This is a dublicate to #11338 cause i can't reopen that bug. I just reviewed the ebuild and there was no MPM-enviroment-varaibale you could export to use e.g. perchild, wich is prefer. At the moment there are many variables similar to VIDEO_CARDS or IMPUT_DEVICES popping up. Maybe a similar variable would save use-flags, but would allow you to choose all and any of the mpms. Afaik the perchild mpm is not any worse than the peruser mpm. So I think there should be an opportunity to select it too. Btw: the perchild mpm does compile on my P4-box
http://httpd.apache.org/docs/2.0/mod/perchild.html This module is not functional. Development of this module is not complete and is not currently active. Do not use perchild unless you are a programmer willing to help fix it.