Apache MPM worker

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Description:	Multi-Processing Module implementing a hybrid + multi-threaded multi-process web server
Status:	MPM
Module Identifier:	mpm_worker_module
Source File:	worker.c

Summary

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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.

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The most important directives used to control this MPM are + ThreadsPerChild, which + controls the number of threads deployed by each child process and + MaxRequestWorkers, which + controls the maximum total number of threads that may be + launched.

Topics

How it Works

Directives

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Bugfix checklist

How it Works

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 ThreadsPerChild directive, as well + as a listener thread which listens for connections and passes them + to a server thread for processing when they arrive.

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Apache HTTP Server always tries to maintain a pool of spare 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 StartServers + 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 MinSpareThreads and MaxSpareThreads. 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 + MaxRequestWorkers directive. + The maximum number of active child processes is determined by + the MaxRequestWorkers + directive divided by the + ThreadsPerChild directive.

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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. ServerLimit + is a hard limit on the number of active child + processes, and must be greater than or equal to the + MaxRequestWorkers + directive divided by the + ThreadsPerChild directive. + ThreadLimit is a hard + limit of the number of server threads, and must be greater than + or equal to the ThreadsPerChild directive.

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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 MaxRequestWorkers 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:

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set the value of + MaxConnectionsPerChild to zero
set the value of + MaxSpareThreads to the same value as + MaxRequestWorkers

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A typical configuration of the process-thread controls in + the worker MPM could look as follows:

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ServerLimit         16
+StartServers         2
+MaxRequestWorkers  150
+MinSpareThreads     25
+MaxSpareThreads     75
+ThreadsPerChild     25

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While the parent process is usually started as root + 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 User and Group 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 suexec is used, + these directives also set the privileges which will be inherited + by CGI scripts.

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MaxConnectionsPerChild + controls how frequently the server recycles processes by killing + old ones and launching new ones.

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This MPM uses the mpm-accept 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 + Mutex directive. The performance hints + documentation has additional information about this mutex.