portage 229754 229750 84 11:25 pts/5 00:03:39 /usr/bin/sassc sass/gtk-dark.scss gtk-dark.css portage 229757 229750 85 11:25 pts/5 00:03:40 /usr/bin/sassc sass/gtk-darker.scss gtk-darker.css both using ~13GB of memory and growing when I decided to kill them because of how they were impacting my ssystem. Reproducible: Always
I had this happen to me too. It started when dev-libs/libsass was upgraded to 3.6.3 on my system. Reverting to dev-libs/libsass-3.6.1 allows x11-themes/arc-theme-20190917 to build quickly without causing the excessive memory usage.
*** This bug has been marked as a duplicate of bug 705682 ***