Hi, On 26/02/14 14:34, Heikki Linnakangas wrote: > That still says "The setting is ignored on other systems". That's not quite > true: as explained later in the section, if you set huge_tlb_pages=on and > the platform doesn't support it, the server will refuse to start.
I added a sentence about it. > "Using huge TLB pages reduces overhead when using large contiguous chunks of > memory, like PostgreSQL does." Sentence added. > That's good advice, but perhaps s/calculate/estimate/. It's just an > approximation, after all. Fixed. New patch version is attached. Best regards, -- Christian Kruse http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Training & Services
diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index 4dc1277..c5c2d8b 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -1120,18 +1120,17 @@ include 'filename'
</para>
<para>
- At present, this feature is supported only on Linux. The setting
- is ignored on other systems.
+ At present, this feature is supported only on Linux. The setting is
+ ignored on other systems when set to <literal>try</literal>.
+ <productname>PostgreSQL</productname> will
+ refuse to start when set to <literal>on</literal>.
</para>
<para>
The use of huge TLB pages results in smaller page tables and
less CPU time spent on memory management, increasing performance. For
more details, see
- <ulink url="https://wiki.debian.org/Hugepages";>the Debian wiki</ulink>.
- Remember that you will need at least shared_buffers / huge page size +
- 1 huge TLB pages. So for example for a system with 6GB shared buffers
- and a hugepage size of 2kb of you will need at least 3156 huge pages.
+ <xref linkend="linux-huge-tlb-pages">.
</para>
<para>
diff --git a/doc/src/sgml/runtime.sgml b/doc/src/sgml/runtime.sgml
index bbb808f..5f9fa61 100644
--- a/doc/src/sgml/runtime.sgml
+++ b/doc/src/sgml/runtime.sgml
@@ -1307,6 +1307,57 @@ echo -1000 > /proc/self/oom_score_adj
</para>
</note>
</sect2>
+
+ <sect2 id="linux-huge-tlb-pages">
+ <title>Linux huge TLB pages</title>
+
+ <para>
+ Using huge TLB pages reduces overhead when using large contiguous chunks
+ of memory, like PostgreSQL does. To enable this feature
+ in <productname>PostgreSQL</productname> you need a kernel
+ with <varname>CONFIG_HUGETLBFS=y</varname> and
+ <varname>CONFIG_HUGETLB_PAGE=y</varname>. You also have to tune the system
+ setting <varname>vm.nr_hugepages</varname>. To estimate the number of
+ necessary huge pages start <productname>PostgreSQL</productname> without
+ huge pages enabled and check the <varname>VmPeak</varname> value from the
+ proc filesystem:
+<programlisting>
+$ <userinput>head -1 /path/to/data/directory/postmaster.pid</userinput>
+4170
+$ <userinput>grep ^VmPeak /proc/4170/status</userinput>
+VmPeak: 6490428 kB
+</programlisting>
+ <literal>6490428</literal> / <literal>2048</literal>
+ (<varname>PAGE_SIZE</varname> is <literal>2MB</literal> in this case) are
+ roughly <literal>3169.154</literal> huge pages, so you will need at
+ least <literal>3170</literal> huge pages:
+<programlisting>
+$ <userinput>sysctl -w vm.nr_hugepages=3170</userinput>
+</programlisting>
+ Sometimes the kernel is not able to allocate the desired number of huge
+ pages, so it might be necessary to repeat that command or to reboot. Don't
+ forget to add an entry to <filename>/etc/sysctl.conf</filename> to persist
+ this setting through reboots.
+ </para>
+
+ <para>
+ The default behavior for huge pages in
+ <productname>PostgreSQL</productname> is to use them when possible and
+ to fallback to normal pages when failing. To enforce the use of huge
+ pages, you can set
+ <link linkend="guc-huge-tlb-pages"><varname>huge_tlb_pages</varname></link>
+ to <literal>on</literal>. Note that in this case
+ <productname>PostgreSQL</productname> will fail to start if not enough huge
+ pages are available.
+ </para>
+
+ <para>
+ For a detailed description of the <productname>Linux</productname> huge
+ pages feature have a look
+ at <ulink url="https://www.kernel.org/doc/Documentation/vm/hugetlbpage.txt";>https://www.kernel.org/doc/Documentation/vm/hugetlbpage.txt</ulink>.
+ </para>
+
+ </sect2>
</sect1>
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