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<Chapter Id="runtime">

</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>