MPM stands for Multi Processing Module which extends apache’s capability to implement hybrid multi processing multi threading in apache web server.

Default mpm can be checked with httpd -l or apachectl -l

The default MPM for Unix is the Prefork module.
The Worker MPM was introduced in Apache2.

Before I explain the difference between Prefork and worker its necessary to understand how it works. So let’s see how it works.

• Prefork MPM

Working operation : – 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.
We can adjust this spare process through the apche conf. For a normal server which is having 256 simultaneous connections can use the default prefork settings.

Perfork is the default module given by apache.

# StartServers: number of server processes to start
# MinSpareServers: minimum number of server processes which are kept spare
# MaxSpareServers: maximum number of server processes which are kept spare
# MaxClients: maximum number of server processes allowed to start
# MaxRequestsPerChild directive sets the limit on the number of requests that an individual child server process will handle. After MaxRequestsPerChild requests, the child process will die. If MaxRequestsPerChild is 0, then the process will never expire

As the name suggests this will pre fork necessary child process while starting apache. It is suitable for websites which avoids threading for compatibility for non-thread-safe libraries . It is also known as the best mpm for isolating each request.

• Worker MPM