Add shared_memory_type GUC.

Since 9.3 we have used anonymous shared mmap for our main shared memory
region, except in EXEC_BACKEND builds.  Provide a GUC so that users
can opt for System V shared memory once again, like in 9.2 and earlier.

A later patch proposes to add huge/large page support for AIX, which
requires System V shared memory and provided the motivation to revive
this possibility.  It may also be useful on some BSDs.

Author: Andres Freund (revived and documented by Thomas Munro)
Discussion: https://postgr.es/m/HE1PR0202MB28126DB4E0B6621CC6A1A91286D90%40HE1PR0202MB2812.eurprd02.prod.outlook.com
Discussion: https://postgr.es/m/2AE143D2-87D3-4AD1-AC78-CE2258230C05%40FreeBSD.org

doc/src/sgml/config.sgml

@@ -1694,6 +1694,31 @@ include_dir 'conf.d'
       </listitem>
      </varlistentry>
 
+     <varlistentry id="guc-shared-memory-type" xreflabel="shared_memory_type">
+      <term><varname>shared_memory_type</varname> (<type>enum</type>)
+      <indexterm>
+       <primary><varname>shared_memory_type</varname> configuration parameter</primary>
+      </indexterm>
+      </term>
+      <listitem>
+       <para>
+        Specifies the shared memory implementation that the server
+        should use for the main shared memory region that holds
+        <productname>PostgreSQL</productname>'s shared buffers and other
+        shared data.  Possible values are <literal>mmap</literal> (for
+        anonymous shared memory allocated using <function>mmap</function>),
+        <literal>sysv</literal> (for System V shared memory allocated via
+        <function>shmget</function>) and <literal>windows</literal> (for Windows
+        shared memory).  Not all values are supported on all platforms; the
+        first supported option is the default for that platform.  The use of
+        the <literal>sysv</literal> option, which is not the default on any
+        platform, is generally discouraged because it typically requires
+        non-default kernel settings to allow for large allocations (see <xref
+        linkend="sysvipc"/>).
+       </para>
+      </listitem>
+     </varlistentry>
+
      <varlistentry id="guc-dynamic-shared-memory-type" xreflabel="dynamic_shared_memory_type">
       <term><varname>dynamic_shared_memory_type</varname> (<type>enum</type>)
       <indexterm>

doc/src/sgml/runtime.sgml

@@ -638,9 +638,12 @@ psql: could not connect to server: No such file or directory
    </para>
 
    <para>
-    Upon starting the server, <productname>PostgreSQL</productname> normally allocates
+    By default, <productname>PostgreSQL</productname> allocates
     a very small amount of System V shared memory, as well as a much larger
-    amount of POSIX (<function>mmap</function>) shared memory.
+    amount of anonymous <function>mmap</function> shared memory.
+    Alternatively, a single large System V shared memory region can be used
+    (see <xref linkend="guc-shared-memory-type"/>).
+    
     In addition a significant number of semaphores, which can be either
     System V or POSIX style, are created at server startup.  Currently,
     POSIX semaphores are used on Linux and FreeBSD systems while other
@@ -752,8 +755,10 @@ psql: could not connect to server: No such file or directory
     <productname>PostgreSQL</productname> requires a few bytes of System V shared memory
     (typically 48 bytes, on 64-bit platforms) for each copy of the server.
     On most modern operating systems, this amount can easily be allocated.
-    However, if you are running many copies of the server, or if other
-    applications are also using System V shared memory, it may be necessary to
+    However, if you are running many copies of the server or you explicitly
+    configure the server to use large amounts of System V shared memory (see
+    <xref linkend="guc-shared-memory-type"/> and <xref
+    linkend="guc-dynamic-shared-memory-type"/>), it may be necessary to
     increase <varname>SHMALL</varname>, which is the total amount of System V shared
     memory system-wide.  Note that <varname>SHMALL</varname> is measured in pages
     rather than bytes on many systems.
@@ -879,7 +884,7 @@ kern.ipc.semmns=512
        </para>
 
        <para>
-        You might also want to configure your kernel to lock shared
+        You might also want to configure your kernel to lock System V shared
         memory into RAM and prevent it from being paged out to swap.
         This can be accomplished using the <command>sysctl</command>
         setting <literal>kern.ipc.shm_use_phys</literal>.
@@ -928,7 +933,7 @@ kern.ipc.semmns=512
        </para>
 
        <para>
-        You might also want to configure your kernel to lock shared
+        You might also want to configure your kernel to lock System V shared
         memory into RAM and prevent it from being paged out to swap.
         This can be accomplished using the <command>sysctl</command>
         setting <literal>kern.ipc.shm_use_phys</literal>.