Multi-Processing Modules (MPMs)

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This document describes what a Multi-Processing Module is and +how they are used by the Apache HTTP Server.

Introduction
Choosing an MPM
MPM Defaults

Introduction

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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.

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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.

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Extending the modular design to this level of the server + allows two important benefits:

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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 + mpm_winnt 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.
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 + worker, while sites requiring + stability or compatibility with older software can use a + prefork. In addition, + special features like serving different hosts under different + userids (perchild) can be + provided.

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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 module index page.

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Choosing an MPM

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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.

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To actually choose the desired MPM, use the argument + --with-mpm=NAME with the + configure script. NAME is the name of the + desired MPM.

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Once the server has been compiled, it is possible to + determine which MPM was chosen by using ./httpd + -l. This command will list every module that is compiled + into the server, including the MPM.

MPM Defaults

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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.

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BeOS	`beos`
Netware	`mpm_netware`
OS/2	`mpmt_os2`
Unix	`prefork`
Windows	`mpm_winnt`