Apache 2 mpm-itk and mpm-prefork modules together

We use Apache a lot in a reverse proxy configuration and used to see memory leaks which were noticeable as the server only had 512MB RAM.

I don't know if we had a semaphore-related leak or something else, but we cured it by reducing the MaxConnectionsPerChild directive to one quarter of the default i.e. 2500. At our traffic levels this meant child processes were recycled about twice an hour, which cured the problem. There was no noticeable performance impact.

The problem looks to be that you have these installed as packages:

apache2-mpm-event 2.4.7-1ubuntu4.9 amd64 transitional event MPM package for apache2
apache2-mpm-prefork 2.4.7-1ubuntu4.9 amd64 transitional prefork MPM package for apache2

When you update, you are specifically telling the server to install the prefork version. If you have a proper install of Apache2 on Ubuntu 14.04, which would NOT have specified the MPM type, a dpkg -l | grep apache2 would include:

apache2 2.4.7-1ubuntu4.9 amd64 Apache HTTP Server

but would NOT show the two packages you have listed.

The proper Apache2 installation on Ubuntu 14.04 is simply:

sudo apt-get install apache2

without any specification of MPM. The basic Multi-Processing Modules are core features of Apache 2.4, and are included with the apache2 install.

You then set MPM as follows:

To determine which MPM is currently in use, run apache2ctl -V. You will see a line such as:

 Server MPM:     prefork

Assuming, for example, that you are running "prefork" (as in the above example apache2ctl -V result above) following the install, the switch to "event" is made with the following commands:

 sudo a2dismod mpm_prefork
 sudo a2enmod mpm_event
 sudo service apache2 restart

which will set up the correct symlink.

Once set up this way, under the correct install, updates WILL NOT change your settings.

NOTE: Your "edits" to *.conf files should be taking place in the /etc/apache2/mods-available directory, where they are protected from over writing during updates. The /etc/apache2/mods-enabled directory will just have symlinks back to /etc/apache2/mods-available.