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<!DOCTYPE modulesynopsis SYSTEM "/style/modulesynopsis.dtd">

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<modulesynopsis metafile="worker.xml.meta">

<name>worker</name>

<description>Multi-Processing Module implementing a hybrid

multi-threaded multi-process web server</description>

<status>MPM</status>

<sourcefile>worker.c</sourcefile>

<identifier>mpm_worker_module</identifier>

<summary>

<p>This Multi-Processing Module (MPM) implements a hybrid

multi-process multi-threaded server. By using threads to serve

requests, it is able to serve a large number of requests with

fewer system resources than a process-based server. However, it

retains much of the stability of a process-based server by

keeping multiple processes available, each with many threads.</p>

<p>The most important directives used to control this MPM are

<directive module="mpm_common">ThreadsPerChild</directive>, which

controls the number of threads deployed by each child process and

<directive module="mpm_common">MaxRequestWorkers</directive>, which

controls the maximum total number of threads that may be

launched.</p>

</summary>

<seealso><a href="/bind.html">Setting which addresses and ports Apache HTTP Server uses</a></seealso>

<section id="how-it-works"><title>How it Works</title>

<p>A single control process (the parent) is responsible for launching

child processes. Each child process creates a fixed number of server

threads as specified in the <directive

module="mpm_common">ThreadsPerChild</directive> directive, as well

as a listener thread which listens for connections and passes them

to a server thread for processing when they arrive.</p>

<p>Apache HTTP Server always tries to maintain a pool of <dfn>spare</dfn> or

idle server threads, which stand ready to serve incoming

requests. In this way, clients do not need to wait for a new

threads or processes to be created before their requests can be

served. The number of processes that will initially launch is

set by the <directive module="mpm_common">StartServers</directive>

directive. During operation, the server assesses the total number

of idle threads in all processes, and forks or kills processes to

keep this number within the boundaries specified by <directive

module="mpm_common">MinSpareThreads</directive> and <directive

module="mpm_common">MaxSpareThreads</directive>. Since this

process is very self-regulating, it is rarely necessary to modify

these directives from their default values. The maximum number of

clients that may be served simultaneously (i.e., the maximum total

number of threads in all processes) is determined by the

<directive module="mpm_common">MaxRequestWorkers</directive> directive.

The maximum number of active child processes is determined by

the <directive module="mpm_common">MaxRequestWorkers</directive>

directive divided by the <directive module="mpm_common">

ThreadsPerChild</directive> directive.</p>

<p>Two directives set hard limits on the number of active child

processes and the number of server threads in a child process,

and can only be changed by fully stopping the server and then

starting it again. <directive module="mpm_common">ServerLimit

</directive> is a hard limit on the number of active child

processes, and must be greater than or equal to the

<directive module="mpm_common">MaxRequestWorkers</directive>

directive divided by the <directive module="mpm_common">

ThreadsPerChild</directive> directive.

<directive module="mpm_common">ThreadLimit</directive> is a hard

limit of the number of server threads, and must be greater than

or equal to the <directive

module="mpm_common">ThreadsPerChild</directive> directive.</p>

<p>In addition to the set of active child processes, there may

be additional child processes which are terminating, but where at

least one server thread is still handling an existing client

connection. Up to <directive

module="mpm_common">MaxRequestWorkers</directive> terminating processes

may be present, though the actual number can be expected to be

much smaller. This behavior can be avoided by disabling the

termination of individual child processes, which is achieved using

the following:</p>

<ul>

<li>set the value of <directive module="mpm_common">

MaxConnectionsPerChild</directive> to zero</li>

<li>set the value of <directive module="mpm_common">

MaxSpareThreads</directive> to the same value as

<directive module="mpm_common">MaxRequestWorkers</directive></li>

</ul>

<p>A typical configuration of the process-thread controls in

the <module>worker</module> MPM could look as follows:</p>

<highlight language="config">

ServerLimit 16

StartServers 2

MaxRequestWorkers 150

MinSpareThreads 25

MaxSpareThreads 75

ThreadsPerChild 25

</highlight>

<p>While the parent process is usually started as <code>root</code>

under Unix in order to bind to port 80, the child processes and threads

are launched by the server as a less-privileged user. The <directive

module="mod_unixd">User</directive> and <directive

module="mod_unixd">Group</directive> directives are used to set

the privileges of the Apache HTTP Server child processes. The child processes

must be able to read all the content that will be served, but

should have as few privileges beyond that as possible. In

addition, unless <program>suexec</program> is used,

these directives also set the privileges which will be inherited

by CGI scripts.</p>

<p><directive module="mpm_common">MaxConnectionsPerChild</directive>

controls how frequently the server recycles processes by killing

old ones and launching new ones.</p>

<p>This MPM uses the <code>mpm-accept</code> mutex to serialize

access to incoming connections when subject to the thundering herd

problem (generally, when there are multiple listening sockets).

The implementation aspects of this mutex can be configured with the

<directive module="core">Mutex</directive> directive. The <a

href="/misc/perf-tuning.html">performance hints</a>

documentation has additional information about this mutex.</p>

</section>

<directivesynopsis location="mpm_common"><name>CoreDumpDirectory</name>

</directivesynopsis>

<directivesynopsis location="mpm_common"><name>EnableExceptionHook</name>

</directivesynopsis>

<directivesynopsis location="mod_unixd"><name>Group</name>

</directivesynopsis>

<directivesynopsis location="mpm_common"><name>PidFile</name>

</directivesynopsis>

<directivesynopsis location="mpm_common"><name>Listen</name>

</directivesynopsis>

<directivesynopsis location="mpm_common"><name>ListenBacklog</name>

</directivesynopsis>

<directivesynopsis location="mpm_common"><name>MaxRequestWorkers</name>

</directivesynopsis>

<directivesynopsis location="mpm_common"><name>MaxMemFree</name>

</directivesynopsis>

<directivesynopsis location="mpm_common"><name>MaxConnectionsPerChild</name>

</directivesynopsis>

<directivesynopsis location="mpm_common"><name>MaxSpareThreads</name>

</directivesynopsis>

<directivesynopsis location="mpm_common"><name>MinSpareThreads</name>

</directivesynopsis>

<directivesynopsis location="mpm_common"><name>ScoreBoardFile</name>

</directivesynopsis>

<directivesynopsis location="mpm_common"><name>ReceiveBufferSize</name>

</directivesynopsis>

<directivesynopsis location="mpm_common"><name>SendBufferSize</name>

</directivesynopsis>

<directivesynopsis location="mpm_common"><name>ServerLimit</name>

</directivesynopsis>

<directivesynopsis location="mpm_common"><name>StartServers</name>

</directivesynopsis>

<directivesynopsis location="mpm_common"><name>ThreadLimit</name>

</directivesynopsis>

<directivesynopsis location="mpm_common"><name>ThreadsPerChild</name>

</directivesynopsis>

<directivesynopsis location="mpm_common"><name>ThreadStackSize</name>

</directivesynopsis>

<directivesynopsis location="mod_unixd"><name>User</name>

</directivesynopsis>

</modulesynopsis>