Apache > HTTP Server > Documentation > Version 2.0 > Modules

Apache MPM perchild

Description:Multi-Processing Module allowing for daemon processes serving requests to be assigned a variety of different userids
Module Identifier:mpm_perchild_module
Source File:perchild.c


This MPM does not currently work on most platforms. Work is ongoing to make it functional.

This Multi-Processing Module (MPM) implements a hybrid multi-process, multi-threaded web server. A fixed number of processes create threads to handle requests. Fluctuations in load are handled by increasing or decreasing the number of threads in each process.



See also


How it works

A single control process launches the number of child processes indicated by the NumServers directive at server startup. Each child process creates threads as specified in the StartThreads directive. The individual threads then listen for connections and serve them when they arrive.

Apache 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 new threads to be created. For each child process, Apache assesses the number of idle threads and creates or destroys threads 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 is determined by multiplying the number of server processes that will be created (NumServers) by the maximum number of threads created in each process (MaxThreadsPerChild).

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 Apache as a less-privileged user. The User and Group directives are used to set the privileges of the Apache 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.

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

Working with different user-IDs

The perchild MPM adds the extra ability to specify that particular processes should serve requests under different user-IDs. These user-IDs can then be associated with specific virtual hosts. You have to use one ChildPerUserID directive for every user/group combination you want to be run. Then you can tie particular virtual hosts to that user and group IDs.

The following example runs 7 child processes. Two of them are run under user1/group1. The next four are run under user2/group2 and the remaining process uses the User and Group of the main server:

Global config

NumServers 7
ChildPerUserID user1 group1 2
ChildPerUserID user2 group2 4

Using unbalanced numbers of processes as above is useful, if the particular virtual hosts produce different load. The assignment to the virtual hosts is easily done as in the example below. In conclusion with the example above the following assumes, that server2 has to serve about twice of the hits of server1.


NameVirtualHost *

<VirtualHost *>
ServerName fallbackhost
# no assignment; use fallback

<VirtualHost *>
ServerName server1
AssignUserID user1 group1

<VirtualHost *>
ServerName server2
AssignUserID user2 group2


AssignUserID Directive

Description:Tie a virtual host to a user and group ID
Syntax:AssignUserID user-id group-id
Context:virtual host

Tie a virtual host to a specific user/group combination. Requests addressed to the virtual host where this directive appears will be served by a process running with the specified user and group ID.

The user and group ID has to be assigned to a number of children in the global server config using the ChildPerUserID directive. See the section above for a configuration example.


ChildPerUserID Directive

Description:Specify user ID and group ID for a number of child processes
Syntax:ChildPerUserID user-id group-id num-children
Context:server config

Specify a user ID and group ID for a number of child processes. The third argument, num-children, is the number of child processes to start with the specified user and group. It does not represent a specific child number. In order to use this directive, the server must be run initially as root. If you start the server as a non-root user, it will fail to change to the lesser privileged user.

If the total number of child processes, found by totaling all of the third arguments to all ChildPerUserID directives in the config file, is less than NumServers, then all remaining children will inherit the User and Group settings from the main server. See the section above for a configuration example.


Don't set user-id (or group-id) to root unless you know exactly what you are doing, and what the dangers are.


MaxThreadsPerChild Directive

Description:Maximum number of threads per child process
Syntax:MaxThreadsPerChild number
Default:MaxThreadsPerChild 64
Context:server config

This directive sets the maximum number of threads that will be created in each child process. To increase this value beyond its default, it is necessary to change the value of the ThreadLimit directive and stop and re-start the server.


NumServers Directive

Description:Total number of children alive at the same time
Syntax:NumServers number
Default:NumServers 2
Context:server config

The NumServers directive determines the number of children alive at the same time. This number should be large enough to handle the requests for the entire site. To increase this value beyond the value of 8, it is necessary to change the value of the ServerLimit directive and stop and re-start the server. See the section above for a configuration example.