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Update shared memory configuration information for Linux

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In particular, the assertion that shmall is sufficiently sized by default
is slowly becoming untrue.
This commit is contained in:
Peter Eisentraut
2010-03-13 11:00:19 +00:00
parent 5473df9eb7
commit 8b2ae44dc8
1 changed files with 36 additions and 25 deletions
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61
doc/src/sgml/runtime.sgml
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@ -1,4 +1,4 @@
<!-- $PostgreSQL: pgsql/doc/src/sgml/runtime.sgml,v 1.431 2010/02/03 17:25:06 momjian Exp $ --> <!-- $PostgreSQL: pgsql/doc/src/sgml/runtime.sgml,v 1.432 2010/03/13 11:00:19 petere Exp $ -->
<chapter Id="runtime"> <chapter Id="runtime">
<title>Server Setup and Operation</title> <title>Server Setup and Operation</title>
@ -522,9 +522,10 @@ psql: could not connect to server: No such file or directory
<quote><systemitem class="osname">System V</> <quote><systemitem class="osname">System V</>
<acronym>IPC</></quote> (together with message queues, which are not <acronym>IPC</></quote> (together with message queues, which are not
relevant for <productname>PostgreSQL</>). Almost all modern relevant for <productname>PostgreSQL</>). Almost all modern
operating systems provide these features, but not all of them have operating systems provide these features, but many of them don't have
them turned on or sufficiently sized by default, especially systems them turned on or sufficiently sized by default, especially as
with a BSD heritage. (On <systemitem class="osname">Windows</>, available RAM and the demands of database applications grow.
(On <systemitem class="osname">Windows</>,
<productname>PostgreSQL</> provides its own replacement <productname>PostgreSQL</> provides its own replacement
implementation of these facilities, so most of this section implementation of these facilities, so most of this section
can be disregarded.) can be disregarded.)
@ -535,6 +536,7 @@ psql: could not connect to server: No such file or directory
<errorname>Illegal system call</> error upon server start. In <errorname>Illegal system call</> error upon server start. In
that case there is no alternative but to reconfigure your that case there is no alternative but to reconfigure your
kernel. <productname>PostgreSQL</> won't work without them. kernel. <productname>PostgreSQL</> won't work without them.
This situation is rare, however, among modern operating systems.
</para> </para>
<para> <para>
@ -546,8 +548,6 @@ psql: could not connect to server: No such file or directory
parameters are named consistently across different systems; <xref parameters are named consistently across different systems; <xref
linkend="sysvipc-parameters"> gives an overview. The methods to set linkend="sysvipc-parameters"> gives an overview. The methods to set
them, however, vary. Suggestions for some platforms are given below. them, however, vary. Suggestions for some platforms are given below.
Be warned that it is often necessary to reboot your machine, and
possibly even recompile the kernel, to change these settings.
</para> </para>
@ -643,15 +643,15 @@ psql: could not connect to server: No such file or directory
avoid the failure. While it is possible to get avoid the failure. While it is possible to get
<productname>PostgreSQL</> to run with <varname>SHMMAX</> as small as <productname>PostgreSQL</> to run with <varname>SHMMAX</> as small as
2 MB, you need considerably more for acceptable performance. Desirable 2 MB, you need considerably more for acceptable performance. Desirable
settings are in the tens to hundreds of megabytes. settings are in the hundreds of megabytes to a few gigabytes.
</para> </para>
<para> <para>
Some systems also have a limit on the total amount of shared memory in Some systems also have a limit on the total amount of shared memory in
the system (<varname>SHMALL</>). Make sure this is large enough the system (<varname>SHMALL</>). Make sure this is large enough
for <productname>PostgreSQL</> plus any other applications that for <productname>PostgreSQL</> plus any other applications that
are using shared memory segments. (Caution: <varname>SHMALL</> are using shared memory segments. Note that <varname>SHMALL</>
is measured in pages rather than bytes on many systems.) is measured in pages rather than bytes on many systems.
</para> </para>
<para> <para>
@ -898,30 +898,41 @@ options SEMMAP=256
<listitem> <listitem>
<para> <para>
The default maximum segment size is 32 MB, which is only adequate The default maximum segment size is 32 MB, which is only adequate
for small <productname>PostgreSQL</productname> installations. for very small <productname>PostgreSQL</productname>
However, the remaining installations. The default maximum total size is 2097152
defaults are quite generously sized, and usually do not require pages. A page is almost always 4096 bytes except in unusual
changes. The maximum shared memory segment size can be changed via the kernel configurations with <quote>huge pages</quote>
<command>sysctl</command> interface. For example, to allow 128 MB, (use <literal>getconf PAGE_SIZE</literal> to verify). That
and explicitly set the maximum total shared memory size to 2097152 makes a default limit of 8 GB, which is often enough, but not
pages (the default): always.
<screen>
<prompt>$</prompt> <userinput>sysctl -w kernel.shmmax=134217728</userinput>
<prompt>$</prompt> <userinput>sysctl -w kernel.shmall=2097152</userinput>
</screen>
In addition these settings can be preserved between reboots in
the file <filename>/etc/sysctl.conf</filename>.
</para> </para>
<para> <para>
Older distributions might not have the <command>sysctl</command> program, The shared memory size settings can be changed via the
<command>sysctl</command> interface. For example, to allow 16 GB:
<screen>
<prompt>$</prompt> <userinput>sysctl -w kernel.shmmax=17179869184</userinput>
<prompt>$</prompt> <userinput>sysctl -w kernel.shmall=4194304</userinput>
</screen>
In addition these settings can be preserved between reboots in
the file <filename>/etc/sysctl.conf</filename>. Doing that is
highly recommended.
</para>
<para>
Ancient distributions might not have the <command>sysctl</command> program,
but equivalent changes can be made by manipulating the but equivalent changes can be made by manipulating the
<filename>/proc</filename> file system: <filename>/proc</filename> file system:
<screen> <screen>
<prompt>$</prompt> <userinput>echo 134217728 &gt;/proc/sys/kernel/shmmax</userinput> <prompt>$</prompt> <userinput>echo 17179869184 &gt;/proc/sys/kernel/shmmax</userinput>
<prompt>$</prompt> <userinput>echo 2097152 &gt;/proc/sys/kernel/shmall</userinput> <prompt>$</prompt> <userinput>echo 4194304 &gt;/proc/sys/kernel/shmall</userinput>
</screen> </screen>
</para> </para>
<para>
The remaining defaults are quite generously sized, and usually
do not require changes.
</para>
</listitem> </listitem>
</varlistentry> </varlistentry>