[ekimekim]

I'm referring specifically to disk IO, which on linux using standard read(2) and write(2) is (almost) always blocking. What you describe is true of socket fds and some other things, but on most systems a file read/write which goes to a real disk will never return EAGAIN.

This is why systems like aio[1] exist, though afaik most systems tend to solve this with a thread pool rather than aio, which can be very complicated to use properly.

[1] http://man7.org/linux/man-pages/man7/aio.7.html

[vertex-four]

Ah, absolutely, I forgot that the state of disk IO on Linux is terrible - although this still isn't quite the case, since there's a network socket involved in copying from disk to socket, so if the socket's buffer becomes full the scheduler will run.

It seems that nginx can use thread pools to offload disk IO, although doesn't unless configured to - by default disk IO will block the worker process. And FreeBSD seems to have a slightly better AIO system it can use, too.