Facebook rewrote Mercurial, while Microsoft has essentially expanded git with their own virtual file system VFSforGit [0] and a bunch of performance improvements.
The official reason is that the "internals of Git" weren't conduce to the kinds of invasive changes they needed/wanted. But I think the truth is closer to being that it was going to be too hard/slow to get those invasive changes past the Git mailing list.
Here's an example of a FB eng reaching out to the mailing list: http://git.661346.n2.nabble.com/Git-performance-results-on-a...
> Have you considered upgrading all of engineering to SSDs? 200+GB SSDs are under $400USD nowadays.
The mailing list piece is from 2012, and describes how git is very slow on a synthetic repo with millions of files and commits. Today, my current place of work has a monorepo that’s approaching the size described in this mailing list, but git seems to be holding up just fine. If you checkout a branch that’s far enough away from master it takes a minute, but add, rebase, commit, status and blame are all negligibly impacted speed-wise. The only issue we run into is rejected non-conflicting pushes to master during peak hours, with maybe several dozens of engineers trying to merge and push master simultaneously.
Does anybody have any insight into what’s changed in git internally since 2012 to support bigger repos?
Over 8 years and tens of those changes, it adds up to a significant performance improvement.
Which is about the same thing, mercurial was built to be at least somewhat pluggable, so facebook could build their extensions independently, and work to get a subset of them integrated into mainline. Git is designed so it can be built on top of, but not really under or within.
At a technical level I believe both Google and Facebook engineers make core changes to mercurial when they need to so I don’t think that’s the root philosophical difference.
Funny, but I'm starting to wonder if there's an affect-based complement to Conway's Law.
Google interestingly also still contributes to mercurial[0], but I don't think has said why externally officially.
But yes, my understanding is that mercurial is much easier to replace parts of the flow without entirely hacking the codebase to bits. Microsoft's solution for git required them to fork the code base, which I still don't think has been fully merged with upstream yet? And as others said, the mercurial community was more open to enterprise contributing things than git can be.