> Python threads are basically "green threads"
That's not accurate.
> they have their place but aren't related to parallelisation
You can parallelize all sorts of things with Python threads--just not some things you'd expect to be able to parallelize, due to the GIL. Waiting on or buffering I/O, calling out to compiled code, doing cryptographic operations--all of those can be parallelized, as (in many cases) they entail releasing the GIL.
> But true multiprocessing is ugly when you have hundreds of cores
Why?
> There is no standard UI convention on most OSes to group those processes per app, in terms of signals or stats or whatever.
I have no idea what this means. What do UIs have to do with process groups? Do you know how many processes your Chrome instance is running on the operating system? There are very solid conventions regarding process management, at least on Unix-ish systems: process groups and parent-child relationships are well established and well understood, as is their relationship with signals and signal handling.