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<chapter Id="runtime">

<indexterm><primary>Linux</><secondary>IPC configuration</></>

<listitem>

<para>

The default shared memory limit (both

<varname>SHMMAX</varname> and <varname>SHMALL</varname>) is 32

MB in 2.2 kernels, but it can be changed in the

<filename>proc</filename> file system (without reboot). For

example, to allow 128 MB:

The default settings are only suitable for small installations

(the default max segment size is 32 MB). However the remaining

defaults are quite generously sized, and usually do not require

changes. The max segment size can be changed via the

<command>sysctl</command> interface. For example, to allow 128 MB,

and explicitly set the maximum total shared memory size to 2097152

pages (the default):

<screen>

<prompt>$</prompt> <userinput>echo 134217728 &gt;/proc/sys/kernel/shmall</userinput>

<prompt>$</prompt> <userinput>echo 134217728 &gt;/proc/sys/kernel/shmmax</userinput>

<prompt>$</prompt> <userinput>systcl -wkernel.shmmax=134217728</userinput>

<prompt>$</prompt> <userinput>systcl -wkernel.shmall=2097152</userinput>

</screen>

You could put these commands into a script run at boot-time.

In addition these settings can be saved between reboots in

<filename>/etc/sysctl.conf.

</para>

<para>

Alternatively, you can use <command>sysctl</command>, if

available, to control these parameters. Look for a file

called <filename>/etc/sysctl.conf</filename> and add lines

like the following to it:

<programlisting>

kernel.shmall = 134217728

kernel.shmmax = 134217728

</programlisting>

This file is usually processed at boot time, but

<command>sysctl</command> can also be called

explicitly later.

</para>

<para>

Other parameters are sufficiently sized for any application. If

you want to see for yourself look in

<filename>/usr/src/linux/include/asm-<replaceable>xxx</>/shmparam.h</>

and <filename>/usr/src/linux/include/linux/sem.h</>.

Older distributions may not have the <command>sysctl</command> program,

but equivalent changes can be made by manipulating the

<filename>/proc</filename> filesystem:

<screen>

<prompt>$</prompt> <userinput>echo 134217728 &gt;/proc/sys/kernel/shmmax</userinput>

<prompt>$</prompt> <userinput>echo 2097152 &gt;/proc/sys/kernel/shmall</userinput>

</screen>

</para>

</listitem>

</varlistentry>