apache/docs/manual/mod/worker.xml

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<modulesynopsis metafile="worker.xml.meta">
<name>worker</name>
<description>Multi-Processing Module implementing a hybrid
    multi-threaded multi-process web server</description>
<status>MPM</status>
<sourcefile>worker.c</sourcefile>
<identifier>mpm_worker_module</identifier>

<summary>
    <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
    less system resources than a process-based server.  Yet 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
    <directive module="mpm_common">ThreadsPerChild</directive>, which
    controls the number of threads deployed by each child process and
    <directive module="mpm_common">MaxClients</directive>, which
    controls the maximum total number of threads that may be
    launched.</p>
</summary>
<seealso><a href="../bind.html">Setting which addresses and ports Apache
uses</a></seealso>

<section id="how-it-works"><title>How it Works</title>
    <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 <directive 
    module="mpm_common">ThreadsPerChild</directive> 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 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 launched is
    set by the <directive module="mpm_common">StartServers</directive>
    directive. Then during operation, Apache assesses the total number
    of idle threads in all processes, and forks or kills processes to
    keep this number within the boundaries specified by <directive
    module="mpm_common">MinSpareThreads</directive> and <directive
    module="mpm_common">MaxSpareThreads</directive>. 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
    <directive module="mpm_common">MaxClients</directive> directive.
    The maximum number of active child processes is determined by
    the <directive module="mpm_common">MaxClients</directive>
    directive divided by the <directive module="mpm_common">
    ThreadsPerChild</directive> 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.  <directive module="mpm_common">ServerLimit
    </directive> is a hard limit on the number of active child 
    processes, and must be greater than or equal to the 
    <directive module="mpm_common">MaxClients</directive>
    directive divided by the <directive module="mpm_common">
    ThreadsPerChild</directive> directive.  
    <directive module="mpm_common">ThreadLimit</directive> is a hard
    limit of the number of server threads, and must be greater than
    or equal to the <directive 
    module="mpm_common">ThreadsPerChild</directive> directive.  If 
    non-default values are specified for these directives, they 
    should appear before other <module>worker</module> directives.</p>

    <p>In addition to a 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 <directive 
    module="mpm_common">MaxClients</directive> 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 by 
    the following:</p>

    <ul>
      <li>set the value of <directive module="mpm_common">
      MaxRequestsPerChild</directive> to zero</li>

      <li>set the value of <directive module="mpm_common">
      MaxSpareThreads</directive> to the same value as
      <directive module="mpm_common">MaxClients</directive></li>
    </ul>

    <p>A typical configuration of the process-thread controls in
    the <module>worker</module> MPM could look as follows:</p>

    <example>
      ServerLimit         16<br />
      StartServers         2<br />
      MaxClients         150<br />
      MinSpareThreads     25<br />
      MaxSpareThreads     75<br />
      ThreadsPerChild     25
    </example>

    <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 Apache as a less-privileged user. The <directive
    module="mpm_common">User</directive> and <directive
    module="mpm_common">Group</directive> directives are used to set
    the privileges of the Apache 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 <a href="../suexec.html">suexec</a> is used,
    these directives also set the privileges which will be inherited
    by CGI scripts.</p>

    <p><directive module="mpm_common">MaxRequestsPerChild</directive>
    controls how frequently the server recycles processes by killing
    old ones and launching new ones.</p>
</section>
 
<directivesynopsis location="mpm_common"><name>AcceptMutex</name>
</directivesynopsis>
<directivesynopsis location="mpm_common"><name>CoreDumpDirectory</name>
</directivesynopsis>
<directivesynopsis location="mpm_common"><name>EnableExceptionHook</name>
</directivesynopsis>
<directivesynopsis location="mpm_common"><name>Group</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>LockFile</name>
</directivesynopsis>
<directivesynopsis location="mpm_common"><name>MaxClients</name>
</directivesynopsis>
<directivesynopsis location="mpm_common"><name>MaxMemFree</name>
</directivesynopsis>
<directivesynopsis location="mpm_common"><name>MaxRequestsPerChild</name>
</directivesynopsis>
<directivesynopsis location="mpm_common"><name>MaxSpareThreads</name>
</directivesynopsis>
<directivesynopsis location="mpm_common"><name>MinSpareThreads</name>
</directivesynopsis>
<directivesynopsis location="mpm_common"><name>ScoreBoardFile</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="mpm_common"><name>ThreadLimit</name>
</directivesynopsis>
<directivesynopsis location="mpm_common"><name>ThreadsPerChild</name>
</directivesynopsis>
<directivesynopsis location="mpm_common"><name>ThreadStackSize</name>
</directivesynopsis>
<directivesynopsis location="mpm_common"><name>User</name>
</directivesynopsis>

</modulesynopsis>