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Commit0dcaea5

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Introduce num_os_semaphores GUC.
The documentation for System V IPC parameters provides complicatedformulas to determine the appropriate values for SEMMNI and SEMMNS.Furthermore, these formulas have often been wrong because folksforget to update them (e.g., when adding a new auxiliary process).This commit introduces a new runtime-computed GUC namednum_os_semaphores that reports the number of semaphores needed forthe configured number of allowed connections, worker processes,etc. This new GUC allows us to simplify the formulas in thedocumentation, 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 forpreconfiguring the necessary operating system resources.Reviewed-by: Tom Lane, Sami Imseih, Andres Freund, Robert HaasDiscussion:https://postgr.es/m/20240517164452.GA1914161%40nathanxps13
1 parent8a53539 commit0dcaea5

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4 files changed

+54
-13
lines changed

4 files changed

+54
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lines changed

‎doc/src/sgml/config.sgml

Lines changed: 18 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -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>

‎doc/src/sgml/runtime.sgml

Lines changed: 19 additions & 12 deletions
Original file line numberDiff line numberDiff line change
@@ -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>
@@ -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
849-
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
858865
of allowed connections is a temporary workaround for failures,
859866
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
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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"/>),andallowed 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>

‎src/backend/storage/ipc/ipci.c

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Original file line numberDiff line numberDiff line change
@@ -372,11 +372,12 @@ InitializeShmemGUCs(void)
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Sizesize_b;
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Sizesize_mb;
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Sizehp_size;
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intnum_semas;
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376377
/*
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* Calculate the shared memory size and round up to the nearest megabyte.
378379
*/
379-
size_b=CalculateShmemSize(NULL);
380+
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,
@@ -395,4 +396,7 @@ InitializeShmemGUCs(void)
395396
SetConfigOption("shared_memory_size_in_huge_pages",buf,
396397
PGC_INTERNAL,PGC_S_DYNAMIC_DEFAULT);
397398
}
399+
400+
sprintf(buf,"%d",num_semas);
401+
SetConfigOption("num_os_semaphores",buf,PGC_INTERNAL,PGC_S_DYNAMIC_DEFAULT);
398402
}

‎src/backend/utils/misc/guc_tables.c

Lines changed: 12 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -591,6 +591,7 @@ static intsegment_size;
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staticintshared_memory_size_mb;
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staticintshared_memory_size_in_huge_pages;
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staticintwal_block_size;
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staticintnum_os_semaphores;
594595
staticbooldata_checksums;
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staticboolinteger_datetimes;
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@@ -2283,6 +2284,17 @@ struct config_int ConfigureNamesInt[] =
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NULL,NULL,NULL
22842285
},
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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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