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21 <p class="apache">Apache HTTP Server Version 2.5</p>
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25 <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>
26 <div id="page-content">
27 <div id="preamble"><h1>Apache MPM prefork</h1>
29 <p><span>Available Languages: </span><a href="../de/mod/prefork.html" hreflang="de" rel="alternate" title="Deutsch"> de </a> |
30 <a href="../en/mod/prefork.html" title="English"> en </a> |
31 <a href="../ja/mod/prefork.html" hreflang="ja" rel="alternate" title="Japanese"> ja </a> |
32 <a href="../tr/mod/prefork.html" hreflang="tr" rel="alternate" title="Türkçe"> tr </a></p>
34 <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>
35 <tr><th><a href="module-dict.html#Status">Status:</a></th><td>MPM</td></tr>
36 <tr><th><a href="module-dict.html#ModuleIdentifier">Module Identifier:</a></th><td>mpm_prefork_module</td></tr>
37 <tr><th><a href="module-dict.html#SourceFile">Source File:</a></th><td>prefork.c</td></tr></table>
40 <p>This Multi-Processing Module (MPM) implements a non-threaded,
41 pre-forking web server. Each server process may answer incoming
42 requests, and a parent process manages the size of the server pool.
43 It is appropriate for sites that need to avoid
44 threading for compatibility with non-thread-safe libraries. It
45 is also the best MPM for isolating each request, so that a problem
46 with a single request will not affect any other.</p>
48 <p>This MPM is very self-regulating, so it is rarely necessary to
49 adjust its configuration directives. Most important is that
50 <code class="directive"><a href="../mod/mpm_common.html#maxrequestworkers">MaxRequestWorkers</a></code> be big enough
51 to handle as many simultaneous requests as you expect to receive, but
52 small enough to assure that there is enough physical RAM for all
55 <div id="quickview"><h3>Topics</h3>
57 <li><img alt="" src="../images/down.gif" /> <a href="#how-it-works">How it Works</a></li>
58 </ul><h3 class="directives">Directives</h3>
60 <li><img alt="" src="../images/right.gif" /> <a href="mpm_common.html#coredumpdirectory">CoreDumpDirectory</a></li>
61 <li><img alt="" src="../images/right.gif" /> <a href="mpm_common.html#enableexceptionhook">EnableExceptionHook</a></li>
62 <li><img alt="" src="../images/right.gif" /> <a href="mod_unixd.html#group">Group</a></li>
63 <li><img alt="" src="../images/right.gif" /> <a href="mpm_common.html#listen">Listen</a></li>
64 <li><img alt="" src="../images/right.gif" /> <a href="mpm_common.html#listenbacklog">ListenBacklog</a></li>
65 <li><img alt="" src="../images/right.gif" /> <a href="mpm_common.html#maxconnectionsperchild">MaxConnectionsPerChild</a></li>
66 <li><img alt="" src="../images/right.gif" /> <a href="mpm_common.html#maxmemfree">MaxMemFree</a></li>
67 <li><img alt="" src="../images/right.gif" /> <a href="mpm_common.html#maxrequestworkers">MaxRequestWorkers</a></li>
68 <li><img alt="" src="../images/down.gif" /> <a href="#maxspareservers">MaxSpareServers</a></li>
69 <li><img alt="" src="../images/down.gif" /> <a href="#minspareservers">MinSpareServers</a></li>
70 <li><img alt="" src="../images/right.gif" /> <a href="mpm_common.html#pidfile">PidFile</a></li>
71 <li><img alt="" src="../images/right.gif" /> <a href="mpm_common.html#receivebuffersize">ReceiveBufferSize</a></li>
72 <li><img alt="" src="../images/right.gif" /> <a href="mpm_common.html#scoreboardfile">ScoreBoardFile</a></li>
73 <li><img alt="" src="../images/right.gif" /> <a href="mpm_common.html#sendbuffersize">SendBufferSize</a></li>
74 <li><img alt="" src="../images/right.gif" /> <a href="mpm_common.html#serverlimit">ServerLimit</a></li>
75 <li><img alt="" src="../images/right.gif" /> <a href="mpm_common.html#startservers">StartServers</a></li>
76 <li><img alt="" src="../images/right.gif" /> <a href="mod_unixd.html#user">User</a></li>
78 <h3>Bugfix checklist</h3><ul class="seealso"><li><a href="https://www.apache.org/dist/httpd/CHANGES_2.4">httpd changelog</a></li><li><a href="https://bz.apache.org/bugzilla/buglist.cgi?bug_status=__open__&list_id=144532&product=Apache%20httpd-2&query_format=specific&order=changeddate%20DESC%2Cpriority%2Cbug_severity&component=mpm_prefork">Known issues</a></li><li><a href="https://bz.apache.org/bugzilla/enter_bug.cgi?product=Apache%20httpd-2&component=mpm_prefork">Report a bug</a></li></ul><h3>See also</h3>
80 <li><a href="../bind.html">Setting which addresses and ports Apache HTTP Server
82 <li><a href="#comments_section">Comments</a></li></ul></div>
83 <div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
85 <h2><a name="how-it-works" id="how-it-works">How it Works</a></h2>
86 <p>A single control process is responsible for launching child
87 processes which listen for connections and serve them when they
88 arrive. Apache httpd always tries to maintain several <dfn>spare</dfn>
89 or idle server processes, which stand ready to serve incoming
90 requests. In this way, clients do not need to wait for a new
91 child processes to be forked before their requests can be
94 <p>The <code class="directive"><a href="../mod/mpm_common.html#startservers">StartServers</a></code>,
95 <code class="directive"><a href="#minspareservers">MinSpareServers</a></code>,
96 <code class="directive"><a href="#maxspareservers">MaxSpareServers</a></code>, and
97 <code class="directive"><a href="../mod/mpm_common.html#maxrequestworkers">MaxRequestWorkers</a></code> regulate how
98 the parent process creates children to serve requests. In general,
99 Apache httpd is very self-regulating, so most sites do not need to
100 adjust these directives from their default values. Sites which
101 need to serve more than 256 simultaneous requests may need to
102 increase <code class="directive"><a href="../mod/mpm_common.html#maxrequestworkers">MaxRequestWorkers</a></code>,
103 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
104 thrashing (swapping memory to disk and back). More information
105 about tuning process creation is provided in the <a href="../misc/perf-tuning.html">performance hints</a>
108 <p>While the parent process is usually started as <code>root</code>
109 under Unix in order to bind to port 80, the child processes are
110 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
111 the privileges of the Apache httpd child processes. The child processes
112 must be able to read all the content that will be served, but
113 should have as few privileges beyond that as possible.</p>
115 <p><code class="directive"><a href="../mod/mpm_common.html#maxconnectionsperchild">MaxConnectionsPerChild</a></code>
116 controls how frequently the server recycles processes by killing
117 old ones and launching new ones.</p>
119 <p>This MPM uses the <code>mpm-accept</code> mutex to serialize
120 access to incoming connections when subject to the thundering herd
121 problem (generally, when there are multiple listening sockets).
