Growing number of open files, even on an idle system

Good evening everyone.

Yesterday I ran into a super slow and non-responsive system, and then managed to get into the already running console. However, no matter whatever I tried to do, it kept on complaining about too many open files. I had to kill a few other programmes to finally get hold of my system.

Today I decided to run a test. So ran lsof in the morning, and then left the machine as it is. No other programme, no other activity. Then ran lsof a while back. the difference is quite interesting.

lsof-timestamp # of open files
lsof_20200407-072000 2157
lsof_20200407-191152 9808

The number jumped from 2157 to 9808 in matter of hours in a “not-actively-in-use” system. Further investigation indicates this was by the unionfs process. Just before hitting the “create topic” button, another check on unionfs yielded
% sudo lsof | grep -i unionfs | wc -l

Anyone know of any functional way to keep it under check?

Best regards.