It doesn't really scale. Kernel level threads are quite expensive in the terms of memory. Erlang's processes take up few hundred bytes. It's rare to have dozen thousands of kernel-level threads running on Linux, while it's quite common for Erlang servers to have that many processes.

Anyone know if this is better in Alpine Linux with musl libc and smaller stacks?

It's not really up to a distribution to lessen this cost. Large part of it is what task descriptor in the kernel takes.

Also, if nothing else, you'll run out of PID numbers, as they're usually still 16 bits, even today, though there was a kernel compilation option to change that, from what I remember.

It can be changed at runtime. From proc(5), system wide limits:

/proc/sys/kernel/pid_max

> PID_MAX_LIMIT, approximately 4 million

/proc/sys/kernel/threads-max

> FUTEX_TID_MASK (0x3fffffff), [approximately 1 billion]

For per-process limit, increase RLIMIT_NPROC.

> Large part of it is what task descriptor in the kernel takes.

4K for kernel stacks, only 2K with future work. That's really not much space at all if you're doing anything interesting with those threads.

> Also, if nothing else, you'll run out of PID numbers, as they're usually still 16 bits, even today

Not for a very long time.

I wrote a server application that runs with about a million goroutines and performs quite well. It's not a webserver, but it responds to "requests" within hundreds of micros. Surely this would not be possible with OS threads.