Thread pool of main thread varies depending on how Julia is started

[carstenbauer]

Depending on whether Julia is started with or without interactive threads, the main thread is in either the interactive or default threadpool:

cb@mm ~
➜ julia -t 3 -E 'Threads.threadpool()'
:default

cb@mm ~
➜ julia -t 3,1 -E 'Threads.threadpool()'
:interactive

I find this surprising. Is this intentional? If so, what is the reasoning behind this?

Note that this is wrongly documented since 1.9 (I created a doc fix PR).

Came up in this issue and also seems to be causing problems (see discourse.julialang.org/t/what-is-julia-doing-with-your-threads/110052/20 and the related issue #53269).

[raminammour]

This was implemented here, and the docfix came sometime after.

[carstenbauer]

This was implemented here, and the docfix came sometime after.

Note that this docfix is orthogonal to what is discussed here. It was just fixing a statement about thread ids, specifically, their order. The relevant (yet to be merged) docfix for the present issue is #53388.

[JeffBezanson]

Yes this is intentional. Thanks for the doc fix. We will have the option of changing this at some point, but in the meantime I don't think it's so strange that if there are any interactive threads, then the main thread is one.

[carstenbauer]

I don't think it's so strange that if there are any interactive threads, then the main thread is one.

The part that is a bit more surprising is that the main thread isn't always an interactive thread. (That's also what Valentin seemed to convey with his post here.)
But ok, I take it that this is not considered to be an issue (although it's still not obvious to me why this is intentional).