2 <!DOCTYPE modulesynopsis SYSTEM "../style/modulesynopsis.dtd">
3 <?xml-stylesheet type="text/xsl" href="../style/manual.en.xsl"?>
4 <!-- $LastChangedRevision$ -->
7 Licensed to the Apache Software Foundation (ASF) under one or more
8 contributor license agreements. See the NOTICE file distributed with
9 this work for additional information regarding copyright ownership.
10 The ASF licenses this file to You under the Apache License, Version 2.0
11 (the "License"); you may not use this file except in compliance with
12 the License. You may obtain a copy of the License at
14 http://www.apache.org/licenses/LICENSE-2.0
16 Unless required by applicable law or agreed to in writing, software
17 distributed under the License is distributed on an "AS IS" BASIS,
18 WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
19 See the License for the specific language governing permissions and
20 limitations under the License.
23 <modulesynopsis metafile="worker.xml.meta">
25 <description>Multi-Processing Module implementing a hybrid
26 multi-threaded multi-process web server</description>
28 <sourcefile>worker.c</sourcefile>
29 <identifier>mpm_worker_module</identifier>
32 <p>This Multi-Processing Module (MPM) implements a hybrid
33 multi-process multi-threaded server. By using threads to serve
34 requests, it is able to serve a large number of requests with
35 fewer system resources than a process-based server. However, it
36 retains much of the stability of a process-based server by
37 keeping multiple processes available, each with many threads.</p>
39 <p>The most important directives used to control this MPM are
40 <directive module="mpm_common">ThreadsPerChild</directive>, which
41 controls the number of threads deployed by each child process and
42 <directive module="mpm_common">MaxRequestWorkers</directive>, which
43 controls the maximum total number of threads that may be
46 <seealso><a href="../bind.html">Setting which addresses and ports Apache HTTP Server uses</a></seealso>
48 <section id="how-it-works"><title>How it Works</title>
49 <p>A single control process (the parent) is responsible for launching
50 child processes. Each child process creates a fixed number of server
51 threads as specified in the <directive
52 module="mpm_common">ThreadsPerChild</directive> directive, as well
53 as a listener thread which listens for connections and passes them
54 to a server thread for processing when they arrive.</p>
56 <p>Apache HTTP Server always tries to maintain a pool of <dfn>spare</dfn> or
57 idle server threads, which stand ready to serve incoming
58 requests. In this way, clients do not need to wait for a new
59 threads or processes to be created before their requests can be
60 served. The number of processes that will initially launched is
61 set by the <directive module="mpm_common">StartServers</directive>
62 directive. During operation, the server assesses the total number
63 of idle threads in all processes, and forks or kills processes to
64 keep this number within the boundaries specified by <directive
65 module="mpm_common">MinSpareThreads</directive> and <directive
66 module="mpm_common">MaxSpareThreads</directive>. Since this
67 process is very self-regulating, it is rarely necessary to modify
68 these directives from their default values. The maximum number of
69 clients that may be served simultaneously (i.e., the maximum total
70 number of threads in all processes) is determined by the
71 <directive module="mpm_common">MaxRequestWorkers</directive> directive.
72 The maximum number of active child processes is determined by
73 the <directive module="mpm_common">MaxRequestWorkers</directive>
74 directive divided by the <directive module="mpm_common">
75 ThreadsPerChild</directive> directive.</p>
77 <p>Two directives set hard limits on the number of active child
78 processes and the number of server threads in a child process,
79 and can only be changed by fully stopping the server and then
80 starting it again. <directive module="mpm_common">ServerLimit
81 </directive> is a hard limit on the number of active child
82 processes, and must be greater than or equal to the
83 <directive module="mpm_common">MaxRequestWorkers</directive>
84 directive divided by the <directive module="mpm_common">
85 ThreadsPerChild</directive> directive.
86 <directive module="mpm_common">ThreadLimit</directive> is a hard
87 limit of the number of server threads, and must be greater than
88 or equal to the <directive
89 module="mpm_common">ThreadsPerChild</directive> directive.</p>
91 <p>In addition to the set of active child processes, there may
92 be additional child processes which are terminating, but where at
93 least one server thread is still handling an existing client
94 connection. Up to <directive
95 module="mpm_common">MaxRequestWorkers</directive> terminating processes
96 may be present, though the actual number can be expected to be
97 much smaller. This behavior can be avoided by disabling the
98 termination of individual child processes, which is achieved using
102 <li>set the value of <directive module="mpm_common">
103 MaxConnectionsPerChild</directive> to zero</li>
105 <li>set the value of <directive module="mpm_common">
106 MaxSpareThreads</directive> to the same value as
107 <directive module="mpm_common">MaxRequestWorkers</directive></li>
110 <p>A typical configuration of the process-thread controls in
111 the <module>worker</module> MPM could look as follows:</p>
116 MaxRequestWorkers 150<br />
117 MinSpareThreads 25<br />
118 MaxSpareThreads 75<br />
122 <p>While the parent process is usually started as <code>root</code>
123 under Unix in order to bind to port 80, the child processes and threads
124 are launched by the server as a less-privileged user. The <directive
125 module="mod_unixd">User</directive> and <directive
126 module="mod_unixd">Group</directive> directives are used to set
127 the privileges of the Apache HTTP Server child processes. The child processes
128 must be able to read all the content that will be served, but
129 should have as few privileges beyond that as possible. In
130 addition, unless <program>suexec</program> is used,
131 these directives also set the privileges which will be inherited
134 <p><directive module="mpm_common">MaxConnectionsPerChild</directive>
135 controls how frequently the server recycles processes by killing
136 old ones and launching new ones.</p>
138 <p>This MPM uses the <code>mpm-accept</code> mutex to serialize
139 access to incoming connections when subject to the thundering herd
140 problem (generally, when there are multiple listening sockets).
141 The implementation aspects of this mutex can be configured with the
142 <directive module="core">Mutex</directive> directive. The <a
143 href="../misc/perf-tuning.html">performance hints</a>
144 documentation has additional information about this mutex.</p>
147 <directivesynopsis location="mpm_common"><name>CoreDumpDirectory</name>
149 <directivesynopsis location="mpm_common"><name>EnableExceptionHook</name>
151 <directivesynopsis location="mod_unixd"><name>Group</name>
153 <directivesynopsis location="mpm_common"><name>PidFile</name>
155 <directivesynopsis location="mpm_common"><name>Listen</name>
157 <directivesynopsis location="mpm_common"><name>ListenBacklog</name>
159 <directivesynopsis location="mpm_common"><name>MaxRequestWorkers</name>
161 <directivesynopsis location="mpm_common"><name>MaxMemFree</name>
163 <directivesynopsis location="mpm_common"><name>MaxConnectionsPerChild</name>
165 <directivesynopsis location="mpm_common"><name>MaxSpareThreads</name>
167 <directivesynopsis location="mpm_common"><name>MinSpareThreads</name>
169 <directivesynopsis location="mpm_common"><name>ScoreBoardFile</name>
171 <directivesynopsis location="mpm_common"><name>ReceiveBufferSize</name>
173 <directivesynopsis location="mpm_common"><name>SendBufferSize</name>
175 <directivesynopsis location="mpm_common"><name>ServerLimit</name>
177 <directivesynopsis location="mpm_common"><name>StartServers</name>
179 <directivesynopsis location="mpm_common"><name>ThreadLimit</name>
181 <directivesynopsis location="mpm_common"><name>ThreadsPerChild</name>
183 <directivesynopsis location="mpm_common"><name>ThreadStackSize</name>
185 <directivesynopsis location="mod_unixd"><name>User</name>