docs/manual/mod/mpm_common.xml

<?xml version="1.0"?>
<?xml-stylesheet type="text/xsl" href="../style/manual.xsl"?>
<modulesynopsis>

<name>mpm_common</name>
<description>A collection of directives that are implemented by
more than one multi-processing module (MPM)</description>
<status>MPM</status>

<directivesynopsis>
<name>CoreDumpDirectory</name>
<description>Sets the directory where Apache attempts to
switch before dumping core</description>
<syntax>CoreDumpDirectory <em>directory</em></syntax>
<default>CoreDumpDirectory <em>ServerRoot</em></default>
<contextlist><context>server config</context></contextlist>
<modulelist><module>worker</module><module>perchild</module>
<module>prefork</module><module>mpm_winnt</module>
</modulelist>

<usage>

    <p>This controls the directory to which Apache attempts to
    switch before dumping core. The default is in the 
    <directive module="core">ServerRoot</directive> directory, however
    since this should not be writable by the user the server runs
    as, core dumps won't normally get written. If you want a core
    dump for debugging, you can use this directive to place it in a
    different location.</p>
</usage>
</directivesynopsis>

<directivesynopsis>
<name>Group</name>
<description>Sets the group under which the server will answer
requests</description>
<syntax>Group <em>unix-group</em></syntax>
<default>Group #-1</default>
<contextlist><context>server config</context><context>virtual host</context>
</contextlist>
<modulelist><module>worker</module><module>perchild</module>
<module>prefork</module></modulelist>

<usage>
    <p>The <directive>Group</directive> directive sets the group under
    which the server will answer requests. In order to use this
    directive, the stand-alone server must be run initially as root.
    <em>Unix-group</em> is one of:</p>

    <dl>
      <dt>A group name</dt>

      <dd>Refers to the given group by name.</dd>

      <dt># followed by a group number.</dt>

      <dd>Refers to a group by its number.</dd>
    </dl>
    <p>It is recommended that you set up a new group specifically for
    running the server. Some admins use user <code>nobody</code>,
    but this is not always possible or desirable.</p>

    <p>Note: if you start the server as a non-root user, it will
    fail to change to the specified group, and will instead
    continue to run as the group of the original user.</p>

    <p>Special note: Use of this directive in &lt;VirtualHost&lt; is
    no longer supported. To implement the <a
    href="../suexec.html">suEXEC wrapper</a> with Apache 2.0, use the
    <directive module="mod_suexec">SuexecUserGroup</directive>
    directive. SECURITY: See <directive
    module="mpm_common">User</directive> for a discussion of the
    security considerations.</p>
</usage>
</directivesynopsis>

<directivesynopsis>
<name>PidFile</name>
<description>Sets the file where the server records the process ID
of the daemon</description>
<syntax>PidFile <em>filename</em></syntax>
<default>PidFile logs/httpd.pid</default>
<contextlist><context>server config</context></contextlist>
<modulelist><module>worker</module><module>perchilde</module>
<module>prefork</module><module>mpm_winnt</module>
</modulelist>

<usage>
    <p>The <directive>PidFile</directive> directive sets the file to
    which the server records the process id of the daemon. If the
    filename does not begin with a slash (/) then it is assumed to be
    relative to the <directive module="core">ServerRoot</directive>.</p>

    <p>It is often useful to be able to send the server a signal,
    so that it closes and then reopens its <directive
    module="core">ErrorLog</directive> and TransferLog, and
    re-reads its configuration files. This is done by sending a
    SIGHUP (kill -1) signal to the process id listed in the
    PidFile.</p>

    <p>The PidFile is subject to the same warnings about log file
    placement and <a
    href="../misc/security_tips.html#serverroot">security</a>.</p>
</usage>
</directivesynopsis>

<directivesynopsis>
<name>Listen</name>
<description>Sets the IP addresses and ports that the server
listens to</description>
<syntax>Listen [<em>IP-address</em>:]<em>portnumber</em></syntax>
<contextlist><context>server config</context></contextlist>
<modulelist><module>worker</module><module>perchild</module>
<module>prefork</module><module>mpm_winnt</module>
</modulelist>

<usage>
    <p>The <directive>Listen</directive> directive instructs Apache to
    listen to only specific IP addresses or ports; by default it
    responds to requests on all IP interfaces. The Listen directive is
    now a required directive. If it is not in the config file, the
    server will fail to start. This is a change from previous versions
    of Apache.</p>

    <p>The Listen directive tells the server to accept incoming
    requests on the specified port or address-and-port combination.
    If only a port number is specified, the server listens to the
    given port on all interfaces. If an IP address is given as well 
    as a port, the server will listen on the given port and
    interface.</p>

