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<title>prefork - Apache HTTP Server Version 2.4</title>
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<p class="apache">Apache HTTP Server Version 2.4</p>
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<div id="preamble"><h1>Apache MPM prefork</h1>
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<table class="module"><tr><th><a href="module-dict.html#Description">Description:</a></th><td>Implements a non-threaded, pre-forking web server</td></tr>
<tr><th><a href="module-dict.html#Status">Status:</a></th><td>MPM</td></tr>
<tr><th><a href="module-dict.html#ModuleIdentifier">Module�Identifier:</a></th><td>mpm_prefork_module</td></tr>
<tr><th><a href="module-dict.html#SourceFile">Source�File:</a></th><td>prefork.c</td></tr></table>
<h3>Summary</h3>
<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
<code class="directive"><a href="../mod/mpm_common.html#maxrequestworkers">MaxRequestWorkers</a></code> 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>
</div>
<div id="quickview"><a href="https://www.apache.org/foundation/contributing.html" class="badge"><img src="https://www.apache.org/images/SupportApache-small.png" alt="Support Apache!" /></a><h3>Topics</h3>
<ul id="topics">
<li><img alt="" src="../images/down.gif" /> <a href="#how-it-works">How it Works</a></li>
</ul><h3 class="directives">Directives</h3>
<ul id="toc">
<li><img alt="" src="../images/right.gif" /> <a href="mpm_common.html#coredumpdirectory">CoreDumpDirectory</a></li>
<li><img alt="" src="../images/right.gif" /> <a href="mpm_common.html#enableexceptionhook">EnableExceptionHook</a></li>
<li><img alt="" src="../images/right.gif" /> <a href="mod_unixd.html#group">Group</a></li>
<li><img alt="" src="../images/right.gif" /> <a href="mpm_common.html#listen">Listen</a></li>
<li><img alt="" src="../images/right.gif" /> <a href="mpm_common.html#listenbacklog">ListenBacklog</a></li>
<li><img alt="" src="../images/right.gif" /> <a href="mpm_common.html#maxconnectionsperchild">MaxConnectionsPerChild</a></li>
<li><img alt="" src="../images/right.gif" /> <a href="mpm_common.html#maxmemfree">MaxMemFree</a></li>
<li><img alt="" src="../images/right.gif" /> <a href="mpm_common.html#maxrequestworkers">MaxRequestWorkers</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#maxspareservers">MaxSpareServers</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#minspareservers">MinSpareServers</a></li>
<li><img alt="" src="../images/right.gif" /> <a href="mpm_common.html#pidfile">PidFile</a></li>
<li><img alt="" src="../images/right.gif" /> <a href="mpm_common.html#receivebuffersize">ReceiveBufferSize</a></li>
<li><img alt="" src="../images/right.gif" /> <a href="mpm_common.html#scoreboardfile">ScoreBoardFile</a></li>
<li><img alt="" src="../images/right.gif" /> <a href="mpm_common.html#sendbuffersize">SendBufferSize</a></li>
<li><img alt="" src="../images/right.gif" /> <a href="mpm_common.html#serverlimit">ServerLimit</a></li>
<li><img alt="" src="../images/right.gif" /> <a href="mpm_common.html#startservers">StartServers</a></li>
<li><img alt="" src="../images/right.gif" /> <a href="mod_unixd.html#user">User</a></li>
</ul>
<h3>Bugfix checklist</h3><ul class="seealso"><li><a href="https://www.apache.org/dist/httpd/CHANGES_2.4">httpd changelog</a></li><li><a href="https://bz.apache.org/bugzilla/buglist.cgi?bug_status=__open__&list_id=144532&product=Apache%20httpd-2&query_format=specific&order=changeddate%20DESC%2Cpriority%2Cbug_severity&component=mpm_prefork">Known issues</a></li><li><a href="https://bz.apache.org/bugzilla/enter_bug.cgi?product=Apache%20httpd-2&component=mpm_prefork">Report a bug</a></li></ul><h3>See also</h3>
<ul class="seealso">
<li><a href="../bind.html">Setting which addresses and ports Apache HTTP Server
uses</a></li>
<li><a href="#comments_section">Comments</a></li></ul></div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="section">
<h2><a name="how-it-works" id="how-it-works">How it Works</a></h2>
<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 <code class="directive"><a href="../mod/mpm_common.html#startservers">StartServers</a></code>,
<code class="directive"><a href="#minspareservers">MinSpareServers</a></code>,
<code class="directive"><a href="#maxspareservers">MaxSpareServers</a></code>, and
<code class="directive"><a href="../mod/mpm_common.html#maxrequestworkers">MaxRequestWorkers</a></code> 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 <code class="directive"><a href="../mod/mpm_common.html#maxrequestworkers">MaxRequestWorkers</a></code>,
