[ruby-core:108925] [Ruby master Misc#18830] Remove MJIT worker thread

Issue #18830 has been reported by k0kubun (Takashi Kokubun).

----------------------------------------
Misc #18830: Remove MJIT worker thread
https://bugs.ruby-lang.org/issues/18830

* Author: k0kubun (Takashi Kokubun)
* Status: Open
* Priority: Normal
----------------------------------------
## Proposal

* Remove MJIT worker thread
  * Compiling ISeq to C should synchronously happen in Ruby threads
  * Unix platforms asynchronously wait for a C compiler process to finish using SIGCHLD, and MSWIN synchronously waits for a C compiler process to finish.

Note: The architecture is similar to what we did for the timer thread [Misc #14937]. Removing the MJIT worker thread using SIGCHLD is what @normalperson proposed before too.

## Motivation

* It's hard to maintain a JIT compiler if it can't touch Ruby VM. Many C functions in CRuby rely on the VM behind the scenes.
* It's hard to maintain a JIT compiler if it can't trigger GC. Not only you cannot use `xmalloc`, but also you have to fight race conditions with GC and SEGV caused by them.

## Assumptions

* The bottleneck of MJIT compilation is the time taken to wait for a C compiler process to finish. Thus compiling ISeq to C will not be a bottleneck. Potentially, this cost could be insignificant even if we rewrite the entire ISeq -> C compiler in Ruby.
* Nobody really uses MJIT on MSWIN. If the maintenance cost becomes too high because of the MSWIN-specific implementation introduced by this change, it's okay to just remove the support.

---

Patch: https://github.com/ruby/ruby/pull/6006

The motivation for this change is not about improving performance by itself, but this makes it slightly faster.

```
$ benchmark-driver benchmark.yml --rbenv 'before --mjit;after --mjit' -v --repeat-count=12 --output=all --alternate
before --mjit: ruby 3.2.0dev (2022-06-15T05:42:09Z master 46706e7808) +MJIT [x86_64-linux]
after --mjit: ruby 3.2.0dev (2022-06-15T07:21:31Z no-mjit-worker 90433da34d) +MJIT [x86_64-linux]
Calculating -------------------------------------
                                before --mjit          after --mjit
Optcarrot Lan_Master.nes    125.3789474179577     129.6361842567196 fps
                            125.7892049665125     130.1237905211524
                            126.3359252053467     131.3352551857975
                            127.2303480033240     131.5128102697965
                            127.2368143596796     131.9221842055012
                            127.3938219177180     131.9712318537562
                            127.8370994014408     132.0527716303878
                            127.9904178998168     132.4032777876095
                            128.0631040346902     132.4425814311906
                            128.3016029706804     132.5687514040249
                            128.8733290749460     132.7843477576762
                            129.5397101458566     133.4416009274654
```



-- 
https://bugs.ruby-lang.org/

Unsubscribe: <mailto:[e-mail]?subject=unsubscribe>
<http://lists.ruby-lang.org/cgi-bin/mailman/options/ruby-core>