
doc/src/sgml/runtime.sgml
View File

@ -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 + 7) / 16)</literal> plus room for other applications</entry>
<entry>at least <literal>ceil(num_os_semaphores / 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 + 7) / 16) * 17</literal> plus room for other applications</entry>
<entry><literal>ceil(num_os_semaphores / 16) * 17</literal> plus room for other applications</entry>
</row>
<row>
@ -839,21 +839,28 @@ psql: error: connection to server on socket "/tmp/.s.PGSQL.5432" failed: No such
<productname>PostgreSQL</productname> uses one semaphore per allowed connection
(<xref linkend="guc-max-connections"/>), allowed autovacuum worker process
(<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
(<xref linkend="guc-max-wal-senders"/>), allowed background
process (<xref linkend="guc-max-worker-processes"/>), etc., in sets of 16.
The runtime-computed parameter <xref linkend="guc-num-os-semaphores"/>
reports the number of semaphores required. This parameter can be viewed
before starting the server with a <command>postgres</command> command like:
<programlisting>
$ <userinput>postgres -D $PGDATA -C num_os_semaphores</userinput>
</programlisting>
</para>
<para>
Each set of 16 semaphores 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</varname>, which consequently must be at least
as high as <varname>max_connections</varname> plus
<varname>autovacuum_max_workers</varname> plus <varname>max_wal_senders</varname>,
plus <varname>max_worker_processes</varname>, plus one extra for each 16
allowed connections plus workers (see the formula in <xref
as high as <literal>num_os_semaphores</literal> plus one extra for
each set of 16 required semaphores (see the formula in <xref
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 + 7) / 16)</literal>.
least <literal>ceil(num_os_semaphores / 16)</literal>.
Lowering the number
of allowed connections is a temporary workaround for failures,
which are usually confusingly worded <quote>No space
@ -885,8 +892,8 @@ psql: error: connection to server on socket "/tmp/.s.PGSQL.5432" failed: No such
same as for System V, that is one semaphore per allowed connection
(<xref linkend="guc-max-connections"/>), allowed autovacuum worker process
(<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"/>).
(<xref linkend="guc-max-wal-senders"/>), allowed background
process (<xref linkend="guc-max-worker-processes"/>), etc.
On the platforms where this option is preferred, there is no specific
kernel limit on the number of POSIX semaphores.
</para>

6
src/backend/storage/ipc/ipci.c
View File

@ -372,11 +372,12 @@ InitializeShmemGUCs(void)
Size size_b;
Size size_mb;
Size hp_size;
int num_semas;
/*
* Calculate the shared memory size and round up to the nearest megabyte.
*/
size_b = CalculateShmemSize(NULL);
size_b = CalculateShmemSize(&num_semas);
size_mb = add_size(size_b, (1024 * 1024) - 1) / (1024 * 1024);
sprintf(buf, "%zu", size_mb);
SetConfigOption("shared_memory_size", buf,
@ -395,4 +396,7 @@ InitializeShmemGUCs(void)
SetConfigOption("shared_memory_size_in_huge_pages", buf,
PGC_INTERNAL, PGC_S_DYNAMIC_DEFAULT);
}
sprintf(buf, "%d", num_semas);
SetConfigOption("num_os_semaphores", buf, PGC_INTERNAL, PGC_S_DYNAMIC_DEFAULT);
}

12
src/backend/utils/misc/guc_tables.c
View File

@ -591,6 +591,7 @@ static int segment_size;
static int shared_memory_size_mb;
static int shared_memory_size_in_huge_pages;
static int wal_block_size;
static int num_os_semaphores;
static bool data_checksums;
static bool integer_datetimes;
@ -2283,6 +2284,17 @@ struct config_int ConfigureNamesInt[] =
NULL, NULL, NULL
},
{
{"num_os_semaphores", PGC_INTERNAL, PRESET_OPTIONS,
gettext_noop("Shows the number of semaphores required for the server."),
NULL,
GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE | GUC_RUNTIME_COMPUTED
},
&num_os_semaphores,
0, 0, INT_MAX,
NULL, NULL, NULL
},
{
{"commit_timestamp_buffers", PGC_POSTMASTER, RESOURCES_MEM,
gettext_noop("Sets the size of the dedicated buffer pool used for the commit timestamp cache."),