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<!DOCTYPE modulesynopsis SYSTEM "/style/modulesynopsis.dtd">

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<modulesynopsis metafile="prefork.xml.meta">

<name>prefork</name>

<description>Implements a non-threaded, pre-forking web server</description>

<status>MPM</status>

<sourcefile>prefork.c</sourcefile>

<identifier>mpm_prefork_module</identifier>

<summary>

<p>This Multi-Processing Module (MPM) implements a non-threaded,

pre-forking web server. Each server process may answer incoming

requests, and a parent process manages the size of the server pool.

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.</p>

<p>This MPM is very self-regulating, so it is rarely necessary to

adjust its configuration directives. Most important is that

<directive module="mpm_common">MaxClients</directive> 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.</p>

</summary>

<seealso><a href="/bind.html">Setting which addresses and ports Apache HTTP Server

uses</a></seealso>

<section id="how-it-works"><title>How it Works</title>

<p>A single control process is responsible for launching child

processes which listen for connections and serve them when they

arrive. Apache httpd always tries to maintain several <dfn>spare</dfn>

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.</p>

<p>The <directive module="mpm_common">StartServers</directive>,

<directive module="prefork">MinSpareServers</directive>,

<directive module="prefork">MaxSpareServers</directive>, and

<directive module="mpm_common">MaxClients</directive> regulate how

the parent process creates children to serve requests. In general,

Apache httpd 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 <directive module="mpm_common">MaxClients</directive>,

while sites with limited memory may need to decrease <directive

module="mpm_common">MaxClients</directive> to keep the server from

thrashing (swapping memory to disk and back). More information

about tuning process creation is provided in the <a

href="/misc/perf-tuning.html">performance hints</a>

documentation.</p>

<p>While the parent process is usually started as <code>root</code>

under Unix in order to bind to port 80, the child processes are

launched by Apache httpd as a less-privileged user. The <directive

module="mod_unixd">User</directive> and <directive

module="mod_unixd">Group</directive> directives are used to set

the privileges of the Apache httpd 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.</p>

<p><directive module="mpm_common">MaxConnectionsPerChild</directive>

controls how frequently the server recycles processes by killing

old ones and launching new ones.</p>

<p>This MPM uses the <code>mpm-accept</code> mutex to serialize

access to incoming connections when subject to the thundering herd

problem (generally, when there are multiple listening sockets).

The implementation aspects of this mutex can be configured with the

<directive module="core">Mutex</directive> directive. The <a

href="/misc/perf-tuning.html">performance hints</a>

documentation has additional information about this mutex.</p>

</section>

<directivesynopsis location="mpm_common"><name>CoreDumpDirectory</name>

</directivesynopsis>

<directivesynopsis location="mpm_common"><name>EnableExceptionHook</name>

</directivesynopsis>

<directivesynopsis location="mpm_common"><name>PidFile</name>

</directivesynopsis>

<directivesynopsis location="mpm_common"><name>Listen</name>

</directivesynopsis>

<directivesynopsis location="mpm_common"><name>ListenBacklog</name>

</directivesynopsis>

<directivesynopsis location="mpm_common"><name>MaxClients</name>

</directivesynopsis>

<directivesynopsis location="mpm_common"><name>MaxMemFree</name>

</directivesynopsis>

<directivesynopsis location="mpm_common"><name>MaxConnectionsPerChild</name>

</directivesynopsis>

<directivesynopsis location="mpm_common"><name>ScoreBoardFile</name>

</directivesynopsis>

<directivesynopsis location="mpm_common"><name>ReceiveBufferSize</name>

</directivesynopsis>

<directivesynopsis location="mpm_common"><name>SendBufferSize</name>

</directivesynopsis>

<directivesynopsis location="mpm_common"><name>ServerLimit</name>

</directivesynopsis>

<directivesynopsis location="mpm_common"><name>StartServers</name>

</directivesynopsis>

<directivesynopsis location="mod_unixd"><name>User</name>

</directivesynopsis>

<directivesynopsis location="mod_unixd"><name>Group</name>

</directivesynopsis>

<directivesynopsis>

<name>MaxSpareServers</name>

<description>Maximum number of idle child server processes</description>

<syntax>MaxSpareServers <var>number</var></syntax>

<default>MaxSpareServers 10</default>

<contextlist><context>server config</context></contextlist>

<usage>

<p>The <directive>MaxSpareServers</directive> directive sets the

desired maximum number of <em>idle</em> child server processes. An

idle process is one which is not handling a request. If there are

more than <directive>MaxSpareServers</directive> idle, then the

parent process will kill off the excess processes.</p>

<p>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. If you are trying to set the value equal to or lower than

<directive module="prefork">MinSpareServers</directive>, Apache HTTP Server

will automatically adjust it to <directive

>MinSpareServers</directive><code> + 1</code>.</p>

</usage>

<seealso><directive module="prefork">MinSpareServers</directive></seealso>

<seealso><directive module="mpm_common">StartServers</directive></seealso>

<seealso><directive module="mpm_common">MaxSpareThreads</directive></seealso>

</directivesynopsis>

<directivesynopsis>

<name>MinSpareServers</name>

<description>Minimum number of idle child server processes</description>

<syntax>MinSpareServers <var>number</var></syntax>

<default>MinSpareServers 5</default>

<contextlist><context>server config</context></contextlist>

<usage>

<p>The <directive>MinSpareServers</directive> directive sets the

desired minimum number of <em>idle</em> child server processes. An

idle process is one which is not handling a request. If there are

fewer than <directive>MinSpareServers</directive> idle, then the parent

process creates new children at a maximum rate of 1 per second.</p>

<p>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.</p>

</usage>

<seealso><directive module="prefork">MaxSpareServers</directive></seealso>

<seealso><directive module="mpm_common">StartServers</directive></seealso>

<seealso><directive module="mpm_common">MinSpareThreads</directive></seealso>

</directivesynopsis>

</modulesynopsis>