122 The implementation aspects of this mutex can be configured with the
123 <code class="directive"><a href="../mod/core.html#mutex">Mutex</a></code> directive. The <a href="../misc/perf-tuning.html">performance hints</a>
124 documentation has additional information about this mutex.</p>
126 <div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
127 <div class="directive-section"><h2><a name="MaxSpareServers" id="MaxSpareServers">MaxSpareServers</a> <a name="maxspareservers" id="maxspareservers">Directive</a></h2>
128 <table class="directive">
129 <tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Maximum number of idle child server processes</td></tr>
130 <tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>MaxSpareServers <var>number</var></code></td></tr>
131 <tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>MaxSpareServers 10</code></td></tr>
132 <tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config</td></tr>
133 <tr><th><a href="directive-dict.html#Status">Status:</a></th><td>MPM</td></tr>
134 <tr><th><a href="directive-dict.html#Module">Module:</a></th><td>prefork</td></tr>
136 <p>The <code class="directive">MaxSpareServers</code> directive sets the
137 desired maximum number of <em>idle</em> child server processes. An
138 idle process is one which is not handling a request. If there are
139 more than <code class="directive">MaxSpareServers</code> idle, then the
140 parent process will kill off the excess processes.</p>
142 <p>Tuning of this parameter should only be necessary on very
143 busy sites. Setting this parameter to a large number is almost
144 always a bad idea. If you are trying to set the value equal to or lower than
145 <code class="directive"><a href="#minspareservers">MinSpareServers</a></code>, Apache HTTP Server
146 will automatically adjust it to <code class="directive">MinSpareServers</code><code> + 1</code>.</p>
150 <li><code class="directive"><a href="#minspareservers">MinSpareServers</a></code></li>
151 <li><code class="directive"><a href="../mod/mpm_common.html#startservers">StartServers</a></code></li>
152 <li><code class="directive"><a href="../mod/mpm_common.html#maxsparethreads">MaxSpareThreads</a></code></li>
155 <div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
156 <div class="directive-section"><h2><a name="MinSpareServers" id="MinSpareServers">MinSpareServers</a> <a name="minspareservers" id="minspareservers">Directive</a></h2>
157 <table class="directive">
158 <tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Minimum number of idle child server processes</td></tr>
159 <tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>MinSpareServers <var>number</var></code></td></tr>
160 <tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>MinSpareServers 5</code></td></tr>
161 <tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config</td></tr>
162 <tr><th><a href="directive-dict.html#Status">Status:</a></th><td>MPM</td></tr>
163 <tr><th><a href="directive-dict.html#Module">Module:</a></th><td>prefork</td></tr>
165 <p>The <code class="directive">MinSpareServers</code> directive sets the
166 desired minimum number of <em>idle</em> child server processes. An
167 idle process is one which is not handling a request. If there are
168 fewer than <code class="directive">MinSpareServers</code> idle, then the parent
169 process creates new children: It will spawn one, wait a second, then spawn
170 two, wait a second, then spawn four, and it will
171 continue exponentially until it is spawning 32 children per second. It will
172 stop whenever it satisfies the <code class="directive">MinSpareServers</code>
175 <p>Tuning of this parameter should only be necessary on very
176 busy sites. Setting this parameter to a large number is almost
177 always a bad idea.</p>
181 <li><code class="directive"><a href="#maxspareservers">MaxSpareServers</a></code></li>
182 <li><code class="directive"><a href="../mod/mpm_common.html#startservers">StartServers</a></code></li>
183 <li><code class="directive"><a href="../mod/mpm_common.html#minsparethreads">MinSpareThreads</a></code></li>
187 <div class="bottomlang">
188 <p><span>Available Languages: </span><a href="../de/mod/prefork.html" hreflang="de" rel="alternate" title="Deutsch"> de </a> |
189 <a href="../en/mod/prefork.html" title="English"> en </a> |
190 <a href="../ja/mod/prefork.html" hreflang="ja" rel="alternate" title="Japanese"> ja </a> |
191 <a href="../tr/mod/prefork.html" hreflang="tr" rel="alternate" title="Türkçe"> tr </a></p>
192 </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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