select on kqueue
Anyone who has written a large sockets app knows that select is really slow with a lot of file descriptors. Various operating systems have solved this in incompatible ways. For example, FreeBSD has kqueue, linux has epoll, solaris has /dev/poll, etc. Apple also has kqueue, as it is somewhat FreeBSD derived. Rather than have duplicate interfaces in the kernel for eventing, they wrapped it all up together.
Select is implemented by creating a kqueue per thread and adding fds from the select bits to the kqueue event set. They actually persist the events and cache the old bits and then only do updates based on the delta. That means if your application is passing in a relatively static set of bits you get the scalability of kqueue! See
http://www.kegel.com/dkftpbench/Poller_bench.html for an old benchmark on kqueue vs epoll vs select vs dev poll.
We're presently trying to get patches out of apple rather than re-invent the wheel here. It has a secondary advantage; kqueue does not have any global locks in the FreeBSD kernel while select does due to historical implementation issues. Fixing the select locking makes it internally just like kqueue except without persistence.
Overall, I think it's very clever and quite exciting.
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