while sites with limited memory may need to decrease <code class="directive"><a href="../mod/mpm_common.html#maxrequestworkers">MaxRequestWorkers</a></code> 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 <code class="directive"><a href="../mod/mod_unixd.html#user">User</a></code> and <code class="directive"><a href="../mod/mod_unixd.html#group">Group</a></code> 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><code class="directive"><a href="../mod/mpm_common.html#maxconnectionsperchild">MaxConnectionsPerChild</a></code>
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
<code class="directive"><a href="../mod/core.html#mutex">Mutex</a></code> directive. The <a href="../misc/perf-tuning.html">performance hints</a>
documentation has additional information about this mutex.</p>
</div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="directive-section"><h2><a name="MaxSpareServers" id="MaxSpareServers">MaxSpareServers</a> <a name="maxspareservers" id="maxspareservers">Directive</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Maximum number of idle child server processes</td></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>MaxSpareServers <var>number</var></code></td></tr>
<tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>MaxSpareServers 10</code></td></tr>
<tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config</td></tr>
<tr><th><a href="directive-dict.html#Status">Status:</a></th><td>MPM</td></tr>
<tr><th><a href="directive-dict.html#Module">Module:</a></th><td>prefork</td></tr>
</table>
<p>The <code class="directive">MaxSpareServers</code> 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 <code class="directive">MaxSpareServers</code> 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
<code class="directive"><a href="#minspareservers">MinSpareServers</a></code>, Apache HTTP Server
will automatically adjust it to <code class="directive">MinSpareServers</code><code> + 1</code>.</p>
<h3>See also</h3>
<ul>
<li><code class="directive"><a href="#minspareservers">MinSpareServers</a></code></li>
<li><code class="directive"><a href="../mod/mpm_common.html#startservers">StartServers</a></code></li>
<li><code class="directive"><a href="../mod/mpm_common.html#maxsparethreads">MaxSpareThreads</a></code></li>
</ul>
</div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="directive-section"><h2><a name="MinSpareServers" id="MinSpareServers">MinSpareServers</a> <a name="minspareservers" id="minspareservers">Directive</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Minimum number of idle child server processes</td></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>MinSpareServers <var>number</var></code></td></tr>
<tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>MinSpareServers 5</code></td></tr>
<tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config</td></tr>
<tr><th><a href="directive-dict.html#Status">Status:</a></th><td>MPM</td></tr>
<tr><th><a href="directive-dict.html#Module">Module:</a></th><td>prefork</td></tr>
</table>
<p>The <code class="directive">MinSpareServers</code> 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 <code class="directive">MinSpareServers</code> 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 <code class="directive">MinSpareServers</code>
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>
<h3>See also</h3>
<ul>
<li><code class="directive"><a href="#maxspareservers">MaxSpareServers</a></code></li>
<li><code class="directive"><a href="../mod/mpm_common.html#startservers">StartServers</a></code></li>
<li><code class="directive"><a href="../mod/mpm_common.html#minsparethreads">MinSpareThreads</a></code></li>
</ul>
</div>
</div>
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</div><div class="top"><a href="#page-header"><img src="../images/up.gif" alt="top" /></a></div><div class="section"><h2><a id="comments_section" name="comments_section">Comments</a></h2><div class="warning"><strong>Notice:</strong><br />This is not a Q&A section. Comments placed here should be pointed towards suggestions on improving the documentation or server, and may be removed again by our moderators if they are either implemented or considered invalid/off-topic. Questions on how to manage the Apache HTTP Server should be directed at either our IRC channel, #httpd, on Freenode, or sent to our <a href="http://httpd.apache.org/lists.html">mailing lists</a>.</div>
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