Replace Trixi.True() by Trixi.Threaded()

[JoshuaLampert]

Split out of #2910.

[github-actions]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.13%. Comparing base (bb36217) to head (af92b27).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2997   +/-   ##
=======================================
  Coverage   97.13%   97.13%           
=======================================
  Files         625      625           
  Lines       48514    48514           
=======================================
  Hits        47122    47122           
  Misses       1392     1392           

Flag	Coverage Δ
unittests	97.13% <ø> (ø)

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[vchuravy]

Technically a breaking change?

[JoshuaLampert]

Technically a breaking change?

Yes, we could also name it True again, but then it would be inconsistent with newer versions of OrdinaryDiffEq.jl. So probably it's better to put into a breaking release.

[ranocha]

I think this is not breaking from our side but from OrdinaryDIffEq.jl. Do they export their Threaded type so that we can replace Trixi.Threaded() by something coming from th OrdinaryDiffEq packages? It is not really clean to use something from Trixi.jl for an API created by the DiffEq ecosystem.

[JoshuaLampert]

No, it is not exported, but only used from FastBroadcast.jl here.

[vchuravy]

From the weekly meeting:

It would make sense to take ownership of this, instead of punting the problem to FastBroadcast/OrdinaryDiffEq.

As an example, we already have a preference for the threading backend, and it would make sense to have a function Trixi.Parallel() (bad name) that looks at the preference. That way we are not reexporting something from FastBroadcast and avoid the scenario where someone turned off Polyester for Trixi, but is still passing Threaded()

[JoshuaLampert]

Ok, could you take over the PR then @vchuravy (sorry I couldn't make it to the meeting today)?