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19 <p class="apache">Apache HTTP Server Version 2.5</p>
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23 <a href="http://www.apache.org/">Apache</a> > <a href="http://httpd.apache.org/">HTTP Server</a> > <a href="http://httpd.apache.org/docs/">Documentation</a> > <a href="../">Version 2.5</a> > <a href="./">Modules</a></div>
24 <div id="page-content">
25 <div id="preamble"><h1>Apache MPM prefork</h1>
27 <p><span>Available Languages: </span><a href="../de/mod/prefork.html" hreflang="de" rel="alternate" title="Deutsch"> de </a> |
28 <a href="../en/mod/prefork.html" title="English"> en </a> |
29 <a href="../ja/mod/prefork.html" hreflang="ja" rel="alternate" title="Japanese"> ja </a> |
30 <a href="../tr/mod/prefork.html" hreflang="tr" rel="alternate" title="Türkçe"> tr </a></p>
32 <table class="module"><tr><th><a href="module-dict.html#Description">Description:</a></th><td>Implements a non-threaded, pre-forking web server</td></tr>
33 <tr><th><a href="module-dict.html#Status">Status:</a></th><td>MPM</td></tr>
34 <tr><th><a href="module-dict.html#ModuleIdentifier">Module Identifier:</a></th><td>mpm_prefork_module</td></tr>
35 <tr><th><a href="module-dict.html#SourceFile">Source File:</a></th><td>prefork.c</td></tr></table>
38 <p>This Multi-Processing Module (MPM) implements a non-threaded,
39 pre-forking web server. Each server process may answer incoming
40 requests, and a parent process manages the size of the server pool.
41 It is appropriate for sites that need to avoid
42 threading for compatibility with non-thread-safe libraries. It
43 is also the best MPM for isolating each request, so that a problem
44 with a single request will not affect any other.</p>
46 <p>This MPM is very self-regulating, so it is rarely necessary to
47 adjust its configuration directives. Most important is that
48 <code class="directive"><a href="../mod/mpm_common.html#maxrequestworkers">MaxRequestWorkers</a></code> be big enough
49 to handle as many simultaneous requests as you expect to receive, but
50 small enough to assure that there is enough physical RAM for all
53 <div id="quickview"><h3 class="directives">Directives</h3>
55 <li><img alt="" src="../images/right.gif" /> <a href="mpm_common.html#coredumpdirectory">CoreDumpDirectory</a></li>
56 <li><img alt="" src="../images/right.gif" /> <a href="mpm_common.html#enableexceptionhook">EnableExceptionHook</a></li>
57 <li><img alt="" src="../images/right.gif" /> <a href="mod_unixd.html#group">Group</a></li>
58 <li><img alt="" src="../images/right.gif" /> <a href="mpm_common.html#listen">Listen</a></li>
59 <li><img alt="" src="../images/right.gif" /> <a href="mpm_common.html#listenbacklog">ListenBacklog</a></li>
60 <li><img alt="" src="../images/right.gif" /> <a href="mpm_common.html#maxconnectionsperchild">MaxConnectionsPerChild</a></li>
61 <li><img alt="" src="../images/right.gif" /> <a href="mpm_common.html#maxmemfree">MaxMemFree</a></li>
62 <li><img alt="" src="../images/right.gif" /> <a href="mpm_common.html#maxrequestworkers">MaxRequestWorkers</a></li>
63 <li><img alt="" src="../images/down.gif" /> <a href="#maxspareservers">MaxSpareServers</a></li>
64 <li><img alt="" src="../images/down.gif" /> <a href="#minspareservers">MinSpareServers</a></li>
65 <li><img alt="" src="../images/right.gif" /> <a href="mpm_common.html#pidfile">PidFile</a></li>
