I thought Google was still using Borg[1] internally but Kubernetes is being built with the lessons learned from Borg. From that paper:

>Now, all Borg tasks run inside a Linux cgroup-based resource container...

I don't work for Google so this is just a guess, but you could probably ping @kelseyhightower on twitter and get a real answer pretty quickly.

[1] http://research.google.com/pubs/pub43438.html