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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 event</h1>
29 <p><span>Available Languages: </span><a href="../en/mod/event.html" title="English"> en </a> |
30 <a href="../fr/mod/event.html" hreflang="fr" rel="alternate" title="Français"> fr </a></p>
32 <table class="module"><tr><th><a href="module-dict.html#Description">Description:</a></th><td>A variant of the <code class="module"><a href="../mod/worker.html">worker</a></code> MPM with the goal
33 of consuming threads only for connections with active processing</td></tr>
34 <tr><th><a href="module-dict.html#Status">Status:</a></th><td>MPM</td></tr>
35 <tr><th><a href="module-dict.html#ModuleIdentifier">Module Identifier:</a></th><td>mpm_event_module</td></tr>
36 <tr><th><a href="module-dict.html#SourceFile">Source File:</a></th><td>event.c</td></tr></table>
39 <p>The <code class="module"><a href="../mod/event.html">event</a></code> Multi-Processing Module (MPM) is
40 designed to allow more requests to be served simultaneously by
41 passing off some processing work to supporting threads, freeing up
42 the main threads to work on new requests. It is based on the
43 <code class="module"><a href="../mod/worker.html">worker</a></code> MPM, which implements a hybrid
44 multi-process multi-threaded server. Run-time configuration
45 directives are identical to those provided by
46 <code class="module"><a href="../mod/worker.html">worker</a></code>.</p>
48 <p>To use the <code class="module"><a href="../mod/event.html">event</a></code> MPM, add
49 <code>--with-mpm=event</code> to the <code class="program"><a href="../programs/configure.html">configure</a></code>
50 script's arguments when building the <code class="program"><a href="../programs/httpd.html">httpd</a></code>.</p>
53 <div id="quickview"><h3>Topics</h3>
55 <li><img alt="" src="../images/down.gif" /> <a href="#how-it-works">How it Works</a></li>
56 <li><img alt="" src="../images/down.gif" /> <a href="#requirements">Requirements</a></li>
57 </ul><h3 class="directives">Directives</h3>
59 <li><img alt="" src="../images/down.gif" /> <a href="#asyncrequestworkerfactor">AsyncRequestWorkerFactor</a></li>
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/right.gif" /> <a href="mpm_common.html#maxsparethreads">MaxSpareThreads</a></li>
69 <li><img alt="" src="../images/right.gif" /> <a href="mpm_common.html#minsparethreads">MinSpareThreads</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#scoreboardfile">ScoreBoardFile</a></li>
72 <li><img alt="" src="../images/right.gif" /> <a href="mpm_common.html#sendbuffersize">SendBufferSize</a></li>
73 <li><img alt="" src="../images/right.gif" /> <a href="mpm_common.html#serverlimit">ServerLimit</a></li>
74 <li><img alt="" src="../images/right.gif" /> <a href="mpm_common.html#startservers">StartServers</a></li>
75 <li><img alt="" src="../images/right.gif" /> <a href="mpm_common.html#threadlimit">ThreadLimit</a></li>
76 <li><img alt="" src="../images/right.gif" /> <a href="mpm_common.html#threadsperchild">ThreadsPerChild</a></li>
77 <li><img alt="" src="../images/right.gif" /> <a href="mpm_common.html#threadstacksize">ThreadStackSize</a></li>
78 <li><img alt="" src="../images/right.gif" /> <a href="mod_unixd.html#user">User</a></li>
82 <li><a href="worker.html">The worker MPM</a></li>
83 </ul><ul class="seealso"><li><a href="#comments_section">Comments</a></li></ul></div>
84 <div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
86 <h2><a name="how-it-works" id="how-it-works">How it Works</a></h2>
87 <p>This MPM tries to fix the 'keep alive problem' in HTTP. After a client
88 completes the first request, the client can keep the connection
89 open, and send further requests using the same socket. This can
90 save significant overhead in creating TCP connections. However,
91 Apache HTTP Server traditionally keeps an entire child process/thread waiting
92 for data from the client, which brings its own disadvantages. To
93 solve this problem, this MPM uses a dedicated thread to handle both
94 the Listening sockets, all sockets that are in a Keep Alive state,
95 and sockets where the handler and protocol filters have done their work
96 and the only remaining thing to do is send the data to the client. The
97 status page of <code class="module"><a href="../mod/mod_status.html">mod_status</a></code> shows how many connections are
98 in the mentioned states.</p>
100 <p>The improved connection handling may not work for certain connection
101 filters that have declared themselves as incompatible with event. In these
102 cases, this MPM will fall back to the behaviour of the
103 <code class="module"><a href="../mod/worker.html">worker</a></code> MPM and reserve one worker thread per connection.
