docs/manual/mod/prefork.xml

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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">MaxRequestWorkers</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">MaxRequestWorkers</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">MaxRequestWorkers</directive>,
    while sites with limited memory may need to decrease <directive
    module="mpm_common">MaxRequestWorkers</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>MaxRequestWorkers</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: It will spawn one, wait a second, then spawn
    two, wait a second, then spawn four, and it will
    continue exponentially until it is spawning 32 children per second. It will
    stop whenever it satisfies the <directive>MinSpareServers</directive>
    setting.</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>