Introduce num_os_semaphores GUC.
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
parent
8a53539bd6
commit
0dcaea5690
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@ -11145,6 +11145,24 @@ dynamic_library_path = 'C:\tools\postgresql;H:\my_project\lib;$libdir'
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</listitem>
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</varlistentry>
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<varlistentry id="guc-num-os-semaphores" xreflabel="num_os_semaphores">
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<term><varname>num_os_semaphores</varname> (<type>integer</type>)
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<indexterm>
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<primary><varname>num_os_semaphores</varname> configuration parameter</primary>
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</indexterm>
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</term>
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<listitem>
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<para>
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Reports the number of semaphores that are needed for the server based
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on the configured number of allowed connections
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(<xref linkend="guc-max-connections"/>), allowed autovacuum worker
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processes (<xref linkend="guc-autovacuum-max-workers"/>), allowed WAL
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sender processes (<xref linkend="guc-max-wal-senders"/>), allowed
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background processes (<xref linkend="guc-max-worker-processes"/>), etc.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry id="guc-segment-size" xreflabel="segment_size">
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<term><varname>segment_size</varname> (<type>integer</type>)
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<indexterm>
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@ -781,13 +781,13 @@ psql: error: connection to server on socket "/tmp/.s.PGSQL.5432" failed: No such
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<row>
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<entry><varname>SEMMNI</varname></entry>
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<entry>Maximum number of semaphore identifiers (i.e., sets)</entry>
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<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>
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<entry>at least <literal>ceil(num_os_semaphores / 16)</literal> plus room for other applications</entry>
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</row>
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<row>
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<entry><varname>SEMMNS</varname></entry>
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<entry>Maximum number of semaphores system-wide</entry>
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<entry><literal>ceil((max_connections + autovacuum_max_workers + max_wal_senders + max_worker_processes + 7) / 16) * 17</literal> plus room for other applications</entry>
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<entry><literal>ceil(num_os_semaphores / 16) * 17</literal> plus room for other applications</entry>
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</row>
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<row>
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@ -839,21 +839,28 @@ psql: error: connection to server on socket "/tmp/.s.PGSQL.5432" failed: No such
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<productname>PostgreSQL</productname> uses one semaphore per allowed connection
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(<xref linkend="guc-max-connections"/>), allowed autovacuum worker process
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(<xref linkend="guc-autovacuum-max-workers"/>), allowed WAL sender process
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(<xref linkend="guc-max-wal-senders"/>), and allowed background
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process (<xref linkend="guc-max-worker-processes"/>), in sets of 16.
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Each such set will
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(<xref linkend="guc-max-wal-senders"/>), allowed background
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process (<xref linkend="guc-max-worker-processes"/>), etc., in sets of 16.
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The runtime-computed parameter <xref linkend="guc-num-os-semaphores"/>
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reports the number of semaphores required. This parameter can be viewed
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before starting the server with a <command>postgres</command> command like:
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<programlisting>
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$ <userinput>postgres -D $PGDATA -C num_os_semaphores</userinput>
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</programlisting>
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</para>
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<para>
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Each set of 16 semaphores will
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also contain a 17th semaphore which contains a <quote>magic
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number</quote>, to detect collision with semaphore sets used by
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other applications. The maximum number of semaphores in the system
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is set by <varname>SEMMNS</varname>, which consequently must be at least
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as high as <varname>max_connections</varname> plus
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<varname>autovacuum_max_workers</varname> plus <varname>max_wal_senders</varname>,
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plus <varname>max_worker_processes</varname>, plus one extra for each 16
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allowed connections plus workers (see the formula in <xref
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as high as <literal>num_os_semaphores</literal> plus one extra for
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each set of 16 required semaphores (see the formula in <xref
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linkend="sysvipc-parameters"/>). The parameter <varname>SEMMNI</varname>
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determines the limit on the number of semaphore sets that can
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exist on the system at one time. Hence this parameter must be at
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least <literal>ceil((max_connections + autovacuum_max_workers + max_wal_senders + max_worker_processes + 7) / 16)</literal>.
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least <literal>ceil(num_os_semaphores / 16)</literal>.
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Lowering the number
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of allowed connections is a temporary workaround for failures,
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which are usually confusingly worded <quote>No space
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@ -885,8 +892,8 @@ psql: error: connection to server on socket "/tmp/.s.PGSQL.5432" failed: No such
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same as for System V, that is one semaphore per allowed connection
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(<xref linkend="guc-max-connections"/>), allowed autovacuum worker process
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(<xref linkend="guc-autovacuum-max-workers"/>), allowed WAL sender process
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(<xref linkend="guc-max-wal-senders"/>), and allowed background
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process (<xref linkend="guc-max-worker-processes"/>).
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(<xref linkend="guc-max-wal-senders"/>), allowed background
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process (<xref linkend="guc-max-worker-processes"/>), etc.
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On the platforms where this option is preferred, there is no specific
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kernel limit on the number of POSIX semaphores.
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</para>
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@ -372,11 +372,12 @@ InitializeShmemGUCs(void)
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Size size_b;
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Size size_mb;
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Size hp_size;
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int num_semas;
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/*
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* Calculate the shared memory size and round up to the nearest megabyte.
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*/
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size_b = CalculateShmemSize(NULL);
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size_b = CalculateShmemSize(&num_semas);
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size_mb = add_size(size_b, (1024 * 1024) - 1) / (1024 * 1024);
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sprintf(buf, "%zu", size_mb);
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SetConfigOption("shared_memory_size", buf,
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@ -395,4 +396,7 @@ InitializeShmemGUCs(void)
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SetConfigOption("shared_memory_size_in_huge_pages", buf,
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PGC_INTERNAL, PGC_S_DYNAMIC_DEFAULT);
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}
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sprintf(buf, "%d", num_semas);
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SetConfigOption("num_os_semaphores", buf, PGC_INTERNAL, PGC_S_DYNAMIC_DEFAULT);
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}
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@ -591,6 +591,7 @@ static int segment_size;
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static int shared_memory_size_mb;
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static int shared_memory_size_in_huge_pages;
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static int wal_block_size;
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static int num_os_semaphores;
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static bool data_checksums;
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static bool integer_datetimes;
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@ -2283,6 +2284,17 @@ struct config_int ConfigureNamesInt[] =
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NULL, NULL, NULL
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},
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{
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{"num_os_semaphores", PGC_INTERNAL, PRESET_OPTIONS,
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gettext_noop("Shows the number of semaphores required for the server."),
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NULL,
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GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE | GUC_RUNTIME_COMPUTED
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},
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&num_os_semaphores,
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0, 0, INT_MAX,
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NULL, NULL, NULL
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},
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{
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{"commit_timestamp_buffers", PGC_POSTMASTER, RESOURCES_MEM,
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gettext_noop("Sets the size of the dedicated buffer pool used for the commit timestamp cache."),
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