    <p>Multiple Listen directives may be used to specify a number
    of addresses and ports to listen to. The server will respond to
    requests from any of the listed addresses and ports.</p>

    <p>For example, to make the server accept connections on both
    port 80 and port 8000, use:</p>
<example>
   Listen 80<br />
   Listen 8000
</example>
    To make the server accept connections on two specified
    interfaces and port numbers, use 
<example>
   Listen 192.170.2.1:80<br />
   Listen 192.170.2.5:8000
</example>
    IPv6 addresses must be surrounded in square brackets, as in the
    following example:
<example>
   Listen [fe80::a00:20ff:fea7:ccea]:80
</example>
</usage>

<seealso><a href="../dns-caveats.html">DNS Issues</a></seealso>
<seealso><a href="../bind.html">Setting
    which addresses and ports Apache uses</a></seealso>
</directivesynopsis>

<directivesynopsis>
<name>ListenBackLog</name>
<description>Maximum length of the queue of pending connections</description>
<syntax>ListenBacklog <em>backlog</em></syntax>
<default>ListenBacklog 511</default>
<contextlist><context>server config</context></contextlist>
<modulelist><module>worker</module><module>perchild</module>
<module>prefork</module><module>mpm_winnt</module>
</modulelist>

<usage>
    <p>The maximum length of the queue of pending connections.
    Generally no tuning is needed or desired, however on some
    systems it is desirable to increase this when under a TCP SYN
    flood attack. See the backlog parameter to the
    <code>listen(2)</code> system call.</p>

    <p>This will often be limited to a smaller number by the
    operating system. This varies from OS to OS. Also note that
    many OSes do not use exactly what is specified as the backlog,
    but use a number based on (but normally larger than) what is
    set.</p>
</usage>
</directivesynopsis>

<directivesynopsis>
<name>LockFile</name>
<description>Location of the accept serialization lock file</description>
<syntax>LockFile <em>filename</em></syntax>
<default>LockFile logs/accept.lock</default>
<contextlist><context>server config</context></contextlist>
<modulelist><module>worker</module><module>perchild</module>
<module>prefork</module></modulelist>

<usage>
    <p>The <directive>LockFile</directive> directive sets the path to
    the lockfile used when Apache is compiled with either
    USE_FCNTL_SERIALIZED_ACCEPT or USE_FLOCK_SERIALIZED_ACCEPT. This
    directive should normally be left at its default value. The main
    reason for changing it is if the <code>logs</code> directory is
    NFS mounted, since <strong>the lockfile must be stored on a local
    disk</strong>.  The PID of the main server process is
    automatically appended to the filename.</p>

    <p><strong>SECURITY:</strong> It is best to avoid putting this
    file in a world writable directory such as
    <code>/var/tmp</code> because someone could create a denial of
    service attack and prevent the server from starting by creating
    a lockfile with the same name as the one the server will try to
    create.</p>
</usage>
</directivesynopsis>

<directivesynopsis>
<name>MaxClients</name>
<description>Maximum number of child processes that will be created
to serve requests</description>
<syntax>MaxClients <em>number</em></syntax>
<default>>MaxClients
    8 (with threads) MaxClients 256</default>
<contextlist><context>server config</context></contextlist>
<modulelist><module>worker</module><module>prefork</module>
</modulelist>

<usage>
    <p>The <directive>MaxClients</directive> directive sets the limit
    on the number of child processes that will be created to serve
    requests. When the server is built without threading, no more than
    this number of clients can be served simultaneously. To configure
    more than 256 clients with the prefork MPM, you must use the
    <directive module="mpm_common">ServerLimit</directive> directive.
    To configure more than 1024 clients with the worker MPM, you must
    use the <directive module="mpm_common">ServerLimit</directive> and
    <directive module="mpm_common">ThreadLimit</directive> directives.</p>

    <p>Any connection attempts over the
    <directive>MaxClients</directive> limit will normally be queued,
    up to a number based on the <directive module="mpm_common"
    >ListenBacklog</directive> directive. Once a child
    process is freed at the end of a different request, the connection
    will then be serviced.</p>

    <p>When the server is compiled with threading, then the maximum
    number of simultaneous requests that can be served is obtained
    from the value of this directive multiplied by
    <directive module="mpm_common">ThreadsPerChild</directive>.</p>
</usage>
</directivesynopsis>

