For all of the source code in the world that is source-controlled, it exists in separate repos. It doesn't exist in one repo, and where there are inter-project dependencies, it is the dependency consumer's responsibility to keep abreast of the changes in the dependency and integrate them as necessary. It is self-organizing.

So, if Google and Facebook have grown beyond what can be done with a single repo, either they are not being mindful of their engineering practices (which puts the public at significant risk), or they are approaching a scale that is more similar to "world scale" than it is "corporate scale".

Which I would also say is putting the public at significant risk.

that is the problem - single logical repo in Perforce/Mercurial does mean single physical repo. Which obviously causes issues at very large scale. Even at normal enterprise scale :)

Git solves it through kind of distributed scaling where many operations can be performed on local physical repos without ever hitting the central. With centralized solutions, like Perforce, Mercurial, etc... most of the operations are performed on central server and your ability to scale vertically hits the ceiling pretty soon.

Compare the simplest case - each dev updating his local workspace/repo would hit the central server in Perforce where is with Git you can (and normally would have) a small set of downstream repos which the updates would be propagated/distributed through. You can branch left and right in Git in your local and team's repos without "master" repo and "central" server involved (while all your branch activity in Perforce is happening right on the central server in the central repo). The same happens for synchronizing your work inside the team - no need to hit the central. Etc...

my bad. 5+ years since i worked with Mercurial. Digging deep into painful memories, my Mercurial PTSD from that time is absence of in-repo branching - need to clone which is a killer for very large repo we had and no partial commits - again a killer aggravated by the above mentioned absence of in-repo branches. Both issues made working with large repo unreasonably and unnecessary hard.

These days, the standard practice is probably to use bookmarks, which are more or less like Git branches.

No. You have the choice to use either named branches or bookmarks, and which one you choose is a matter of your workflow. Note that even a Git-like workflow does not necessarily require the use of bookmarks.

Mercurial is still designed under the assumption that people will use named branches and the Git-like aspects of bookmarks exist to ease transition from Git: http://www.selenic.com/pipermail/mercurial/2014-January/0464...

Personally, I use Fossil-like private branches [1] in order to facilitate local work that I may still want to reorganize before pushing it to a shared server; I use bookmarks solely for local tags.

[1] Emulated by putting a named branch in the secret phase. I actually have a simple extension to automatically make all branches that start with a dot (e.g, ".tempstuff") secret.

What do you mean by partial commit? I can guess but I would rather know. :-)

I think you're incorrectly putting mercurial and perforce in the same bucket. Mercurial is a lot more like Git than perforce in that it's a DVCS. Facebook made it scale by using a lot of centralized caching, but most operations (diff, looking through history, branching, etc) can be done locally.