Add huge_page_size setting for use on Linux.

This allows the huge page size to be set explicitly.  The default is 0,
meaning it will use the system default, as before.

Author: Odin Ugedal <odin@ugedal.com>
Discussion: https://postgr.es/m/20200608154639.20254-1-odin%40ugedal.com

2 files changed, 62 insertions, 20 deletions

diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index b353c616830..e0ea397ed40 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -1582,6 +1582,33 @@ include_dir 'conf.d'
       </listitem>
      </varlistentry>
 
+     <varlistentry id="guc-huge-page-size" xreflabel="huge_page_size">
+      <term><varname>huge_page_size</varname> (<type>integer</type>)
+      <indexterm>
+       <primary><varname>huge_page_size</varname> configuration parameter</primary>
+      </indexterm>
+      </term>
+      <listitem>
+       <para>
+        Controls the size of huge pages, when they are enabled with
+        <xref linkend="guc-huge-pages"/>.
+        The default is zero (<literal>0</literal>).
+        When set to <literal>0</literal>, the default huge page size on the
+        system will be used.
+       </para>
+       <para>
+        Some commonly available page sizes on modern 64 bit server architectures include:
+        <literal>2MB</literal> and <literal>1GB</literal> (Intel and AMD), <literal>16MB</literal> and
+        <literal>16GB</literal> (IBM POWER), and <literal>64kB</literal>, <literal>2MB</literal>,
+        <literal>32MB</literal> and <literal>1GB</literal> (ARM). For more information
+        about usage and support, see <xref linkend="linux-huge-pages"/>.
+       </para>
+       <para>
+        Non-default settings are currently supported only on Linux.
+       </para>
+      </listitem>
+     </varlistentry>
+
      <varlistentry id="guc-temp-buffers" xreflabel="temp_buffers">
       <term><varname>temp_buffers</varname> (<type>integer</type>)
       <indexterm>
diff --git a/doc/src/sgml/runtime.sgml b/doc/src/sgml/runtime.sgml
index 937bb2e8ac9..e09cb55efcd 100644
--- a/doc/src/sgml/runtime.sgml
+++ b/doc/src/sgml/runtime.sgml
@@ -1391,13 +1391,14 @@ export PG_OOM_ADJUST_VALUE=0
     using large values of <xref linkend="guc-shared-buffers"/>.  To use this
     feature in <productname>PostgreSQL</productname> you need a kernel
     with <varname>CONFIG_HUGETLBFS=y</varname> and
-    <varname>CONFIG_HUGETLB_PAGE=y</varname>. You will also have to adjust
-    the kernel setting <varname>vm.nr_hugepages</varname>. To estimate the
-    number of huge pages needed, start <productname>PostgreSQL</productname>
-    without huge pages enabled and check the
-    postmaster's anonymous shared memory segment size, as well as the system's
-    huge page size, using the <filename>/proc</filename> file system.  This might
-    look like:
+    <varname>CONFIG_HUGETLB_PAGE=y</varname>. You will also have to configure
+    the operating system to provide enough huge pages of the desired size.
+    To estimate the number of huge pages needed, start
+    <productname>PostgreSQL</productname> without huge pages enabled and check
+    the postmaster's anonymous shared memory segment size, as well as the
+    system's default and supported huge page sizes, using the
+    <filename>/proc</filename> and <filename>/sys</filename> file systems.
+    This might look like:
 <programlisting>
 $ <userinput>head -1 $PGDATA/postmaster.pid</userinput>
 4170
@@ -1405,27 +1406,40 @@ $ <userinput>pmap 4170 | awk '/rw-s/ &amp;&amp; /zero/ {print $2}'</userinput>
 6490428K
 $ <userinput>grep ^Hugepagesize /proc/meminfo</userinput>
 Hugepagesize:       2048 kB
+$ <userinput>ls /sys/kernel/mm/hugepages</userinput>
+hugepages-1048576kB  hugepages-2048kB
 </programlisting>
+
+     In this example the default is 2MB, but you can also explicitly request
+     either 2MB or 1GB with <xref linkend="guc-huge-page-size"/>.
+
+     Assuming <literal>2MB</literal> huge pages,
      <literal>6490428</literal> / <literal>2048</literal> gives approximately
      <literal>3169.154</literal>, so in this example we need at
-     least <literal>3170</literal> huge pages, which we can set with:
+     least <literal>3170</literal> huge pages.  A larger setting would be
+     appropriate if other programs on the machine also need huge pages.
+     We can set this with:
+<programlisting>
+# <userinput>sysctl -w vm.nr_hugepages=3170</userinput>
+</programlisting>
+     Don't forget to add this setting to <filename>/etc/sysctl.conf</filename>
+     so that it is reapplied after reboots.  For non-default huge page sizes,
+     we can instead use:
 <programlisting>
-$ <userinput>sysctl -w vm.nr_hugepages=3170</userinput>
+# <userinput>echo 3170 > /sys/kernel/mm/hugepages/hugepages-2048kB/nr_hugepages</userinput>
 </programlisting>
-    A larger setting would be appropriate if other programs on the machine
-    also need huge pages.  Don't forget to add this setting
-    to <filename>/etc/sysctl.conf</filename> so that it will be reapplied
-    after reboots.
+    It is also possible to provide these settings at boot time using
+    kernel parameters such as <literal>hugepagesz=2M hugepages=3170</literal>.
    </para>
 
    <para>
     Sometimes the kernel is not able to allocate the desired number of huge
-    pages immediately, so it might be necessary to repeat the command or to
-    reboot.  (Immediately after a reboot, most of the machine's memory
-    should be available to convert into huge pages.)  To verify the huge
-    page allocation situation, use:
+    pages immediately due to fragmentation, so it might be necessary
+    to repeat the command or to reboot.  (Immediately after a reboot, most of
+    the machine's memory should be available to convert into huge pages.)
+    To verify the huge page allocation situation for a given size, use:
 <programlisting>
-$ <userinput>grep Huge /proc/meminfo</userinput>
+$ <userinput>cat /sys/kernel/mm/hugepages/hugepages-2048kB/nr_hugepages</userinput>
 </programlisting>
    </para>
 
@@ -1438,8 +1452,9 @@ $ <userinput>grep Huge /proc/meminfo</userinput>
 
    <para>
     The default behavior for huge pages in
-    <productname>PostgreSQL</productname> is to use them when possible and
-    to fall back to normal pages when failing. To enforce the use of huge
+    <productname>PostgreSQL</productname> is to use them when possible, with
+    the system's default huge page size, and
+    to fall back to normal pages on failure. To enforce the use of huge
     pages, you can set <xref linkend="guc-huge-pages"/>
     to <literal>on</literal> in <filename>postgresql.conf</filename>.
     Note that with this setting <productname>PostgreSQL</productname> will fail to