How does GHC runtime deal with file I/O?

Question

Some papers on GHC runtime internals mention that it uses epoll/kqueue/poll to detect whether a file descriptor is ready to read/write.

I can understand how it's done for socket I/O. But what about disk file access? The poll syscall doesn't work with ordinary files, only with socket I/O; true?

The only option I can imagine here is using a thread pool for blocking syscalls, one thread-per-request...

Answer 1

In the unthreaded RTS, the whole runtime will block. In the threaded RTS, it will do safe foreign calls like this via thread pool, so the capability will not block.