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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> |