Apache HTTP Server Version 2.0
Apache HTTP Server Version 2.0
| Description: | Multi-Processing Module implementing a hybrid multi-threaded multi-process web server |
|---|---|
| Status: | MPM |
| Module Identifier: | mpm_worker_module |
| Source File: | worker.c |
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.
CoreDumpDirectory Group Listen ListenBacklog LockFile MaxClients MaxMemFree MaxRequestsPerChild MaxSpareThreads MinSpareThreads PidFile ScoreBoardFile SendBufferSize ServerLimit StartServers ThreadLimit ThreadsPerChild User
How it Works
Each process has a fixed number of threads. The server adjusts to handle load by increasing or decreasing the number of processes.
A single control process is responsible for launching child
processes. Each child process creates a fixed number of threads as
specified in the ThreadsPerChild 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 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,
while the maximum number of processes that can be launched is set
by the ServerLimit
directive. ServerLimit
multiplied by ThreadsPerChild must be greater
than or equal to MaxClients
A typical configuration of the process-thread controls in
the worker MPM could look as follows:
StartServers 2 MaxClients 150 MinSpareThreads 25 MaxSpareThreads 75 ThreadsPerChild 25 ServerLimit 16
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.