I had similar problem, memory was increasing during the day and after one day almost everything was exhausted, even swap...
I run the next command on my Linux (centos 3.x):
ps -eo pid,ppid,rss,vsize,pcpu,pmem,cmd -ww --sort=pid
and found that I have tons of process like this:
/usr/bin/python2.2 /usr/local/psa/admin/sbin/agent_runner xxxxxxxxx
where xxx my local path to tmp folder for backups..
I instantly reminded that I also have problem with backup - could not backup even through plesk panel.
I have croned backup on each domain (~20)
The solutions for backup was to run chmod 777 on:
/var/lib/psa/dumps and
/var/lib/psa/dumps/tmp
Backup started to work.
Then I cleaned up
/var/lib/psa/dumps/tmp from a few thousands of files - as I understood plesk tried to continue after failure
So my theory is backup agent added every night even more unfinished backups to tmp directory (since they never were finished).
The backup agent on regular base looked at the tmp directory and tried with backups, that caused hang them in the memory...
I hope that it is over now - I mean memory leak