Apache Prefork vs Worker MPM

Question

Looking at the Apache config file  I see Prefork and Worker MPM defined  What is the difference and which one is Apache using

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For CentOS 6 x and 7 x  including Amazon Linux  use   sudo httpd -V   This will show you which of the MPMs are configured  Either prefork  worker  or event  Prefork is the earlier  threadsafe model  Worker is multi-threaded  and event supports php-mpm which is supposed to be a better system for handling threads and requests    However  your results may vary  based on configuration  I ve seen a lot of instability in php-mpm and not any speed improvements  An aggressive spider can exhaust the maximum child processes in php-mpm quite easily   The setting for prefork  worker  or event is set in sudo nano  etc httpd conf modules d 00-mpm conf  for CentOS 6 x 7 x Apache 2 4      Select the MPM module which should be used by uncommenting exactly   one of the following LoadModule lines     prefork MPM  Implements a non-threaded  pre-forking web server   See  http   httpd apache org docs 2 4 mod prefork html  LoadModule mpm prefork module modules mod mpm prefork so    worker MPM  Multi-Processing Module implementing a hybrid   multi-threaded multi-process web server   See  http   httpd apache org docs 2 4 mod worker html  LoadModule mpm worker module modules mod mpm worker so    event MPM  A variant of the worker MPM with the goal of consuming   threads only for connections with active processing   See  http   httpd apache org docs 2 4 mod event html  LoadModule mpm event module modules mod mpm event so

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Apache s Multi-Processing Modules  MPMs  are responsible for binding to network ports on the machine  accepting requests  and dispatching children to handle the requests  http   httpd apache org docs 2 2 mpm html    They re like any other Apache module  except that just one and only one MPM must be loaded into the server at any time  MPMs are chosen during configuration and compiled into the server by using the argument --with-mpm NAME with the configure script where NAME is the name of the desired MPM   Apache will use a default MPM for each operating system unless a different one is choosen at compile-time  for instance on Windows mpm winnt is used by default   Here s the list of operating systems and their default MPMs    BeOS beos Netware  mpm netware  OS 2 mpmt os2  Unix Linux   prefork  update for Apache version   2 4  prefork  worker  or event  depending on platform capabilities  Windows mpm winnt   To check what modules are compiled into the server use the command-line option -l  here is the documentation   For instance on a Windows installation you might get something like    gt  httpd -l Compiled in modules    core c   mod win32 c   mpm winnt c   http core c   mod so c   As of version 2 2 this is the list of available core features and MPM modules    core - Core Apache HTTP Server features that are always available mpm common - A collection of directives that are implemented by more than one multi-processing module  MPM  beos - This Multi-Processing Module is optimized for BeOS  event - An experimental variant of the standard worker MPM mpm netware Multi-Processing Module implementing an exclusively threaded web server optimized for Novell NetWare mpmt os2 Hybrid multi-process  multi-threaded MPM for OS 2 prefork Implements a non-threaded  pre-forking web server mpm winnt - This Multi-Processing Module is optimized for Windows NT  worker - Multi-Processing Module implementing a hybrid multi-threaded multi-process web server   Now  to the difference between prefork and worker   The prefork 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    The worker MPM implements a hybrid multi-process multi-threaded server and gives better performance  hence it should be preferred unless one is using other modules that contain non-thread-safe libraries  see also this discussion or this on Serverfault

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Its easy to switch between prefork or worker mpm in Apache 2 4 on RHEL7  Check MPM type by executing   sudo httpd -V  Server version  Apache 2 4 6  Red Hat Enterprise Linux  Server built    Jul 26 2017 04 45 44 Server s Module Magic Number  20120211 24 Server loaded   APR 1 4 8  APR-UTIL 1 5 2 Compiled using  APR 1 4 8  APR-UTIL 1 5 2 Architecture    64-bit Server MPM      prefork   threaded      no     forked      yes  variable process count  Server compiled with      -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 256  -D HTTPD ROOT   etc httpd   -D SUEXEC BIN   usr sbin suexec   -D DEFAULT PIDLOG   run httpd httpd pid   -D DEFAULT SCOREBOARD  logs apache runtime status   -D DEFAULT ERRORLOG  logs error log   -D AP TYPES CONFIG FILE  conf mime types   -D SERVER CONFIG FILE  conf httpd conf    Now to change MPM edit following file and uncomment required MPM    etc httpd conf modules d 00-mpm conf     Select the MPM module which should be used by uncommenting exactly   one of the following LoadModule lines     prefork MPM  Implements a non-threaded  pre-forking web server   See  http   httpd apache org docs 2 4 mod prefork html LoadModule mpm prefork module modules mod mpm prefork so    worker MPM  Multi-Processing Module implementing a hybrid   multi-threaded multi-process web server   See  http   httpd apache org docs 2 4 mod worker html    LoadModule mpm worker module modules mod mpm worker so    event MPM  A variant of the worker MPM with the goal of consuming   threads only for connections with active processing   See  http   httpd apache org docs 2 4 mod event html    LoadModule mpm event module modules mod mpm event so

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Take a look at this for more detail  It refers to how Apache handles multiple requests  Preforking  which is the default  starts a number of Apache processes  2 by default here  though I believe one can configure this through httpd conf   Worker MPM will start a new thread per request  which I would guess  is more memory efficient  Historically  Apache has used prefork  so it s a better-tested model  Threading was only added in 2 0

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Apache has 2 types of MPM  Multi-Processing Modules  defined   1 Prefork 2  Worker  By default  Apacke is configured in preforked mode i e  non-threaded pre-forking web server  That means that each Apache child process contains a single thread and handles one request at a time  Because of that  it consumes more resources   Apache also has the worker MPM that turns Apache into a multi-process  multi-threaded web server  Worker MPM uses multiple child processes with many threads each

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You can tell whether Apache is using preform or worker by issuing the following command  apache2ctl -l   In the resulting output  look for mentions of prefork c or worker c

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Prefork and worker are two type of MPM apache provides  Both have their merits and demerits   By default mpm is prefork which is thread safe   Prefork MPM uses multiple child processes with one thread each and each process handles one connection at a time   Worker MPM uses multiple child processes with many threads each  Each thread handles one connection at a time   For more details you can visit https   httpd apache org docs 2 4 mpm html and https   httpd apache org docs 2 4 mod prefork html

[apache] Apache Prefork vs Worker MPM

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