Well, based on Mach 2.5, which contained BSD 4.4 and Mach kernel code.
Google development is done on Linux but Mac laptops at Google run MacOS. Laptops (or chromebooks, there's a mix of both) aren't used for development (except via ssh, etc); they are used for email, web, etc.
> IIRC Google has more of the latter and fewer of the former.
Yes, you are correct. More details here:
"The best projects will have a very high number of independent work units, a high CPU to I/O ratio, and no inter-process communication (commonly described as Embarrassingly or Pleasantly Parallel). The higher the CPU to I/O rate, the better the match with the system. Programs must be developed in C/C++ and compiled via Native Client. Awardees will be able to consult an on-site engineering team."
Native Client: http://en.wikipedia.org/wiki/Google_Native_Client