Summary: Just as surely as Microsoft rules the desktop with Windows and Apple rules tablets with the iPad so Linux rules supercomputers of every type and sort.
Once more, Linux kicks rumps and takes names when it comes to supercomputers.
A few decades back I was working at Goddard Space Flight Center. I’m sorry to say that I left just before some people I’d met, Don Becker and Thomas Sterling, built the first Linux cluster, Beowulf. They didn’t know it, but by making a cheap cluster from 16 486DXs processors and 10Mbps Ethernet, they were creating the ancestor to today’s Linux supercomputers. Now, not 20 years later, well over 90% of the world top 500 supercomputers are running Linux.