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Update information about Linux's overcommit memory behavior.

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Andrew Dunstan
This commit is contained in:
Bruce Momjian 2003-09-05 02:54:09 +00:00
parent 5b49f9025a
commit b0ba70df83
1 changed files with 53 additions and 6 deletions
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doc/src/sgml/runtime.sgml
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<!--
$Header: /cvsroot/pgsql/doc/src/sgml/runtime.sgml,v 1.201 2003/08/31 17:32:19 petere Exp $
$Header: /cvsroot/pgsql/doc/src/sgml/runtime.sgml,v 1.202 2003/09/05 02:54:09 momjian Exp $
-->
<Chapter Id="runtime">
@ -3081,14 +3081,61 @@ kernel.shmmax = 134217728
</para>
<para>
Linux has poor default memory overcommit behavior. Rather than
failing if it can not reserve enough memory, it returns success,
but later fails when the memory can't be mapped and terminates
the application with <literal>kill -9</>. To prevent unpredictable
process termination, use:
Linux kernel version 2.4.* has poor default memory overcommit
behavior, which can result in the postmaster being killed by the
kernel due to memory demands by another process if the system
runs out of memory.
</para>
<para>
The symptom of this occuring is a kernel message looking like
this (consult your system documentation and configuration on
where to look for such a message):
<programlisting>
Out of Memory: Killed process 12345 (postmaster).
</programlisting>
</para>
<para>
To avoid this situation, run <productname>PostgreSQL</productname>
on a machine where you
can be sure that other processes will not run the machine out
of memory. If your kernel supports strict and/or paranoid modes
of overcommit handling, you can also relieve this problem by
altering the system's default behaviour. This can be determined
by examining the function <function>vm_enough_memory</>
in the file <filename>mm/mmap.c</>in the kernel source.
If this file reveals that strict and/or paranoid modes are
supported by your kernel, turn one of these modes on by using
<programlisting>
sysctl -w vm.overcommit_memory=2
</programlisting>
for strict mode or
<programlisting>
sysctl -w vm.overcommit_memory=3
</programlisting>
for paranoid mode, or placing an equivalent entry in
<filename>/etc/sysctl.conf</>.
</para>
<note>
<para>
Warning: using these settings in a kernel which does not support
these modes will almost certainly increase the danger of the
kernel killing the postmaster, rather than reducing it.
If in any doubt, consult a kernel expert or your kernel vendor.
</para>
</note>
<para>
These modes are expected to be supported in all 2.6 and later
kernels. Some vendor 2.4 kernels may also support these modes.
However, it is known that some vendor documents suggest that
they support them while examination of the kernel source reveals
that they do not.
</para>
<para>
Note, you will need enough swap space to cover all your memory needs.
</para>
</listitem>