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| <!DOCTYPE modulesynopsis SYSTEM "../style/modulesynopsis.dtd"> |
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| <modulesynopsis metafile="prefork.xml.meta"> |
| |
| <name>prefork</name> |
| <description>Implements a non-threaded, pre-forking web server</description> |
| <status>MPM</status> |
| <sourcefile>prefork.c</sourcefile> |
| <identifier>mpm_prefork_module</identifier> |
| |
| <summary> |
| <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 |
| <directive module="mpm_common">MaxRequestWorkers</directive> 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> |
| </summary> |
| <seealso><a href="../bind.html">Setting which addresses and ports Apache HTTP Server |
| uses</a></seealso> |
| |
| <section id="how-it-works"><title>How it Works</title> |
| <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 <directive module="mpm_common">StartServers</directive>, |
| <directive module="prefork">MinSpareServers</directive>, |
| <directive module="prefork">MaxSpareServers</directive>, and |
| <directive module="mpm_common">MaxRequestWorkers</directive> 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 <directive module="mpm_common">MaxRequestWorkers</directive>, |
| while sites with limited memory may need to decrease <directive |
| module="mpm_common">MaxRequestWorkers</directive> 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 <directive |
| module="mod_unixd">User</directive> and <directive |
| module="mod_unixd">Group</directive> 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><directive module="mpm_common">MaxConnectionsPerChild</directive> |
| 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 |
| <directive module="core">Mutex</directive> directive. The <a |
| href="../misc/perf-tuning.html">performance hints</a> |
| documentation has additional information about this mutex.</p> |
| </section> |
| |
| <directivesynopsis location="mpm_common"><name>CoreDumpDirectory</name> |
| </directivesynopsis> |
| <directivesynopsis location="mpm_common"><name>EnableExceptionHook</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>MaxRequestWorkers</name> |
| </directivesynopsis> |
| <directivesynopsis location="mpm_common"><name>MaxMemFree</name> |
| </directivesynopsis> |
| <directivesynopsis location="mpm_common"><name>MaxConnectionsPerChild</name> |
| </directivesynopsis> |
| <directivesynopsis location="mpm_common"><name>ScoreBoardFile</name> |
| </directivesynopsis> |
| <directivesynopsis location="mpm_common"><name>ReceiveBufferSize</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="mod_unixd"><name>User</name> |
| </directivesynopsis> |
| <directivesynopsis location="mod_unixd"><name>Group</name> |
| </directivesynopsis> |
| |
| <directivesynopsis> |
| <name>MaxSpareServers</name> |
| <description>Maximum number of idle child server processes</description> |
| <syntax>MaxSpareServers <var>number</var></syntax> |
| <default>MaxSpareServers 10</default> |
| <contextlist><context>server config</context></contextlist> |
| |
| <usage> |
| <p>The <directive>MaxSpareServers</directive> 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 <directive>MaxSpareServers</directive> 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 |
| <directive module="prefork">MinSpareServers</directive>, Apache HTTP Server |
| will automatically adjust it to <directive |
| >MinSpareServers</directive><code> + 1</code>.</p> |
| </usage> |
| <seealso><directive module="prefork">MinSpareServers</directive></seealso> |
| <seealso><directive module="mpm_common">StartServers</directive></seealso> |
| <seealso><directive module="mpm_common">MaxSpareThreads</directive></seealso> |
| </directivesynopsis> |
| |
| <directivesynopsis> |
| <name>MinSpareServers</name> |
| <description>Minimum number of idle child server processes</description> |
| <syntax>MinSpareServers <var>number</var></syntax> |
| <default>MinSpareServers 5</default> |
| <contextlist><context>server config</context></contextlist> |
| |
| <usage> |
| <p>The <directive>MinSpareServers</directive> 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 <directive>MinSpareServers</directive> 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 <directive>MinSpareServers</directive> |
| 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> |
| </usage> |
| <seealso><directive module="prefork">MaxSpareServers</directive></seealso> |
| <seealso><directive module="mpm_common">StartServers</directive></seealso> |
| <seealso><directive module="mpm_common">MinSpareThreads</directive></seealso> |
| </directivesynopsis> |
| |
| </modulesynopsis> |
| |