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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__&amp;list_id=144532&amp;product=Apache%20httpd-2&amp;query_format=specific&amp;order=changeddate%20DESC%2Cpriority%2Cbug_severity&amp;component=mpm_worker">Known issues</a></li><li><a href="https://bz.apache.org/bugzilla/enter_bug.cgi?product=Apache%20httpd-2&amp;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>
</div>
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