Quoting https://www.open-mpi.org/doc/v2.0/man1/mpirun.1.php#sect22

> The Open MPI team strongly advises against executing mpirun as the root user. MPI applications should be run as regular (non-root) users.
>
> Reflecting this advice, mpirun will refuse to run as root by default. To override this default, you can add the --allow-run-as-root option to the mpirun command line.

Apparently mpich allows the user to run as root without complaining - which is an example of why openmpi is probably a more thorough community developed implementation. There are other mpi implementations too - I've no idea how many others are as careful as openmpi.

When we start preparing documentation, we could add a comment for those users who do everything as root ("noone else but me uses my computer") or those who do things in a chroot gaol. I didn't quite see where package-specific documentation would make sense, so I'm posting this here so it doesn't get forgotten.

However, I'm not fully sure about this being worth documenting - if we try documenting every possible mistake, including those easily found by duckduckgoing or qwanting, then we'll be wasting time doing what other groups/networks already do better. People should look for info in the place they expect (and preferably without totalitarian search engines). E.g. we're not going to distribute tutorials in C or fortran or shell.

Comment: In preparing Debian binary executables, chroot is normally used, since the idea is to simulate a root user installing libraries and so on. So --allow-run-as-root is enabled during after building the mpgrafic binary, so that a minimal reproducible "science run" can be tested. (An alternative could be to create a dummy user, run user-level tests, and then rm the dummy user.)