Apache HTTP Server Version 2.0

Apache Module prefork

Description: Implements a non-threaded, pre-forking web server Status: MPM Module Identifier: mpm_prefork_module

Summary

This Multi-Processing Module (MPM) implements a non-threaded, pre-forking web server that handles requests in a manner similar to Apache 1.3. It is appropriate for sites that need to avoid threading for compatibility with non-thread-safe libraries. It is also the best MPM for isolating each request, so that a problem with a single request will not affect any other.

This MPM is very self-regulating, so it is rarely necessary to adjust its configuration directives. Most important is that MaxClients be big enough to handle as many simultaneous requests as you expect to receive, but small enough to assure that there is enough physical RAM for all processes.

Directives

• AcceptMutex
• CoreDumpDirectory
• Group
• Listen
• ListenBacklog
• LockFile
• MaxClients
• MaxRequestsPerChild
• MaxSpareServers
• MaxSpareServers
• MinSpareServers
• MinSpareServers
• PidFile
• ScoreBoardFile
• SendBufferSize
• ServerLimit
• StartServers
• User

See also

• Setting which addresses and ports Apache uses

How it Works

A single control process is responsible for launching child processes which listen for connections and serve them when they arrive. Apache always tries to maintain several spare or idle server processes, which stand ready to serve incoming requests. In this way, clients do not need to wait for a new child processes to be forked before their requests can be served.

The StartServers, MinSpareServers, MaxSpareServers, and MaxClients regulate how the parent process creates children to serve requests. In general, Apache is very self-regulating, so most sites do not need to adjust these directives from their default values. Sites which need to serve more than 256 simultaneous requests may need to increase MaxClients, while sites with limited memory may need to decrease MaxClients to keep the server from thrashing (swapping memory to disk and back). More information about tuning process creation is provided in the performance hints documentation.

While the parent process is usually started as root under Unix in order to bind to port 80, the child processes are launched by Apache as a less-privileged user. The User and Group directives are used to set the privileges of the Apache child processes. The child processes must be able to read all the content that will be served, but should have as few privileges beyond that as possible.

MaxRequestsPerChild controls how frequently the server recycles processes by killing old ones and launching new ones.

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AcceptMutex Directive

Description: Method that Apache uses to serialize multiple children accepting requests on network sockets Syntax: AcceptMutex default|method Default: AcceptMutex default Context: server config Status: MPM Module: prefork

The AcceptMutex directives sets the method that Apache uses to serialize multiple children accepting requests on network sockets. Prior to Apache 2.0, the method was selectable only at compile time. The optimal method to use is highly architecture and platform dependent. For further details, see the performance tuning documentation.

If this directive is set to default, then the compile-time selected default will be used. Other possible methods are listed below. Note that not all methods are available on all platforms. If a method is specified which is not available, a message will be written to the error log listing the available methods.

flock

uses the flock(2) system call to lock the file defined by the LockFile directive.

fcntl

uses the fnctl(2) system call to lock the file defined by the LockFile directive.

sysvsem

uses SySV-style semaphores to implement the mutex.

pthread

uses POSIX mutexes as implemented by the POSIX Threads (PThreads) specification.

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MaxSpareServers Directive

Description: Maximum number of idle child server processes Syntax: MaxSpareServers number Default: MaxSpareServers 10 Context: server config Status: MPM Module: prefork

The MaxSpareServers directive sets the desired maximum number of idle child server processes. An idle process is one which is not handling a request. If there are more than MaxSpareServers idle, then the parent process will kill off the excess processes.

Tuning of this parameter should only be necessary on very busy sites. Setting this parameter to a large number is almost always a bad idea.

See also

• MinSpareServers
• StartServers

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MinSpareServers Directive

Description: Minimum number of idle child server processes Syntax: MinSpareServers number Default: MinSpareServers 5 Context: server config Status: MPM Module: prefork

The MinSpareServers directive sets the desired minimum number of idle child server processes. An idle process is one which is not handling a request. If there are fewer than MinSpareServers idle, then the parent process creates new children at a maximum rate of 1 per second.

Tuning of this parameter should only be necessary on very busy sites. Setting this parameter to a large number is almost always a bad idea.

See also

• MaxSpareServers
• StartServers

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Apache HTTP Server Version 2.0