Date: Thu, 6 Jun 2024 14:21:38 -0500
From: Nathan Bossart <nathandbossart@gmail.com>
To: "Imseih (AWS), Sami" <simseih@amazon.com>
Cc: Tom Lane <tgl@sss.pgh.pa.us>, Justin Pryzby <pryzby@telsasoft.com>,
	"pgsql-hackers@postgresql.org" <pgsql-hackers@postgresql.org>
Subject: Re: problems with "Shared Memory and Semaphores" section of docs
Message-ID: <ZmIMQqOyDNLVsplz@nathan>
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Here is a rebased version of the patch for v18 that adds a runtime-computed
GUC.  As I noted earlier, there still isn't a consensus on this approach.

-- 
nathan

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	filename="v2-0001-add-semaphores_required-GUC.patch"

From 74f638f7df9c51f5ab47b282bb7107c4ba6cb5b6 Mon Sep 17 00:00:00 2001
From: Nathan Bossart <nathan@postgresql.org>
Date: Tue, 21 May 2024 14:02:22 -0500
Subject: [PATCH v2 1/1] add semaphores_required GUC

---
 doc/src/sgml/config.sgml            | 14 +++++++++++
 doc/src/sgml/runtime.sgml           | 39 ++++++++++++++++-------------
 src/backend/storage/ipc/ipci.c      |  6 ++++-
 src/backend/utils/misc/guc_tables.c | 12 +++++++++
 4 files changed, 53 insertions(+), 18 deletions(-)

diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index 698169afdb..f6afc941df 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -11215,6 +11215,20 @@ dynamic_library_path = 'C:\tools\postgresql;H:\my_project\lib;$libdir'
       </listitem>
      </varlistentry>
 
+     <varlistentry id="guc-semaphores-required" xreflabel="semaphores_required">
+      <term><varname>semaphores_required</varname> (<type>integer</type>)
+      <indexterm>
+       <primary><varname>semaphores_required</varname> configuration parameter</primary>
+      </indexterm>
+      </term>
+      <listitem>
+       <para>
+        Reports the number of semaphores that are needed for the server based
+        on the number of allowed connections, worker processes, etc.
+       </para>
+      </listitem>
+     </varlistentry>
+
      <varlistentry id="guc-ssl-library" xreflabel="ssl_library">
       <term><varname>ssl_library</varname> (<type>string</type>)
       <indexterm>
diff --git a/doc/src/sgml/runtime.sgml b/doc/src/sgml/runtime.sgml
index 2f7c618886..c387f302d7 100644
--- a/doc/src/sgml/runtime.sgml
+++ b/doc/src/sgml/runtime.sgml
@@ -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(semaphores_required / 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(semaphores_required / 16) * 17</literal> plus room for other applications</entry>
        </row>
 
        <row>
@@ -836,30 +836,38 @@ psql: error: connection to server on socket "/tmp/.s.PGSQL.5432" failed: No such
 
    <para>
     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"/>), allowed WAL sender process
-    (<xref linkend="guc-max-wal-senders"/>), and allowed background
-    process (<xref linkend="guc-max-worker-processes"/>), in sets of 16.
+    <productname>PostgreSQL</productname> uses one semaphore per allowed connection,
+    worker process, etc., 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</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 <xref linkend="guc-semaphores-required"/> 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(semaphores_required / 16)</literal>.
     Lowering the number
     of allowed connections is a temporary workaround for failures,
     which are usually confusingly worded <quote>No space
     left on device</quote>, from the function <function>semget</function>.
    </para>
 
+   <para>
+    The number of semaphores required by <productname>PostgreSQL</productname>
+    is provided by the runtime-computed parameter
+    <varname>semaphores_required</varname>, which can be determined before
+    starting the server with a <command>postgres</command> command like:
+<programlisting>
+$ <userinput>postgres -D $PGDATA -C semaphores_required</userinput>
+</programlisting>
+    The value of <varname>semaphores_required</varname> should be input into
+    the aforementioned formulas to determine appropriate values for
+    <varname>SEMMNI</varname> and <varname>SEMMNS</varname>.
+   </para>
+
    <para>
     In some cases it might also be necessary to increase
     <varname>SEMMAP</varname> to be at least on the order of
@@ -882,11 +890,8 @@ psql: error: connection to server on socket "/tmp/.s.PGSQL.5432" failed: No such
 
    <para>
     When using POSIX semaphores, the number of semaphores needed is the
-    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"/>).
+    same as for System V, that is one semaphore per allowed connection,
+    worker process, etc.
     On the platforms where this option is preferred, there is no specific
     kernel limit on the number of POSIX semaphores.
    </para>
diff --git a/src/backend/storage/ipc/ipci.c b/src/backend/storage/ipc/ipci.c
index 521ed5418c..3e030accac 100644
--- a/src/backend/storage/ipc/ipci.c
+++ b/src/backend/storage/ipc/ipci.c
@@ -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("semaphores_required", buf, PGC_INTERNAL, PGC_S_DYNAMIC_DEFAULT);
 }
diff --git a/src/backend/utils/misc/guc_tables.c b/src/backend/utils/misc/guc_tables.c
index 46c258be28..86157394d8 100644
--- a/src/backend/utils/misc/guc_tables.c
+++ b/src/backend/utils/misc/guc_tables.c
@@ -599,6 +599,7 @@ 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	semaphores_required;
 static bool data_checksums;
 static bool integer_datetimes;
 
@@ -2291,6 +2292,17 @@ struct config_int ConfigureNamesInt[] =
 		NULL, NULL, NULL
 	},
 
+	{
+		{"semaphores_required", 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
+		},
+		&semaphores_required,
+		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."),
-- 
2.39.3 (Apple Git-146)


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