[ruby-core:120132] [Ruby master Misc#16124] Let the transient heap belong to objspace

["ko1 (Koichi Sasada) via ruby-core" <ruby-core@ml.ruby-lang.org>]

Issue #16124 has been updated by ko1 (Koichi Sasada).

Status changed from Assigned to Rejected

transient heap was removed.

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Misc #16124: Let the transient heap belong to objspace
https://bugs.ruby-lang.org/issues/16124#change-110882

* Author: methodmissing (Lourens Naudé)
* Status: Rejected
* Assignee: ko1 (Koichi Sasada)
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As per comment from Nobu in https://github.com/ruby/ruby/pull/2303#issuecomment-523248875 , I took an initial stab @ a tighter integration between objspace and the transient heap in https://github.com/ruby/ruby/pull/2400

### Benefits

* Multi-VM (MVM) friendly - ( vm -> objspace -> theap )
* The 32MB (current size) arena lazy allocated on ruby init is now properly freed on shutdown as well
* It feels strange that the evacuation from the current global theap is to objspace, whereas the space evacuated from is a global arena.

### Not so great

* A fast reference to a global variable `global_transient_heap` becomes a function call to `rb_objspace_get_theap()` and related pointer chasing from vm -> objspace -> theap
* Some internal transient heap structs moved to the header file now leaks into all other reference sites where this source file (`transient_heap.c`) as previously just used for API
* I'm not sure exactly of the boundary Koichi had in mind for the GC compile module and how tightly it should (or shouldn't) be coupled to the transient heap. `struct rb_objspace*` declarations elsewhere for example reveals nothing about the structure members for example, whereas with this PR a lot of transient heap internals are exposed via the header file now
* Also possible to move `transient_heap.c` into `gc.c` - I feel theap is not an experimental feature anymore and has been stable for quite some time with plausible performance benefits. The downside of that is `gc.c` is quite dense already, but then all ruby heap management concerns belong to one compile unit.

In a similar vein the global method cache could perhaps belong to the VM instance as well, effectively better alignment with MVM and also easier to have a balanced VM setup and teardown sequence without anything left dangling on ruby shutdown.

Thoughts?



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