Apache MPM worker
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
See also
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 a 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 MaxClient 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:
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.
