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<title>Multi-Processing Modules (MPMs)</title>

<summary>

<p>This document describes what a Multi-Processing Module is and

how they are used by the Apache HTTP Server.</p>

</summary>

<section id="introduction"><title>Introduction</title>

<p>The Apache HTTP Server is designed to be a powerful and

flexible web server that can work on a very wide variety of

platforms in a range of different environments. Different

platforms and different environments often require different

features, or may have different ways of implementing the same

feature most efficiently. Apache has always accommodated a wide

variety of environments through its modular design. This design

allows the webmaster to choose which features will be included

in the server by selecting which modules to load either at

compile-time or at run-time.</p>

<p>Apache 2.0 extends this modular design to the most basic

functions of a web server. The server ships with a selection of

Multi-Processing Modules (MPMs) which are responsible for

binding to network ports on the machine, accepting requests,

and dispatching children to handle the requests.</p>

<p>Extending the modular design to this level of the server

allows two important benefits:</p>

<ul>

<li>Apache can more cleanly and efficiently support a wide

variety of operating systems. In particular, the Windows

version of Apache is now much more efficient, since

<module>mpm_winnt</module> can use native

networking features in place of the POSIX layer used in

Apache 1.3. This benefit also extends to other operating

systems that implement specialized MPMs.</li>

<li>The server can be better customized for the needs of the

particular site. For example, sites that need a great deal of

scalability can choose to use a threaded MPM like

<module>worker</module>, while sites requiring

stability or compatibility with older software can use a

<module>prefork</module>. In addition,

special features like serving different hosts under different

userids (<module>perchild</module>) can be

provided.</li>

</ul>

<p>At the user level, MPMs appear much like other Apache

modules. The main difference is that one and only one MPM must

be loaded into the server at any time. The list of available

MPMs appears on the <a href="mod/">module index page</a>.</p>

</section>

<section id="choosing"><title>Choosing an MPM</title>

<p>MPMs must be chosen during configuration, and compiled into

the server. Compilers are capable of optimizing a lot of

functions if threads are used, but only if they know that

threads are being used. Because some MPMs use threads on Unix

and others don't, Apache will always perform better if the MPM

is chosen at configuration time and built into Apache.</p>

<p>To actually choose the desired MPM, use the argument

--with-mpm= <em>NAME</em> with the /configure script.

<em>NAME</em> is the name of the desired MPM.</p>

<p>Once the server has been compiled, it is possible to

determine which MPM was chosen by using <code>/httpd

-l</code>. This command will list every module that is compiled

into the server, including the MPM.</p>

</section>

<section id="defaults"><title>MPM Defaults</title>

<p>The following table lists the default MPMs for various operating

systems. This will be the MPM selected if you do not make another

choice at compile-time.</p>

<table>

<columnspec><column width=".2"/><column width=".2"/></columnspec>

<tr><td>BeOS</td><td><module>beos</module></td></tr>

<tr><td>Netware</td><td><module>mpm_netware</module></td></tr>

<tr><td>OS/2</td><td><module>mpmt_os2</module></td></tr>

<tr><td>Unix</td><td><module>prefork</module></td></tr>

<tr><td>Windows</td><td><module>mpm_winnt</module></td></tr>

</table>

</section>

</manualpage>