CentOS in the top500 supercomputers
Browsing through the top500 supercomputers list, I noticed that in the OS listing, 5 supercomputers are running specifically CentOS (1%) while 389 are running some sort of Linux (not specified).
From the Linux list undoubtedly more are using CentOS, but the remarkable fact is that this known 1% CentOS is the same amount as the 5 Windows supercomputers.
So if we assume from the 389 Linux supercomputers, more are using CentOS, CentOS outnumbers Windows for supercomputers. We simply don't know by what factor.
If only more organisations would be more specific to what exactly they are running.