66 <li><img alt="" src="../images/right.gif" /> <a href="mpm_common.html#receivebuffersize">ReceiveBufferSize</a></li>
67 <li><img alt="" src="../images/right.gif" /> <a href="mpm_common.html#scoreboardfile">ScoreBoardFile</a></li>
68 <li><img alt="" src="../images/right.gif" /> <a href="mpm_common.html#sendbuffersize">SendBufferSize</a></li>
69 <li><img alt="" src="../images/right.gif" /> <a href="mpm_common.html#serverlimit">ServerLimit</a></li>
70 <li><img alt="" src="../images/right.gif" /> <a href="mpm_common.html#startservers">StartServers</a></li>
71 <li><img alt="" src="../images/right.gif" /> <a href="mod_unixd.html#user">User</a></li>
75 <li><img alt="" src="../images/down.gif" /> <a href="#how-it-works">How it Works</a></li>
76 </ul><h3>See also</h3>
78 <li><a href="../bind.html">Setting which addresses and ports Apache HTTP Server
80 </ul><ul class="seealso"><li><a href="#comments_section">Comments</a></li></ul></div>
81 <div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
83 <h2><a name="how-it-works" id="how-it-works">How it Works</a></h2>
84 <p>A single control process is responsible for launching child
85 processes which listen for connections and serve them when they
86 arrive. Apache httpd always tries to maintain several <dfn>spare</dfn>
87 or idle server processes, which stand ready to serve incoming
88 requests. In this way, clients do not need to wait for a new
89 child processes to be forked before their requests can be
92 <p>The <code class="directive"><a href="../mod/mpm_common.html#startservers">StartServers</a></code>,
93 <code class="directive"><a href="#minspareservers">MinSpareServers</a></code>,
94 <code class="directive"><a href="#maxspareservers">MaxSpareServers</a></code>, and
95 <code class="directive"><a href="../mod/mpm_common.html#maxrequestworkers">MaxRequestWorkers</a></code> regulate how
96 the parent process creates children to serve requests. In general,
97 Apache httpd is very self-regulating, so most sites do not need to
98 adjust these directives from their default values. Sites which
99 need to serve more than 256 simultaneous requests may need to
100 increase <code class="directive"><a href="../mod/mpm_common.html#maxrequestworkers">MaxRequestWorkers</a></code>,
101 while sites with limited memory may need to decrease <code class="directive"><a href="../mod/mpm_common.html#maxrequestworkers">MaxRequestWorkers</a></code> to keep the server from
102 thrashing (swapping memory to disk and back). More information
103 about tuning process creation is provided in the <a href="../misc/perf-tuning.html">performance hints</a>
106 <p>While the parent process is usually started as <code>root</code>
107 under Unix in order to bind to port 80, the child processes are
108 launched by Apache httpd 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
109 the privileges of the Apache httpd child processes. The child processes
110 must be able to read all the content that will be served, but
111 should have as few privileges beyond that as possible.</p>
113 <p><code class="directive"><a href="../mod/mpm_common.html#maxconnectionsperchild">MaxConnectionsPerChild</a></code>
114 controls how frequently the server recycles processes by killing
115 old ones and launching new ones.</p>
117 <p>This MPM uses the <code>mpm-accept</code> mutex to serialize
118 access to incoming connections when subject to the thundering herd
119 problem (generally, when there are multiple listening sockets).
