"Each task maps to a set of Linux processes running in a container on a machine . The vast majority of the Borg workload does not run inside virtual machines (VMs), 1 There are a few exceptions for each of these relationships. because we don't want to pay the cost of virtualization. Also, the system was designed at a time when we had a considerable investment in processors with no virtualization support in hardware."
Source: The borg paper.
Google reveling a little of what it was up to a few years ago. The rest of the industry is looking in awe and quietly taking notes,
EDIT: I guess that what didn't make sense to me (before looking at the paper) didn't make sense to Google either ;)