Difference between prefork MPM and worker MPM in apache

What are MPM?

'Multi Processing Modules' aka MPM are modules that extends apache's capability to implement a hybrid multi-process multi-threaded server.

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

MPM uses a multi-process and multi-threaded structure.
Multi-process --> multi PIDs (use 'ps -aef' to find out)
Multi-thread --> more connections per PID. (use 'lsof' to find out. 'netstat -an' don't really see everything.)

The parent process (the one belonging to root) is started up which in turn start up the child processes.

Each child process creates a fixed number of threads as specified in the ThreadsPerChild directive.

Apache always try to maintain a pool of spare threads, which stand ready to serve incoming requests. The number of processes that will initially launched is set by the StartServers directive. Apache will try to keep the number of spare threads within the boundaries specified by MinSpareThreads and MaxSpareThreads.

The maximum number of clients that may be served simultaneously will equal to the maximum total number of threads in all processes. This is set using MaxClients directive.

Therefore, no of processes or PIDs you can have is

no of processes = MaxClients / ThreadsPerChild

Comparing Worker MPM and Prefork MPM,

Worker MPM
- worker MPM uses multiple child processes with many thread each.
- Each thread handle one connection at a time.
- Good for high-traffic, smaller memory footprint.

Prefork MPM
- prefork MPM uses multiple child processes with one thread each.
- Each process handle one connection at a time. uses more memory.
- Good for non-thread-safe third party modules.

Prefork MPM is prefered for better compatibility with older softwares or for better stability although it uses more memories.

Note that we can have one and only MPM module loaded in apache at any one time.

How to check which MPM is compiled?

# httpd -l
Compiled in modules:
core.c
prefork.c
http_core.c
mod_so.c

Reference: http://httpd.apache.org/docs/2.0/mod/worker.html