[badosu]

I was fascinated when I first learned of [FreeBSD Jails], I wonder if right before containerization became a thing the concept was developed further for its requirements (could it have been?) it would have offered a more efficient containerization platform.

FreeBSD Jails: https://docs.freebsd.org/en/books/handbook/jails/

[eqvinox]

Jails are entirely different conceptually from UML; they share the host kernel and are roughly analogous to containers/namespaces. UML is an entirely separate kernel, running as user mode process.