They could have done it using C, Ruby, Python, or any other language. kqueue is what makes FreeBSD (and OSX) awesome at concurrency.
The main advantage Erlang has over C/Python/Ruby/etc. is that asynchronous IO is the default throughout all its libraries, and it has a novel technique for handling errors. Its asynchronous design is ultimately about fault tolerance, not raw speed. Also, it can automatically and intelligently handle a lot of asynchronous control flow that node.js makes you manage by hand (which is so 70s!).
You can make event-driven asynchronous systems pretty smoothly in languages with first class coroutines/continuations (like Lua and Scheme), but most libraries aren't written with that use case in mind. Erlang's pervasive immutability also makes actual parallelism easier.
With that many connections, another big issue is space usage -- keeping buffers, object overhead, etc. low per connection. Some languages fare far, far better than others here.
But as awesome as kqueue is, OS X apparently broke it: http://pod.tst.eu/http://cvs.schmorp.de/libev/ev.pod#OS_X_AN...
Here's a more interesting bug in OSX: kqueue will sometimes return the wrong number for the listen backlog for a socket under high load.
That is more akin to AIO Linux system, then? Otherwise, epoll/poll/select just notifies users when data is available but the actual copy is done by the user. Surprisingly this can make a huge difference when streaming large amounts of data.
http://blog.lighttpd.net/articles/2006/11/12/lighty-1-5-0-an...
We have argued here before and I have gotten downvoted into oblivion for being pedantic and distinguishing between asynchronous IO and non-blocking IO but it looks like that extra user-space memcpy can make a huge difference.
Anyway, the idea was to avoid context switches by waiting/parsing in kernel-side till there was enough data for the client to do something else that just another gimme_more_data()-call back to the kernel.
It could even be applied to other methods than kqueue, so perhaps I remember a bit wrong that this was just for kqueues.
The accept filter concept was pioneered by David Filo at Yahoo! and
refined to be a loadable module system by Alfred Perlstein.
From the cited article on ports of the libev event loop library: "The whole thing is a bug if you ask me - basically any system interface you touch is broken, whether it is locales, poll, kqueue or even the OpenGL drivers." with no particular details on what is broken in Mac OS X.
Issues with porting to AIX, Solaris and Windows are also discussed in that article, and with reports of errors, though with no specific details provided for those platforms.
Without error details, there is also insufficient information around whether alternatives or workarounds or fixes might exist, or whether there were bug reports and reproducers logged that would allow the vendors to address the (unspecified) errors.