Following the instructions found here I modified my
/etc/default/grub with this line:
on a kubuntu 14.04 and a kubuntu 15.04 machines. The reason was trying to limit RAM usage of a dockerized Chrome (see here).
This caused a blank screen with only the cursor after logging in on the kubuntu 15.04 machine, as described here. I tried purging the nVidia drivers but it didn't work.
The only solution was to remove the line, remove the ~/.kde folder and restart.
Why is that? Is it possible to limit a process resources? Does this approach really conflict with Plasma 5 for some reason or I am missing something?