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The Apache HTTP Server Reference Manual
by Apache Software Foundation
Paperback (6"x9"), 862 pages
ISBN 9781906966034
RRP £19.95 ($29.95)

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3.81  Apache Module event


An experimental variant of the standard worker MPM



Module Identifier:


Source File:



Warning 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.


AcceptMutex (p. 999)

CoreDumpDirectory (p. 1000)

EnableExceptionHook (p. 1001)

Group (p. 1002)

Listen (p. 1003)

ListenBacklog (p. 1006)

LockFile (p. 1006)

MaxClients (p. 1007)

MaxMemFree (p. 1007)

MaxRequestsPerChild (p. 1008)

MaxSpareThreads (p. 1009)

MinSpareThreads (p. 1010)

PidFile (p. 1010)

ScoreBoardFile (p. 1012)

SendBufferSize (p. 1013)

ServerLimit (p. 1013)

StartServers (p. 1014)

ThreadLimit (p. 1015)

ThreadStackSize (p. 1016)

ThreadsPerChild (p. 1017)

User (p. 1017)

See also:

  • The worker MPM (p. 1034)

3.81.1  How it Works

This MPM tries to fix the ‘keep alive problem’ in HTTP. After a client completes the first request, the client can keep the connection open, and send further requests using the same socket. This can save signifigant overhead in creating TCP connections. However, Apache traditionally keeps an entire child process/thread waiting for data from the client, which brings its own disadvantages. To solve this problem, this MPM uses a dedicated thread to handle both the Listening sockets, and all sockets that are in a Keep Alive state.

The MPM assumes that the underlying apr_pollset implementation is reasonably threadsafe. This enables the MPM to avoid excessive high level locking, or having to wake up the listener thread in order to send it a keep-alive socket. This is currently only compatible with KQueue and EPoll.

3.81.2  Requirements

This MPM depends on APR’s atomic compare-and-swap operations for thread synchronization. If you are compiling for an x86 target and you don’t need to support 386s, or you are compiling for a SPARC and you don’t need to run on pre-UltraSPARC chips, add --enable-nonportable-atomics=yes to the configure script’s arguments. This will cause APR to implement atomic operations using efficient opcodes not available in older CPUs.

This MPM does not perform well on older platforms which lack good threading, but the requirement for EPoll or KQueue makes this moot.

  • To use this MPM on FreeBSD, FreeBSD 5.3 or higher is recommended. However, it is possible to run this MPM on FreeBSD 5.2.1, if you use libkse (see man libmap.conf).
  • For NetBSD, at least version 2.0 is recommended.
  • For Linux, a 2.6 kernel is recommended. It is also necessary to ensure that your version of glibc has been compiled with support for EPoll.

ISBN 9781906966034The Apache HTTP Server Reference ManualSee the print edition