<directivesynopsis>
<name>MaxRequestPerChild</name>
<description>Limit on the number of requests that an individual child server
will handle during its life</description>
<syntax>MaxRequestsPerChild <em>number</em></syntax>
<default>MaxRequestsPerChild 10000</default>
<contextlist><context>server config</context></contextlist>
<modulelist><module>worker</module><module>perchild</module>
<module>prefork</module><module>mpm_winnt</module>
</modulelist>

<usage>
    <p>The <directive>MaxRequestsPerChild</directive> directive sets
    the limit on the number of requests that an individual child
    server process will handle. After
    <directive>MaxRequestsPerChild</directive> requests, the child
    process will die. If <directive>MaxRequestsPerChild</directive> is
    0, then the process will never expire.</p>

    <p>Setting <directive>MaxRequestsPerChild</directive> to a
    non-zero limit has two beneficial effects:</p>

    <ul>
      <li>it limits the amount of memory that process can consume
      by (accidental) memory leakage;</li>

      <li>by giving processes a finite lifetime, it helps reduce
      the number of processes when the server load reduces.</li>
    </ul>

    <p><strong>NOTE:</strong> For <em>KeepAlive</em> requests, only
    the first request is counted towards this limit. In effect, it
    changes the behavior to limit the number of
    <em>connections</em> per child.</p>
</usage>
</directivesynopsis>

<directivesynopsis>
<name>MaxSpareThreads</name>
<description>Maximum number of idle threads</description>
<syntax>MaxSpareThreads <em>number</em></syntax>
<default>MaxSpareThreads 10 (Perchild) or 500 (worker)</default>
<contextlist><context>server config</context></contextlist>
<modulelist><module>worker</module><module>perchild</module>
</modulelist>

<usage>
    <p>Maximum number of idle threads. Different MPMs deal with this
    directive differently. <module>perchild</module> monitors the
    number of idle threads on a per-child basis. If there are too many
    idle threads in that child, the server will begin to kill threads
    within that child.</p>

    <p><module>worker</module> deals with idle threads on a
    server-wide basis. If there are too many idle threads in the
    server then child processes are killed until the number of idle
    threads is less than this number.</p>

</usage>
<seealso><directive module="mpm_common">MinSpareThreads</directive></seealso>
<seealso><directive module="mpm_common">StartServers</directive></seealso>
</directivesynopsis>

<directivesynopsis>
<name>MaxThreadsPerChild</name>
<description>Maximum number of threads per child process</description>
<syntax>MaxThreadsPerChild <em>number</em></syntax>
<default>MaxThreadsPerChild 64</default>
<contextlist><context>server config</context></contextlist>
<modulelist><module>worker</module><module>perchild</module>
</modulelist>

<usage>
    <p>Maximum number of threads per child. For MPMs with a
    variable number of threads per child, this directive sets the
    maximum number of threads that will be created in each child
    process. To increase this value beyond its default, it is
    necessary to change the value of the compile-time define
    <code>HARD_THREAD_LIMIT</code> and recompile the server.</p>
</usage>
</directivesynopsis>

<directivesynopsis>
<name>MinSpareThreads</name>
<description>Minimum number of idle threads available to handle request
spikes</description>
<syntax>MinSpareServers <em>number</em></syntax>
<default>MinSpareThreads 5 (Perchild) or 250 (worker)</default>
<contextlist><context>server config</context></contextlist>
<modulelist><module>worker</module><module>perchild</module>
</modulelist>

<usage>
    <p>Minimum number of idle threads to handle request spikes.
    Different MPMs deal with this directive
    differently. <module>perchild</module> monitors the number of idle
    threads on a per-child basis. If there aren't enough idle threads
    in that child, the server will begin to create new threads within
    that child.</p>

    <p><module>worker</module> deals with idle threads on a
    server-wide basis. If there aren't enough idle threads in the
    server then child processes are created until the number of idle
    threads is greater than number.</p>
</usage>
<seealso><directive module="mpm_common">MaxSpareThreads</directive></seealso>
<seealso><directive module="mpm_common">StartServers</directive></seealso>
</directivesynopsis>

<directivesynopsis>
<name>NumServers</name>
<description>Total number of children alive at the same time</description>
<syntax>NumServers <em>number</em></syntax>
<default>NumServers 2</default>
<contextlist><context>server config</context></contextlist>
<modulelist><module>perchild</module></modulelist>

<usage>
    <p>Number of children alive at the same time. MPMs that use
    this directive do not dynamically create new child processes so
    this number should be large enough to handle the requests for
    the entire site.</p>
</usage>
</directivesynopsis>

