Why HN was slow and how Rtm fixed it

> when [MzScheme] wants to find a thread to run, it asks the O/S kernel about each socket in turn to see if any input is ready

They've never heard of select()? </snark>

But really, is there some reason that it's hard to collect up all the fds at once or something?

Read C10K? Both select() and poll() have this problem internally. You have to use one of the more advanced techniques available if you really want to scale. epoll(), kqueue() or friends.