[kyrra]

See my reply elsewhere in this post to the ACM article. It's custom in-house based on their own distributed databases. The front-end UI is based on perforce, but there is a Git front-end (but logically it behaves a lot like perforce).

As the ACM paper calls out, files are loaded asynchronously like on access, so you only have the files you need on your desktop. From a developer's point of view, you can see all the files in the entire repo at all times. And with how CITC (client in the cloud) works, I can move from a laptop to a desktop without any effort (all of the files I have in a working state on one are immediately available everywhere I can access CITC).

As far as complexity... how would a large company solve these problems without these large and complex systems? Google is probably one of the more efficient ways I've seen large companies work. When you want all your devs to share code, thing get hard at scale.