<directivesynopsis>
<name>ScoreBoardFile</name>
<description>Location of the file used to store coordination data for
the child processes</description>
<syntax>ScoreBoardFile <em>file-path</em></syntax>
<default>ScoreBoardFile logs/apache_status</default>
<contextlist><context>server config</context></contextlist>
<modulelist><module>worker</module><module>perchild</module>
<module>prefork</module></modulelist>

<usage>
    <p>The <directive>ScoreBoardFile</directive> directive is required
    on some architectures to place a file that the server will use to
    communicate between its children and the parent. The easiest way
    to find out if your architecture requires a scoreboard file is to
    run Apache and see if it creates the file named by the
    directive. If your architecture requires it then you must ensure
    that this file is not used at the same time by more than one
    invocation of Apache.</p>

    <p>If you have to use a <directive>ScoreBoardFile</directive> then
    you may see improved speed by placing it on a RAM disk. But be
    careful that you heed the same warnings about log file placement
    and <a href="../misc/security_tips.html">security</a>.</p>
</usage>
<seealso><a
    href="../stopping.html">Stopping and Restarting Apache</a></seealso>
</directivesynopsis>

<directivesynopsis>
<name>SendBufferSize</name>
<description>TCP buffer size</description>
<syntax>SendBufferSize <em>bytes</em></syntax>
<contextlist><context>server config</context></contextlist>
<modulelist><module>worker</module><module>perchild</module>
<module>prefork</module><module>mpm_winnt</module>
</modulelist>

<usage>
    <p>The server will set the TCP buffer size to the number of bytes
    specified. Very useful to increase past standard OS defaults on
    high speed high latency (<em>i.e.</em>, 100ms or so, such as
    transcontinental fast pipes).</p>
</usage>
</directivesynopsis>

<directivesynopsis>
<name>ServerLimit</name>
<description>Upper limit on configurable number of processes</description>
<syntax>ServerLimit <em>number</em></syntax>
<default>ServerLimit 256 (prefork), ServerLimit 16 (worker)</default>
<contextlist><context>server config</context></contextlist>
<modulelist><module>worker</module><module>prefork</module>
</modulelist>

<usage>
    <p>For the <module>prefork</module> MPM, this directive sets the
    maximum configured value for <directive
    module="mpm_common">MaxClients</directive> for the lifetime of the
    Apache process.  For the worker MPM, this directive in combination
    with <directive module="mpm_common">ThreadLimit</directive> sets
    the maximum configured value for <directive
    module="mpm_common">MaxClients</directive> for the lifetime of the
    Apache process.  Any attempts to change this directive during a
    restart will be ignored, but <directive
    module="mpm_common">MaxClients</directive> can be modified during
    a restart.</p>

    <p>Special care must be taken when using this directive.  If
    <directive>ServerLimit</directive> is set to a value much higher
    than necessary, extra, unused shared memory will be allocated.  If
    both <directive>ServerLimit</directive> and <directive
    module="mpm_common">MaxClients</directive> are set to values
    higher than the system can handle, Apache may not start or the
    system may become unstable.</p>

    <p>With the <module>prefork</module> MPM, use this directive only
    if you need to set <directive
    module="mpm_common">MaxClients</directive> higher higher than 256.
    Do not set the value of this directive any higher than what you
    might want to set <directive
    module="mpm_common">MaxClients</directive> to.</p>

    <p>With the <module>worker</module> MPM, use this directive only
    if your <directive module="mpm_common">MaxClients</directive> and
    <directive module="mpm_common">ThreadsPerChild</directive>
    settings require more than 16 server processes.  Do not set the
    value of this directive any higher than the number of server
    processes required by what you may want for <directive
    module="mpm_common">MaxClients </directive> and <directive
    module="mpm_common">ThreadsPerChild</directive>.</p>
</usage>
</directivesynopsis>

<directivesynopsis>
<name>StartServers</name>
<description>Number of child server processes created at startup</description>
<syntax>StartServers <em>number</em></syntax>
<default>StartServers 5</default>
<contextlist><context>server config</context></contextlist>
<modulelist><module>worker</module></modulelist>

<usage>
    <p>The <directive>StartServers</directive> directive sets the
    number of child server processes created on startup. As the number
    of processes is dynamically controlled depending on the load,
    there is usually little reason to adjust this parameter.</p>
</usage>
<seealso><directive module="mpm_common">MinSpareThreads</directive></seealso>
<seealso><directive module="mpm_common">MaxSpareThreads</directive></seealso>
</directivesynopsis>

