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Some editorializing on the Linux memory overcommit docs, from Neil Conway.
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<!--
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$Header: /cvsroot/pgsql/doc/src/sgml/runtime.sgml,v 1.218 2003/11/14 15:43:22 tgl Exp $
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$Header: /cvsroot/pgsql/doc/src/sgml/runtime.sgml,v 1.219 2003/11/16 03:38:34 tgl Exp $
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-->
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<Chapter Id="runtime">
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@ -1294,7 +1294,7 @@ SET ENABLE_SEQSCAN TO OFF;
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<para>
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Unfortunately, there is no well-defined method for determining
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ideal values for the family of <quote>cost</quote> variables that
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below. You are encouraged to experiment and share
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appear below. You are encouraged to experiment and share
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your findings.
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</para>
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</note>
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@ -3267,35 +3267,36 @@ default:\
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<title>Linux Memory Overcommit</title>
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<para>
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Linux kernels of version 2.4.* have a poor default memory
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overcommit behavior, which can result in the PostgreSQL server
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(<filename>postmaster</filename> process) being killed by the
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kernel if the memory demands of another process cause the system
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to run out of memory.
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In Linux 2.4, the default virtual memory behavior is not
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optimal for <productname>PostgreSQL</productname>. Because of the
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way that the kernel implements memory overcommit, the kernel may
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terminate the <productname>PostgreSQL</productname> server (the
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<filename>postmaster</filename> process) if the memory demands of
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another process cause the system to run out of virtual memory.
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</para>
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<para>
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If this happens, you will see a kernel message looking like this
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(consult your system documentation and configuration on where to
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look for such a message):
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If this happens, you will see a kernel message that looks like
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this (consult your system documentation and configuration on where
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to look for such a message):
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<programlisting>
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Out of Memory: Killed process 12345 (postmaster).
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</programlisting>
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And, of course, you will find that your database server has
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disappeared.
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This indicates that the <filename>postmaster</filename> process
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has been terminated due to memory pressure;
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<productname>PostgreSQL</productname> will need to be restarted.
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</para>
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<para>
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To avoid this situation, run <productname>PostgreSQL</productname>
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on a machine where you can be sure that other processes will not
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run the machine out of memory. If your kernel supports the strict
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and/or paranoid modes of overcommit handling, you can also relieve
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this problem by altering the system's default behaviour. This can
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be determined by examining the function
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<function>vm_enough_memory</> in the file <filename>mm/mmap.c</>
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in the kernel source. If this file reveals that the strict and/or
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paranoid modes are supported by your kernel, turn one of these
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modes on by using
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and/or paranoid modes of overcommit handling, you can also relieve this
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problem by altering the system's default behaviour. This can be
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determined by examining the function <function>vm_enough_memory</>
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in the file <filename>mm/mmap.c</> in the kernel source. If this
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file reveals that the strict or paranoid modes are supported by
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your kernel, turn one of these modes on by using
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<programlisting>
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sysctl -w vm.overcommit_memory=2
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</programlisting>
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@ -3304,12 +3305,16 @@ sysctl -w vm.overcommit_memory=2
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sysctl -w vm.overcommit_memory=3
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</programlisting>
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for paranoid mode, or placing an equivalent entry in
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<filename>/etc/sysctl.conf</>.
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<filename>/etc/sysctl.conf</>. For more information on memory
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overcommit handling and the meaning of these kernel settings,
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consult the file
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<filename>Documentation/vm/overcommit-accounting</filename> in a
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recent version of the source code for the Linux kernel.
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</para>
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<caution>
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<para>
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Using these settings in a kernel which does not support these
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Using these settings with a kernel which does not support these
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modes will almost certainly increase the danger of the kernel
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killing the database server, rather than reducing it. If in any
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doubt, consult a kernel expert or your kernel vendor.
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@ -3317,15 +3322,12 @@ sysctl -w vm.overcommit_memory=3
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</caution>
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<para>
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These modes are expected to be supported in all 2.6 and later
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kernels. Some vendor 2.4 kernels may also support these modes. It
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is, however, known that some vendor documents suggest that they
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support them while examination of the kernel source reveals that
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they do not.
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</para>
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<para>
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Note, you will need enough swap space to cover all your memory needs.
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The method described above for changing the kernel's overcommit
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behavior should work with Linux kernel versions 2.6 and later. In addition,
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some versions of the 2.4 kernel distributed by some vendors may
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also support this functionality. However, it is known that some
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vendor documents suggest that they support them while examination
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of the kernel source reveals that they do not.
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</para>
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</sect2>
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</sect1>
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