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<p class="apache">Apache HTTP Server Version 2.4</p>
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<a href="http://www.apache.org/">Apache</a> > <a href="http://httpd.apache.org/">HTTP Server</a> > <a href="http://httpd.apache.org/docs/">Documentation</a> > <a href="../">Version 2.4</a> > <a href="./">Modules</a></div>
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<div id="preamble"><h1>Apache MPM worker</h1>
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<table class="module"><tr><th><a href="module-dict.html#Description">Description:</a></th><td>Multi-Processing Module implementing a hybrid
multi-threaded multi-process 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_worker_module</td></tr>
<tr><th><a href="module-dict.html#SourceFile">Source�File:</a></th><td>worker.c</td></tr></table>
<h3>Summary</h3>
<p>This Multi-Processing Module (MPM) implements a hybrid
multi-process multi-threaded server. By using threads to serve
requests, it is able to serve a large number of requests with
fewer system resources than a process-based server. However, it
retains much of the stability of a process-based server by
keeping multiple processes available, each with many threads.</p>
<p>The most important directives used to control this MPM are
<code class="directive"><a href="../mod/mpm_common.html#threadsperchild">ThreadsPerChild</a></code>, which
controls the number of threads deployed by each child process and
<code class="directive"><a href="../mod/mpm_common.html#maxrequestworkers">MaxRequestWorkers</a></code>, which
controls the maximum total number of threads that may be
launched.</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/right.gif" /> <a href="mpm_common.html#maxsparethreads">MaxSpareThreads</a></li>
<li><img alt="" src="../images/right.gif" /> <a href="mpm_common.html#minsparethreads">MinSpareThreads</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="mpm_common.html#threadlimit">ThreadLimit</a></li>
<li><img alt="" src="../images/right.gif" /> <a href="mpm_common.html#threadsperchild">ThreadsPerChild</a></li>
<li><img alt="" src="../images/right.gif" /> <a href="mpm_common.html#threadstacksize">ThreadStackSize</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_worker">Known issues</a></li><li><a href="https://bz.apache.org/bugzilla/enter_bug.cgi?product=Apache%20httpd-2&component=mpm_worker">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>
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<div class="section">
<h2><a name="how-it-works" id="how-it-works">How it Works</a></h2>
<p>A single control process (the parent) is responsible for launching
child processes. Each child process creates a fixed number of server
threads as specified in the <code class="directive"><a href="../mod/mpm_common.html#threadsperchild">ThreadsPerChild</a></code> directive, as well
as a listener thread which listens for connections and passes them
to a server thread for processing when they arrive.</p>
<p>Apache HTTP Server always tries to maintain a pool of <dfn>spare</dfn> or
idle server threads, which stand ready to serve incoming
requests. In this way, clients do not need to wait for a new
threads or processes to be created before their requests can be
served. The number of processes that will initially launch is
set by the <code class="directive"><a href="../mod/mpm_common.html#startservers">StartServers</a></code>
directive. During operation, the server assesses the total number
of idle threads in all processes, and forks or kills processes to
keep this number within the boundaries specified by <code class="directive"><a href="../mod/mpm_common.html#minsparethreads">MinSpareThreads</a></code> and <code class="directive"><a href="../mod/mpm_common.html#maxsparethreads">MaxSpareThreads</a></code>. Since this
process is very self-regulating, it is rarely necessary to modify
these directives from their default values. The maximum number of
clients that may be served simultaneously (i.e., the maximum total
number of threads in all processes) is determined by the
<code class="directive"><a href="../mod/mpm_common.html#maxrequestworkers">MaxRequestWorkers</a></code> directive.
The maximum number of active child processes is determined by
the <code class="directive"><a href="../mod/mpm_common.html#maxrequestworkers">MaxRequestWorkers</a></code>
directive divided by the <code class="directive"><a href="../mod/mpm_common.html#threadsperchild">
ThreadsPerChild</a></code> directive.</p>
<p>Two directives set hard limits on the number of active child
processes and the number of server threads in a child process,
and can only be changed by fully stopping the server and then
starting it again. <code class="directive"><a href="../mod/mpm_common.html#serverlimit">ServerLimit
</a></code> is a hard limit on the number of active child
processes, and must be greater than or equal to the
<code class="directive"><a href="../mod/mpm_common.html#maxrequestworkers">MaxRequestWorkers</a></code>
directive divided by the <code class="directive"><a href="../mod/mpm_common.html#threadsperchild">
ThreadsPerChild</a></code> directive.
<code class="directive"><a href="../mod/mpm_common.html#threadlimit">ThreadLimit</a></code> is a hard
limit of the number of server threads, and must be greater than
or equal to the <code class="directive"><a href="../mod/mpm_common.html#threadsperchild">ThreadsPerChild</a></code> directive.</p>
<p>In addition to the set of active child processes, there may
be additional child processes which are terminating, but where at
least one server thread is still handling an existing client
connection. Up to <code class="directive"><a href="../mod/mpm_common.html#maxrequestworkers">MaxRequestWorkers</a></code> terminating processes
may be present, though the actual number can be expected to be
much smaller. This behavior can be avoided by disabling the
termination of individual child processes, which is achieved using
the following:</p>
<ul>
<li>set the value of <code class="directive"><a href="../mod/mpm_common.html#maxconnectionsperchild">
MaxConnectionsPerChild</a></code> to zero</li>
<li>set the value of <code class="directive"><a href="../mod/mpm_common.html#maxsparethreads">
MaxSpareThreads</a></code> to the same value as
<code class="directive"><a href="../mod/mpm_common.html#maxrequestworkers">MaxRequestWorkers</a></code></li>
</ul>
<p>A typical configuration of the process-thread controls in
the <code class="module"><a href="../mod/worker.html">worker</a></code> MPM could look as follows:</p>
<pre class="prettyprint lang-config">ServerLimit 16
StartServers 2
MaxRequestWorkers 150
MinSpareThreads 25
MaxSpareThreads 75
ThreadsPerChild 25</pre>
<p>While the parent process is usually started as <code>root</code>
under Unix in order to bind to port 80, the child processes and threads
are launched by the server 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 HTTP Server 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. In
addition, unless <code class="program"><a href="../programs/suexec.html">suexec</a></code> is used,
these directives also set the privileges which will be inherited
by CGI scripts.</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>
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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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