Hosting the kernel git repositories has many more implications than simply managing a raw number of contributors. The kernel repostiory and source is much, much larger than Ruby on Rails.

in which sense? I have yet to see an argument why it would be more of a "test of GH's infrastructure" than RoR.

The kernel source has about 10 times more files. The total filesize is about 10 times larger as well.

  linux-3.1-rc3$ ls -R -l |wc -l
  46855
  linux-3.1-rc3$ du -h -s
  500M

  rails$ ls -R -l |wc -l
  4141
  rails$ du -h -s
  59M	.

If you mean hacking on it using github, sure. Otherwise, https://lwn.net/Articles/451243/

yes, I meant hacking on it when I said "hacking on it".

It definitely does, Linux is used by far more people than Ruby on Rails is.

I'm not sure that usage indicates even rough numbers of hackers. Hacking OS code is a bit more daunting than hacking RoR code (at least in popular opinion), therefore I would think that more users of RoR are likely to hack on it.

That said, Linux is a huge project with tons of contributors.

I suspect it's likely to be:

RoR: Large number of contributors with relatively small patches, few full time contributors who are paid just to work on RoR.

Linux: Possibly smaller number of overall contributors but supplying more patches each, and more patches in total. More contributors who are paid to work on the kernel on a full or part time basis.

The numbers for the kernel are well known (and they are hard to beat for any open source projects): https://lwn.net/Articles/395961/ (for every version ~1.1k devs and around 10k changesets)

Indeed. I don't recall the exact numbers, though I've heard both Linux and RoR numbers, I think that Linux has factors more developers committing to it than RoR.

Linux is, perhaps, the most rapidly changing piece of software in existence