Automatically add threads when calling @spawn

[tknopp]

When starting Julia with only one thread and calling @spawn, no additional thread is created. I know that this is because the number of threads is currently static so take this as a feature request, how I would expect the system to work in an ideal world.

[mbauman]

The pre-requisite here is #16134.

[JeffBezanson]

This isn't what @spawn means; it's not for creating hardware threads but for exposing parallelism to whatever threads are available. I think this can be combined with #16350, as a request for threading features for interactivity.

[tknopp]

responded to the spawn meaning on discourse: https://discourse.julialang.org/t/threading-1-3-success-story/27111/6

[mbauman]

Is not the point here that there are program structures that demand either pre-emptive (software) threads or hardware threads and would otherwise deadlock? Whether or not we call that @spawn, do we want to support this more directly (beyond just @assert nthreads() > 1)?

[StefanKarpinski]

There's always the radical alternative of using LD_PRELOAD to replace all blocking libc calls with versions that yield back into the Julia scheduler. No threads required.

[JeffBezanson]

Is not the point here that there are program structures that demand either pre-emptive (software) threads or hardware threads and would otherwise deadlock?

Yes, of course such programs exist. Adding a pthread_create-style API would take things in a violently different direction, but I suppose it's not out of the question. However I closed this because @spawn can't be the interface to that.