120 The implementation aspects of this mutex can be configured with the
121 <code class="directive"><a href="../mod/core.html#mutex">Mutex</a></code> directive. The <a href="../misc/perf-tuning.html">performance hints</a>
122 documentation has additional information about this mutex.</p>
124 <div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
125 <div class="directive-section"><h2><a name="MaxSpareServers" id="MaxSpareServers">MaxSpareServers</a> <a name="maxspareservers" id="maxspareservers">Directive</a></h2>
126 <table class="directive">
127 <tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Maximum number of idle child server processes</td></tr>
128 <tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>MaxSpareServers <var>number</var></code></td></tr>
129 <tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>MaxSpareServers 10</code></td></tr>
130 <tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config</td></tr>
131 <tr><th><a href="directive-dict.html#Status">Status:</a></th><td>MPM</td></tr>
132 <tr><th><a href="directive-dict.html#Module">Module:</a></th><td>prefork</td></tr>
134 <p>The <code class="directive">MaxSpareServers</code> directive sets the
135 desired maximum number of <em>idle</em> child server processes. An
136 idle process is one which is not handling a request. If there are
137 more than <code class="directive">MaxSpareServers</code> idle, then the
138 parent process will kill off the excess processes.</p>
140 <p>Tuning of this parameter should only be necessary on very
141 busy sites. Setting this parameter to a large number is almost
142 always a bad idea. If you are trying to set the value equal to or lower than
143 <code class="directive"><a href="#minspareservers">MinSpareServers</a></code>, Apache HTTP Server
144 will automatically adjust it to <code class="directive">MinSpareServers</code><code> + 1</code>.</p>
148 <li><code class="directive"><a href="#minspareservers">MinSpareServers</a></code></li>
149 <li><code class="directive"><a href="../mod/mpm_common.html#startservers">StartServers</a></code></li>
150 <li><code class="directive"><a href="../mod/mpm_common.html#maxsparethreads">MaxSpareThreads</a></code></li>
153 <div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
154 <div class="directive-section"><h2><a name="MinSpareServers" id="MinSpareServers">MinSpareServers</a> <a name="minspareservers" id="minspareservers">Directive</a></h2>
155 <table class="directive">
156 <tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Minimum number of idle child server processes</td></tr>
157 <tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>MinSpareServers <var>number</var></code></td></tr>
158 <tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>MinSpareServers 5</code></td></tr>
159 <tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config</td></tr>
160 <tr><th><a href="directive-dict.html#Status">Status:</a></th><td>MPM</td></tr>
161 <tr><th><a href="directive-dict.html#Module">Module:</a></th><td>prefork</td></tr>
163 <p>The <code class="directive">MinSpareServers</code> directive sets the
164 desired minimum number of <em>idle</em> child server processes. An
165 idle process is one which is not handling a request. If there are
166 fewer than <code class="directive">MinSpareServers</code> idle, then the parent
167 process creates new children: It will spawn one, wait a second, then spawn
168 two, wait a second, then spawn four, and it will
169 continue exponentially until it is spawning 32 children per second. It will
170 stop whenever it satisfies the <code class="directive">MinSpareServers</code>
173 <p>Tuning of this parameter should only be necessary on very
174 busy sites. Setting this parameter to a large number is almost
175 always a bad idea.</p>
179 <li><code class="directive"><a href="#maxspareservers">MaxSpareServers</a></code></li>
180 <li><code class="directive"><a href="../mod/mpm_common.html#startservers">StartServers</a></code></li>
181 <li><code class="directive"><a href="../mod/mpm_common.html#minsparethreads">MinSpareThreads</a></code></li>
185 <div class="bottomlang">
186 <p><span>Available Languages: </span><a href="../de/mod/prefork.html" hreflang="de" rel="alternate" title="Deutsch"> de </a> |
187 <a href="../en/mod/prefork.html" title="English"> en </a> |
188 <a href="../ja/mod/prefork.html" hreflang="ja" rel="alternate" title="Japanese"> ja </a> |
189 <a href="../tr/mod/prefork.html" hreflang="tr" rel="alternate" title="Türkçe"> tr </a></p>
190 </div><div class="top"><a href="#page-header"><img src="../images/up.gif" alt="top" /></a></div><div class="section"><h2><a id="comments_section" name="comments_section">Comments</a></h2><div class="warning"><strong>Notice:</strong><br />This is not a Q&A section. Comments placed here should be pointed towards suggestions on improving the documentation or server, and may be removed again by our moderators if they are either implemented or considered invalid/off-topic. Questions on how to manage the Apache HTTP Server should be directed at either our IRC channel, #httpd, on Freenode, or sent to our <a href="http://httpd.apache.org/lists.html">mailing lists</a>.</div>
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