<directivesynopsis>
<name>StartThreads</name>
<description>Nubmer of threads each child creates on startup</description>
<syntax>StartThreads <em>number</em></syntax>
<default>StartThreads 5</default>
<contextlist><context>server config</context></contextlist>
<modulelist><module>perchild</module></modulelist>

<usage>
    <p>Number of threads each child creates on startup. As the
    number of threads is dynamically controlled depending on the
    load, there is usually little reason to adjust this
    parameter.</p>
</usage>
</directivesynopsis>

<directivesynopsis>
<name>ThreadLimit</name>
<description>Sets the upper limit on the configurable number of threads
per child process</description>
<syntax>ThreadLimit <em>number</em></syntax>
<default>ThreadLimit 64</default>
<contextlist><context>server config</context></contextlist>
<modulelist><module>worker</module></modulelist>

<usage>
    <p>This directive sets the maximum configured value for <directive
    module="mpm_common">ThreadsPerChild</directive> for the lifetime
    of the Apache process.  Any attempts to change this directive
    during a restart will be ignored, but <directive
    module="mpm_common">ThreadsPerChild</directive> can be modified
    during a restart up to the value of this directive.</p>

    <p>Special care must be taken when using this directive.  If
    <directive>ThreadLimit</directive> is set to a value much higher
    than <directive module="mpm_common">ThreadsPerChild</directive>,
    extra unused shared memory will be allocated.  If both
    <directive>ThreadLimit</directive> and <directive
    module="mpm_common">ThreadsPerChild</directive> are set to values
    higher than the system can handle, Apache may not start or the
    system may become unstable.</p>

    <p>Use this directive only if you need to set <directive
    module="mpm_common">ThreadsPerChild</directive> higher than 64.  Do
    not set the value of this directive any higher than what you might
    want to set <directive
    module="mpm_common">ThreadsPerChild</directive> to.</p>
</usage>
</directivesynopsis>

<directivesynopsis>
<name>ThreadsPerChild</name>
<description>Number of threads created by each child process</description>
<syntax>ThreadsPerChild <em>number</em></syntax>
<default>ThreadsPerChild 50</default>
<contextlist><context>server config</context></contextlist>
<modulelist><module>worker</module><module>mpm_winnt</module>
</modulelist>

<usage>
    <p>This directive sets the number of threads created by each
    child process. The child creates these threads at startup and
    never creates more. if using an MPM like mpmt_winnt, where
    there is only one child process, this number should be high
    enough to handle the entire load of the server. If using an MPM
    like worker, where there are multiple child processes, the
    total number of threads should be high enough to handle the
    common load on the server.</p>
</usage>
</directivesynopsis>

<directivesynopsis>
<name>User</name>
<description>The userid under which the server will answer
requests</description>
<syntax>User <em>unix-userid</em></syntax>
<default>User #-1</default>
<contextlist><context>server config</context><context>virtual host</context>
</contextlist>
<modulelist><module>worker</module><module>perchild</module>
<module>prefork</module></modulelist>

<usage>
    <p>The <directive>User</directive> directive sets the userid as
    which the server will answer requests. In order to use this
    directive, the standalone server must be run initially as
    root. <em>Unix-userid</em> is one of:</p>

    <dl>
      <dt>A username</dt>

      <dd>Refers to the given user by name.</dd>

      <dt># followed by a user number.</dt>

      <dd>Refers to a user by their number.</dd>
    </dl>

    <p>The user should have no privileges which result in it being
    able to access files which are not intended to be visible to the
    outside world, and similarly, the user should not be able to
    execute code which is not meant for httpd requests. It is
    recommended that you set up a new user and group specifically for
    running the server. Some admins use user <code>nobody</code>, but
    this is not always possible or desirable. For example
    <module>mod_proxy</module>'s cache, when enabled, must be
    accessible to this user (see <directive
    module="mod_proxy">CacheRoot</directive>).</p>

    <p>Notes: If you start the server as a non-root user, it will
    fail to change to the lesser privileged user, and will instead
    continue to run as that original user. If you do start the
    server as root, then it is normal for the parent process to
    remain running as root.</p>

    <p>Special note: Use of this directive in <directive module="core"
    type="section">VirtualHost</directive> is no longer supported. To
    configure your server for <a href="mod_suexec.html">suexec</a> use
    <directive module="mod_suexec">SuexecUserGroup</directive>.</p>

<note><title>Security</title> <p>Don't set <directive>User</directive>
(or <directive module="mpm_common">Group</directive>) to
<code>root</code> unless you know exactly what you are doing, and what
the dangers are.</p></note>
</usage>
</directivesynopsis>

</modulesynopsis>