On an ubuntu box with 4GB, I am running apache with some mpm + mysqld. Furthermore, there is a postfix, mongod and ocassionaly some imagemagick/convert processes active.
On 99% of the days, that's fine. However, sometimes (not at peak times, but e.g. at 3am in the morning - although backup process start few hours late) the server runs out of RAM. Then it's killing processes and unfortunately it's starting with apache and/or mysqld :-(
Do I have a chance to define which process is "important" or high-priority? E.g. i'd define 1) apache 2) mysql 3) postfix 4) convert 5) mongod <- as far as i know, mongod is very polite and will release ram for other applications if needed.
Btw. the system is not running out of memory because of ghost processes; i use some cleaner scripts which make sure that unused ram is recycled :-)