How to Check which MPM (Multi Processing Module) Apache is Running

MPM (Multi processing module) is a component of apache server, that determines how multiple clients or connections shall be handled.

Apache is flexible by design and there are different mpms that can be used with apache depending on the needs. Some MPMs create a separate process to handle each request, whereas some create separate threads.

Some mpms run separate processes with separate uid/gid allowing for improved security.

Some of the common mpms used by Apache web server on Linux:

• 1. Worker
• 2. Prefork
• 3. mpm-ITK
• 4. Peruser

Each of the mpm has its own pros and cons. Apache doc page on mpms is here.

Apache can run only one of them at a time. Mpms are not modules that are loaded by apache. Instead they are compiled into apache.

To check which mpm apache is using run the following command :

Ubuntu

On Ubuntu Apache is called apache2. First locate the apache2 binary.

$ whereis apache2
apache2: /usr/sbin/apache2 /etc/apache2 /usr/lib/apache2 /usr/lib64/apache2 /usr/share/apache2 /usr/share/man/man8/apache2.8.gz

Now run the apache binary with the "l" option. It provides a list of all the modules that have been compiled into apache.

$ /usr/sbin/apache2 -l
Compiled in modules:
  core.c
  mod_log_config.c
  mod_logio.c
  worker.c
  http_core.c
  mod_so.c

In the above output it can be see that worker.c has been compiled into apache. So mpm worker is being used.

Another way to determine the mpm is by executing the apache binary with the "V" option :

$ /usr/sbin/apache2 -V
Server version: Apache/2.2.17 (Ubuntu)
Server built:   Nov  3 2011 02:15:48
Server's Module Magic Number: 20051115:25
Server loaded:  APR 1.4.2, APR-Util 1.3.9
Compiled using: APR 1.4.2, APR-Util 1.3.9
Architecture:   64-bit
Server MPM:     Worker
  threaded:     yes (fixed thread count)
    forked:     yes (variable process count)
Server compiled with....
 -D APACHE_MPM_DIR="server/mpm/worker"
 -D APR_HAS_SENDFILE
 -D APR_HAS_MMAP
 -D APR_HAVE_IPV6 (IPv4-mapped addresses enabled)
 -D APR_USE_SYSVSEM_SERIALIZE
 -D APR_USE_PTHREAD_SERIALIZE
 -D SINGLE_LISTEN_UNSERIALIZED_ACCEPT
 -D APR_HAS_OTHER_CHILD
 -D AP_HAVE_RELIABLE_PIPED_LOGS
 -D DYNAMIC_MODULE_LIMIT=128
 -D HTTPD_ROOT="/etc/apache2"
 -D SUEXEC_BIN="/usr/lib/apache2/suexec"
 -D DEFAULT_PIDLOG="/var/run/apache2.pid"
 -D DEFAULT_SCOREBOARD="logs/apache_runtime_status"
 -D DEFAULT_ERRORLOG="logs/error_log"
 -D AP_TYPES_CONFIG_FILE="mime.types"
 -D SERVER_CONFIG_FILE="apache2.conf"

It provides various information about the MPM (name , threaded , forked) :

Server MPM:     Worker
  threaded:     yes (fixed thread count)
    forked:     yes (variable process count)

A shorter syntax for the above command using grep

$ /usr/sbin/apache2 -V | grep MPM
Server MPM:     Worker
 -D APACHE_MPM_DIR="server/mpm/worker"

The process of checking MPMs on Ubuntu should work the same way as on Debian and similar systems.

CentOS

On CentOS Apache is called httpd

# whereis httpd
httpd: /usr/sbin/httpd /etc/httpd.old /etc/httpd

List the apache details

# /usr/sbin/httpd -l
/usr/sbin/httpd: line 63: ulimit: open files: cannot modify limit: Operation not permitted
/usr/sbin/httpd: line 64: ulimit: open files: cannot modify limit: Operation not permitted
/usr/sbin/httpd: line 65: ulimit: open files: cannot modify limit: Operation not permitted
Compiled in modules:
  core.c
  mod_proxy_balancer.c
  mod_ssl.c
  prefork.c
  http_core.c
  mod_mime.c
......

It shows the prefork.c So mpm prefork is running.

# /usr/sbin/httpd -V
/usr/sbin/httpd: line 63: ulimit: open files: cannot modify limit: Operation not permitted
/usr/sbin/httpd: line 64: ulimit: open files: cannot modify limit: Operation not permitted
/usr/sbin/httpd: line 65: ulimit: open files: cannot modify limit: Operation not permitted
Server version: Apache/2.2.17 (Unix)
Server built:   Feb 12 2011 11:30:46
Cpanel::Easy::Apache v3.2.0 rev5291
Server's Module Magic Number: 20051115:25
Server loaded:  APR 1.4.2, APR-Util 1.3.10
Compiled using: APR 1.4.2, APR-Util 1.3.10
Architecture:   32-bit
Server MPM:     Prefork
  threaded:     no
    forked:     yes (variable process count)
Server compiled with....
 -D APACHE_MPM_DIR="server/mpm/prefork"
 -D APR_HAS_SENDFILE
 -D APR_HAS_MMAP
 -D APR_HAVE_IPV6 (IPv4-mapped addresses enabled)
 -D APR_USE_SYSVSEM_SERIALIZE
 -D APR_USE_PTHREAD_SERIALIZE
 -D SINGLE_LISTEN_UNSERIALIZED_ACCEPT
 -D APR_HAS_OTHER_CHILD
 -D AP_HAVE_RELIABLE_PIPED_LOGS
 -D DYNAMIC_MODULE_LIMIT=128
 -D HTTPD_ROOT="/usr/local/apache"
 -D SUEXEC_BIN="/usr/local/apache/bin/suexec"
 -D DEFAULT_PIDLOG="logs/httpd.pid"
 -D DEFAULT_SCOREBOARD="logs/apache_runtime_status"
 -D DEFAULT_LOCKFILE="logs/accept.lock"
 -D DEFAULT_ERRORLOG="logs/error_log"
 -D AP_TYPES_CONFIG_FILE="conf/mime.types"
 -D SERVER_CONFIG_FILE="conf/httpd.conf"

The mpm information is :

Server MPM:     Prefork
  threaded:     no
    forked:     yes (variable process count)

Conclusion

Each MPM has its own advantages and disadvantages.

The Prefork MPM creates multiple child processes each of which handle a request. This is fast and efficient, however does not scale well with huge number of concurrent requests.

The Worker MPM on the other hand creates child processes and threads which can handle multiple concurrent requests a lot efficiently than prefork.

The following discussion on stackoverflow explain the benefits and drawbacks of different apache mpms:
https://serverfault.com/questions/383526/how-do-i-select-which-apache-mpm-to-use

If you have any feedback or questions, let us know in the comments below.