Apache HTTP Server Version 2.2
Apache MPM prefork
| Description: | Implements a non-threaded, pre-forking web server |
|---|---|
| Status: | MPM |
| Module Identifier: | mpm_prefork_module |
| Source File: | prefork.c |
Summary
This Multi-Processing Module (MPM) implements a non-threaded, pre-forking web server that handles requests in a manner similar to Apache 1.3. It is appropriate for sites that need to avoid threading for compatibility with non-thread-safe libraries. It is also the best MPM for isolating each request, so that a problem with a single request will not affect any other.
This MPM is very self-regulating, so it is rarely necessary to
adjust its configuration directives. Most important is that
MaxClients be big enough to
handle as many simultaneous requests as you expect to receive, but
small enough to assure that there is enough physical RAM for all
processes.
How it Works
A single control process is responsible for launching child processes which listen for connections and serve them when they arrive. Apache always tries to maintain several spare or idle server processes, which stand ready to serve incoming requests. In this way, clients do not need to wait for a new child processes to be forked before their requests can be served.
The StartServers,
MinSpareServers,
MaxSpareServers, and
MaxClients regulate how
the parent process creates children to serve requests. In general,
Apache is very self-regulating, so most sites do not need to
adjust these directives from their default values. Sites which
need to serve more than 256 simultaneous requests may need to
increase MaxClients,
while sites with limited memory may need to decrease MaxClients to keep the server from
thrashing (swapping memory to disk and back). More information
about tuning process creation is provided in the performance hints
documentation.
While the parent process is usually started as root
under Unix in order to bind to port 80, the child processes 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.
MaxRequestsPerChild
controls how frequently the server recycles processes by killing
old ones and launching new ones.
MaxSpareServers Directive
| Description: | Maximum number of idle child server processes |
|---|---|
| Syntax: | MaxSpareServers number |
| Default: | MaxSpareServers 10 |
| Context: | server config |
| Status: | MPM |
| Module: | prefork |
The MaxSpareServers directive sets the
desired maximum number of idle child server processes. An
idle process is one which is not handling a request. If there are
more than MaxSpareServers idle, then the
parent process will kill off the excess processes.
Tuning of this parameter should only be necessary on very
busy sites. Setting this parameter to a large number is almost
always a bad idea. If you are trying to set the value equal to or lower than
MinSpareServers, Apache
will automatically adjust it to MinSpareServers + 1.
See also
MinSpareServers Directive
| Description: | Minimum number of idle child server processes |
|---|---|
| Syntax: | MinSpareServers number |
| Default: | MinSpareServers 5 |
| Context: | server config |
| Status: | MPM |
| Module: | prefork |
The MinSpareServers directive sets the
desired minimum number of idle child server processes. An
idle process is one which is not handling a request. If there are
fewer than MinSpareServers idle, then the parent
process creates new children at a maximum rate of 1 per second.
Tuning of this parameter should only be necessary on very busy sites. Setting this parameter to a large number is almost always a bad idea.