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
MPM Defaults
Building an MPM as a static module
Building an MPM as a DSO module

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 httpd 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 HTTP Server 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 httpd can more cleanly and efficiently support a wide + variety of operating systems. In particular, the Windows + version of the server is now much more efficient, since + mpm_winnt can use native + networking features in place of the POSIX layer used in + Apache httpd 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 or event, while sites requiring + stability or compatibility with older software can use a + prefork.

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At the user level, MPMs appear much like other Apache httpd + 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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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.

+ + + + + +

Netware	`mpm_netware`
OS/2	`mpmt_os2`
Unix	`prefork`, `worker`, or + `event`, depending on platform capabilities
Windows	`mpm_winnt`

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Here, 'Unix' is used to mean Unix-like operating systems, such as +Linux, BSD, Solaris, Mac OS X, etc.

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In the case of Unix, the decision as to which MPM is installed is +based on two questions:

1. Does the system support threads?

2. Does the system support thread-safe polling (Specifically, the +kqueue and epoll functions)?

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If the answer to both questions is 'yes', the default MPM is +event.

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If The answer to #1 is 'yes', but the answer to #2 is 'no', the +default will be worker.

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If the answer to both questions is 'no', then the default MPM will be +prefork.

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In practical terms, this means that the default will almost always be +event, as all modern operating systems support these +two features.

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Building an MPM as a static module

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MPMs can be built as static modules on all platforms. A single MPM + is chosen at build time and linked into the server. The server must + be rebuilt in order to change the MPM.

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To override the default MPM choice, use the + --with-mpm=NAME option of 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.

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Building an MPM as a DSO module

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On Unix and similar platforms, MPMs can be built as DSO modules and + dynamically loaded into the server in the same manner as other DSO + modules. Building MPMs as DSO modules allows the MPM to be changed by + updating the LoadModule directive + for the MPM instead of by rebuilding the server.

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LoadModule mpm_prefork_module modules/mod_mpm_prefork.so

+ + +

Attempting to LoadModule + more than one MPM will result in a startup failure with the + following error.

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AH00534: httpd: Configuration error: More than one MPM + loaded.

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This feature is enabled using the + --enable-mpms-shared option of the configure + script. + With argument all, all possible MPMs for the platform + will be installed. Alternately, a list of MPMs can be specified as the + argument.

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The default MPM, either selected automatically or specified with the + --with-mpm option of the configure + script, will be loaded in the generated server configuration file. Edit the + LoadModule directive to select a + different MPM.

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Multi-Processing Modules (MPMs)

See also

Introduction

MPM Defaults

Building an MPM as a static module

Building an MPM as a DSO module

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