Friday, 17 May 2013

Apache: Prefork MPM vs Worker MPM

Difference between Prefork and Worker MPM modules.

prefork

worker

(mpm_winnt This Multi-Processing Module is optimized for Windows NT.)

(mpm_netware Multi-Processing Module implementing an exclusively threaded web server optimized for Novell NetWare)

Prefork MPM:

A prefork mpm handles requests just like apche 1.3. 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.

Working:

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

Worker MPM:

A worker mpm is an Multi-Processing Module (MPM) which implements a hybrid multi-process multi-threaded server. By using threads to serve requests, it is able to serve a large number of requests with fewer system resources than a process-based server.

The most important directives used to control this MPM are ThreadsPerChild, which controls the number of threads deployed by each child process and MaxClients, which controls the maximum total number of threads that may be launched.

Strength : Memory usage and performanance wise its better than prefork

Weakness : worker will not work properly with languages like php

Working:

A single control process (the parent) is responsible for launching child processes. Each child process creates a fixed number of server threads as specified in the ThreadsPerChild directive, as well as a listener thread which listens for connections and passes them to a server thread for processing when they arrive.

Apache always tries to maintain a group of spare or idle server threads, which stand ready to serve incoming requests. In this way, clients do not need to wait for a new threads or processes to be created before their requests can be served. The number of processes that will initially launched is set by the StartServers directive. During operation, Apache assesses the total number of idle threads in all processes, and forks or kills processes to keep this number within the boundaries specified by MinSpareThreads and MaxSpareThreads. Since this process is very self-regulating, it is rarely necessary to modify these directives from their default values. The maximum number of clients that may be served simultaneously (i.e., the maximum total number of threads in all processes) is determined by the MaxClients directive. The maximum number of active child processes is determined by the MaxClients directive divided by the ThreadsPerChild directive

Event MPM(This MPM is experimental, so it may or may not work as expected):

The event Multi-Processing Module (MPM) is designed to allow more requests to be served simultaneously by passing off some processing work to supporting threads, freeing up the main threads to work on new requests. It is based on the worker MPM, which implements a hybrid multi-process multi-threaded server. Run-time configuration directives are identical to those provided by worker.

To use the event MPM, add --with-mpm=event to the configure script's arguments when building the httpd.

Worker MPM:
StartServers 35
MaxClients 256
MinSpareThreads 30
MaxSpareThreads 305
ThreadsPerChild 255
MaxRequestsPerChild 0

Prefork MPM:

StartServers 20
MinSpareServers 50
MaxSpareServers 100
MaxClients 200
MaxRequestsPerChild 20000

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