Apache HTTP Server Version 2.0

Apache > HTTP Server > Documentation > Version 2.0 > Modules

# 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

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 less system resources than a process-based server. Yet it retains much of the stability of a process-based server by keeping multiple processes available, each with many threads.

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

### Directives

### Topics

- How it Works

### See also

- Setting which addresses and ports Apache uses

## 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.

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 a new threads or processes to be created before their requests can be served. The number of processes that will initially launched is set by the `StartServers` directive. Then during operation, Apache 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 `MaxClients` directive. The maximum number of active child processes is determined by the `MaxClients` directive divided by the `ThreadsPerChild` directive.

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 `MaxClients` 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. If non-default values are specified for these directives, they should appear before other `worker` directives.

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 `MaxClients` 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 by the following:

- set the value of `MaxRequestsPerChild` to zero
- set the value of `MaxSpareThreads` to the same value as `MaxClients`

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

`ServerLimit 16
StartServers 2
MaxClients 150
MinSpareThreads 25
MaxSpareThreads 75
ThreadsPerChild 25`

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.

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