Apache MPM worker

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 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.

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.

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:

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

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

ServerLimit         16
StartServers         2
MaxRequestWorkers  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 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.

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

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.