104 All modules shipped with the server are compatible with the event MPM.</p>
106 <p>A similar restriction is currently present for requests involving an
107 output filter that needs to read and/or modify the whole response body,
108 like for example mod_ssl, mod_deflate, or mod_include. If the
109 connection to the client blocks while the filter is processing the
110 data, and the amount of data produced by the filter is too big to be
111 buffered in memory, the thread used for the request is not freed while
112 httpd waits until the pending data is sent to the client.</p>
114 <p>The MPM assumes that the underlying <code>apr_pollset</code>
115 implementation is reasonably threadsafe. This enables the MPM to
116 avoid excessive high level locking, or having to wake up the listener
117 thread in order to send it a keep-alive socket. This is currently
118 only compatible with KQueue and EPoll.</p>
120 </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
121 <div class="section">
122 <h2><a name="requirements" id="requirements">Requirements</a></h2>
123 <p>This MPM depends on <a class="glossarylink" href="../glossary.html#apr" title="see glossary">APR</a>'s atomic
124 compare-and-swap operations for thread synchronization. If you are
125 compiling for an x86 target and you don't need to support 386s, or
126 you are compiling for a SPARC and you don't need to run on
127 pre-UltraSPARC chips, add
128 <code>--enable-nonportable-atomics=yes</code> to the
129 <code class="program"><a href="../programs/configure.html">configure</a></code> script's arguments. This will cause
130 APR to implement atomic operations using efficient opcodes not
131 available in older CPUs.</p>
133 <p>This MPM does not perform well on older platforms which lack good
134 threading, but the requirement for EPoll or KQueue makes this
139 <li>To use this MPM on FreeBSD, FreeBSD 5.3 or higher is recommended.
140 However, it is possible to run this MPM on FreeBSD 5.2.1, if you
141 use <code>libkse</code> (see <code>man libmap.conf</code>).</li>
143 <li>For NetBSD, at least version 2.0 is recommended.</li>
145 <li>For Linux, a 2.6 kernel is recommended. It is also necessary to
146 ensure that your version of <code>glibc</code> has been compiled
147 with support for EPoll.</li>
151 <div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
152 <div class="directive-section"><h2><a name="AsyncRequestWorkerFactor" id="AsyncRequestWorkerFactor">AsyncRequestWorkerFactor</a> <a name="asyncrequestworkerfactor" id="asyncrequestworkerfactor">Directive</a></h2>
153 <table class="directive">
154 <tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Limit concurrent connections per process</td></tr>
155 <tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>AsyncRequestWorkerFactor <var>factor</var></code></td></tr>
156 <tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>2</code></td></tr>
157 <tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config</td></tr>
158 <tr><th><a href="directive-dict.html#Status">Status:</a></th><td>MPM</td></tr>
159 <tr><th><a href="directive-dict.html#Module">Module:</a></th><td>event</td></tr>
160 <tr><th><a href="directive-dict.html#Compatibility">Compatibility:</a></th><td>Available in version 2.3.13 and later</td></tr>
162 <p>The event MPM handles some connections in an asynchronous way, where
163 request worker threads are only allocated for short periods of time as
164 needed, and other connections with one request worker thread reserved per
165 connection. This can lead to situations where all workers are tied up and
166 no worker thread is available to handle new work on established async
169 <p>To mitigate this problem, the event MPM does two things: Firstly, it
170 limits the number of connections accepted per process, depending on the
171 number of idle request workers. Secondly, if all workers are busy, it will
172 close connections in keep-alive state even if the keep-alive timeout has
173 not expired. This allows the respective clients to reconnect to a
174 different process which may still have worker threads available.</p>
176 <p>This directive can be used to fine-tune the per-process connection
177 limit. A process will only accept new connections if the current number of
178 connections (not counting connections in the "closing" state) is lower
181 <p class="indent"><strong>
182 <code class="directive"><a href="../mod/mpm_common.html#threadsperchild">ThreadsPerChild</a></code> +
183 (<code class="directive">AsyncRequestWorkerFactor</code> *
184 <var>number of idle workers</var>)
187 <p>This means the absolute maximum numbers of concurrent connections is:</p>
189 <p class="indent"><strong>
190 (<code class="directive">AsyncRequestWorkerFactor</code> + 1) *
191 <code class="directive"><a href="../mod/mpm_common.html#maxrequestworkers">MaxRequestWorkers</a></code>
194 <p><code class="directive"><a href="../mod/mpm_common.html#maxrequestworkers">MaxRequestWorkers</a></code> was called
195 <code class="directive">MaxClients</code> prior to version 2.3.13. The above value
196 shows that the old name did not accurately describe its meaning for the event MPM.</p>
198 <p><code class="directive">AsyncRequestWorkerFactor</code> can take non-integer
199 arguments, e.g "1.5".</p>
204 <div class="bottomlang">
205 <p><span>Available Languages: </span><a href="../en/mod/event.html" title="English"> en </a> |
206 <a href="../fr/mod/event.html" hreflang="fr" rel="alternate" title="Français"> fr </a></p>
207 </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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