Since i faced issues with apache2 processes eating all my memory within short time, i found a workaround like this cronjob:
0,15,30,45 * * * * [ -x /usr/lib/php5/maxlifetime ] && [ -d /var/lib/php5 ] && find /var/lib/php5/ -depth -mindepth 1 -maxdepth 1 -type f -cmin +$(/usr/lib/php5/maxlifetime) ! -execdir fuser -s {} 2>/dev/null \; -delete
Before my RAM load is 2GB and after cronjob ran, it's 500MB load => 1.5GB free.
The cronjob is set to run on 0,15,30,45 every hour. Isn't there a proper solution which keeps the apache2 in check to avoid running out of memory within any 15min?
I don't see any issues here; it certainly doesn't look like you're running out of memory, with only 608MB used. I suspect you need to read Linux Ate My RAM.
See also this question and answer.