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7 --><title>prefork - Apache HTTP Server</title><link href="../style/css/manual.css" rel="stylesheet" media="all" type="text/css" title="Main stylesheet" /><link href="../images/favicon.ico" rel="shortcut icon" /></head><body><div id="page-header"><p class="menu"><a href="../mod/">Modules</a> | <a href="../mod/directives.html">Directives</a> | <a href="../faq/">FAQ</a> | <a href="../glossary.html">Glossary</a> | <a href="../sitemap.html">Sitemap</a></p><p class="apache">Apache HTTP Server Version 2.0</p><img alt="" src="../images/feather.gif" /></div><div class="up"><a href="./"><img title="<-" alt="<-" src="../images/left.gif" /></a></div><div id="path"><a href="http://www.apache.org/">Apache</a> > <a href="http://httpd.apache.org/">HTTP Server</a> > <a href="http://httpd.apache.org/docs-project/">Documentation</a> > <a href="../">Version 2.0</a> > <a href="./">Modules</a></div><div id="page-content"><div id="preamble"><h1>Apache Module prefork</h1><table class="module"><tr><th>Description:
8 </th><td>Implements a non-threaded, pre-forking web server</td></tr><tr><th><a href="module-dict.html#Status">Status:
9 </a></th><td>MPM</td></tr><tr><th><a href="module-dict.html#ModuleIdentifier">Module Identifier:
10 </a></th><td>mpm_prefork_module</td></tr></table><h3>Summary</h3>
11 <p>This Multi-Processing Module (MPM) implements a non-threaded,
12 pre-forking web server that handles requests in a manner similar
13 to Apache 1.3. It is appropriate for sites that need to avoid
14 threading for compatibility with non-thread-safe libraries. It
15 is also the best MPM for isolating each request, so that a problem
16 with a single request will not affect any other.</p>
18 <p>This MPM is very self-regulating, so it is rarely necessary to
19 adjust its configuration directives. Most important is that
20 <code class="directive"><a href="../mod/mpm_common.html#maxclients">MaxClients</a></code> be big enough to
21 handle as many simultaneous requests as you expect to receive, but
22 small enough to assure that there is enough physical RAM for all
25 </div><div id="quickview"><h3 class="directives">Directives</h3><ul id="toc"><li><img alt="" src="../images/down.gif" /> <a href="#acceptmutex">AcceptMutex</a></li><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#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#lockfile">LockFile</a></li><li><img alt="" src="../images/right.gif" /> <a href="mpm_common.html#maxclients">MaxClients</a></li><li><img alt="" src="../images/right.gif" /> <a href="mpm_common.html#maxrequestsperchild">MaxRequestsPerChild</a></li><li><img alt="" src="../images/down.gif" /> <a href="#maxspareservers">MaxSpareServers</a></li><li><img alt="" src="../images/down.gif" /> <a href="#minspareservers">MinSpareServers</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#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#user">User</a></li></ul><h3>Topics</h3><ul id="topics"><li><img alt="" src="../images/down.gif" /> How it Works</li></ul><h3>See also</h3><ul class="seealso"><li><a href="../bind.html">Setting which addresses and
26 ports Apache uses</a></li></ul></div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div><div class="section"><h2>How it Works</h2>
27 <p>A single control process is responsible for launching child
28 processes which listen for connections and serve them when they
29 arrive. Apache always tries to maintain several <em>spare</em>
30 or idle server processes, which stand ready to serve incoming
31 requests. In this way, clients do not need to wait for a new
32 child processes to be forked before their requests can be
35 <p>The <code class="directive"><a href="../mod/mpm_common.html#startservers">StartServers</a></code>,
36 <code class="directive"><a href="#minspareservers">MinSpareServers</a></code>,
37 <code class="directive"><a href="#maxspareservers">MaxSpareServers</a></code>, and
38 <code class="directive"><a href="../mod/mpm_common.html#maxclients">MaxClients</a></code> regulate how
39 the parent process creates children to serve requests. In general,
40 Apache is very self-regulating, so most sites do not need to
41 adjust these directives from their default values. Sites which
42 need to serve more than 256 simultaneous requests may need to
43 increase <code class="directive"><a href="../mod/mpm_common.html#maxclients">MaxClients</a></code>,
44 while sites with limited memory may need to decrease <code class="directive"><a href="../mod/mpm_common.html#maxclients">MaxClients</a></code> to keep the server from
45 thrashing (swapping memory to disk and back). More information
46 about tuning process creation is provided in the <a href="../misc/perf-tuning.html">performance hints</a>
49 <p>While the parent process is usually started as root under Unix
50 in order to bind to port 80, the child processes are launched by
51 Apache as a less-privileged user. The <code class="directive"><a href="../mod/mpm_common.html#user">User</a></code> and <code class="directive"><a href="../mod/mpm_common.html#group">Group</a></code> directives are used to set
52 the privileges of the Apache child processes. The child processes
53 must be able to read all the content that will be served, but
54 should have as few privileges beyond that as possible.</p>
56 <p><code class="directive"><a href="../mod/mpm_common.html#maxrequestsperchild">MaxRequestsPerChild</a></code>
57 controls how frequently the server recycles processes by killing
58 old ones and launching new ones.</p>
