[HACKERS] max_files_per_processes vs others uses of file descriptors

Hi,

currently the default max_files_per_process is 1000. fd.c uses close to
that many (- NUM_RESERVED_FDS/10). count_usable_fds() makes sure that at
server start there's at most that many fds available, but that doesn't
mean that much for runtime.

These days there's a number of other consumers of
fds. E.g. postgres_fdw, epoll, ...  All these aren't accounted for by
fd.c.

Given how close max_files_per_process is to the default linux limit of
1024 fds, I wonder if we shouldn't increase NUM_RESERVED_FDS by quite a
bit?

Greetings,

Andres Freund