59 </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div><div class="directive-section"><h2><a name="AcceptMutex" id="AcceptMutex">AcceptMutex</a> <a name="acceptmutex" id="acceptmutex">Directive</a></h2><table class="directive"><tr><th>Description:
60 </th><td>Method that Apache uses to serialize multiple children
61 accepting requests on network sockets</td></tr><tr><th><a href="directive-dict.html#Syntax">Syntax:
62 </a></th><td>AcceptMutex default|<em>method</em></td></tr><tr><th><a href="directive-dict.html#Default">Default:
63 </a></th><td><code>AcceptMutex default</code></td></tr><tr><th><a href="directive-dict.html#Context">Context:
64 </a></th><td>server config</td></tr><tr><th><a href="directive-dict.html#Status">Status:
65 </a></th><td>MPM</td></tr><tr><th><a href="directive-dict.html#Module">Module:
66 </a></th><td>prefork</td></tr></table>
67 <p>The <code class="directive">AcceptMutex</code> directives sets the
68 method that Apache uses to serialize multiple children accepting
69 requests on network sockets. Prior to Apache 2.0, the method was
70 selectable only at compile time. The optimal method to use is
71 highly architecture and platform dependent. For further details,
72 see the <a href="../misc/perf-tuning.html">performance tuning</a>
75 <p>If this directive is set to <code>default</code>, then the
76 compile-time selected default will be used. Other possible
77 methods are listed below. Note that not all methods are
78 available on all platforms. If a method is specified which is
79 not available, a message will be written to the error log
80 listing the available methods.</p>
83 <dt><code>flock</code></dt>
85 <dd>uses the <code>flock(2)</code> system call to lock the
86 file defined by the <code class="directive"><a href="../mod/mpm_common.html#lockfile">LockFile</a></code> directive.</dd>
88 <dt><code>fcntl</code></dt>
90 <dd>uses the <code>fnctl(2)</code> system call to lock the
91 file defined by the <code class="directive"><a href="../mod/mpm_common.html#lockfile">LockFile</a></code> directive.</dd>
93 <dt><code>sysvsem</code></dt>
95 <dd>uses SySV-style semaphores to implement the mutex.</dd>
97 <dt><code>pthread</code></dt>
99 <dd>uses POSIX mutexes as implemented by the POSIX Threads
100 (PThreads) specification.</dd>
102 </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div><div class="directive-section"><h2><a name="MaxSpareServers" id="MaxSpareServers">MaxSpareServers</a> <a name="maxspareservers" id="maxspareservers">Directive</a></h2><table class="directive"><tr><th>Description:
103 </th><td>Maximum number of idle child server processes</td></tr><tr><th><a href="directive-dict.html#Syntax">Syntax:
104 </a></th><td>MaxSpareServers <em>number</em><br /></td></tr><tr><th><a href="directive-dict.html#Default">Default:
105 </a></th><td><code>MaxSpareServers 10</code></td></tr><tr><th><a href="directive-dict.html#Context">Context:
106 </a></th><td>server config</td></tr><tr><th><a href="directive-dict.html#Status">Status:
107 </a></th><td>MPM</td></tr><tr><th><a href="directive-dict.html#Module">Module:
108 </a></th><td>prefork</td></tr></table>
109 <p>The <code class="directive">MaxSpareServers</code> directive sets the
110 desired maximum number of <em>idle</em> child server processes. An
111 idle process is one which is not handling a request. If there are
112 more than MaxSpareServers idle, then the parent process will kill
113 off the excess processes.</p>
115 <p>Tuning of this parameter should only be necessary on very
116 busy sites. Setting this parameter to a large number is almost
117 always a bad idea.</p>
118 <h3>See also</h3><ul><li><code class="directive"><a href="#minspareservers">MinSpareServers</a></code></li><li><code class="directive"><a href="../mod/mpm_common.html#startservers">StartServers</a></code></li></ul></div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div><div class="directive-section"><h2><a name="MinSpareServers" id="MinSpareServers">MinSpareServers</a> <a name="minspareservers" id="minspareservers">Directive</a></h2><table class="directive"><tr><th>Description:
119 </th><td>Minimum number of idle child server processes</td></tr><tr><th><a href="directive-dict.html#Syntax">Syntax:
120 </a></th><td>MinSpareServers <em>number</em></td></tr><tr><th><a href="directive-dict.html#Default">Default:
121 </a></th><td><code>MinSpareServers 5</code></td></tr><tr><th><a href="directive-dict.html#Context">Context:
122 </a></th><td>server config</td></tr><tr><th><a href="directive-dict.html#Status">Status:
123 </a></th><td>MPM</td></tr><tr><th><a href="directive-dict.html#Module">Module:
124 </a></th><td>prefork</td></tr></table>
125 <p>The <code class="directive">MinSpareServers</code> directive sets the
126 desired minimum number of <em>idle</em> child server processes. An
127 idle process is one which is not handling a request. If there are
128 fewer than MinSpareServers idle, then the parent process creates
129 new children at a maximum rate of 1 per second.</p>
131 <p>Tuning of this parameter should only be necessary on very
132 busy sites. Setting this parameter to a large number is almost
133 always a bad idea.</p>
134 <h3>See also</h3><ul><li><code class="directive"><a href="#maxspareservers">MaxSpareServers</a></code></li><li><code class="directive"><a href="../mod/mpm_common.html#startservers">StartServers</a></code></li></ul></div></div><div id="footer"><p class="apache">Maintained by the <a href="http://httpd.apache.org/docs-project/">Apache HTTP Server Documentation Project</a></p><p class="menu"><a href="../mod/">Modules</a> | <a href="../mod/directives.html">Directives</a> | <a href="../faq/">FAQ</a> | <a href="../glossary.html">Glossary</a> | <a href="../sitemap.html">Sitemap</a></p></div></body></html>