[ruby-core:118340] [Ruby master Bug#20462] Native threads are no longer reused

Issue #20462 has been updated by byroot (Jean Boussier).

Backport changed from 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN to 3.1: DONTNEED, 3.2: DONTNEED, 3.3: UNKNOWN

be1bbd5b7d40ad863ab35097765d3754726bbd54 very explicitly removes the cache. What's unclear is whether it was intended, or if the intent was to reimplement it differently and it was forgotten.

@ko1 could you let us know what the intent was?

----------------------------------------
Bug #20462: Native threads are no longer reused
https://bugs.ruby-lang.org/issues/20462#change-108846

* Author: tenderlovemaking (Aaron Patterson)
* Status: Open
* Backport: 3.1: DONTNEED, 3.2: DONTNEED, 3.3: UNKNOWN
----------------------------------------
Ruby used to reuse native threads in order to amortize the cost of making a pthread.

For example this program:

```ruby
ntids = 1000.times.map {
  Thread.new {
    Thread.current.native_thread_id
  }.value
}

p ntids.uniq.length
```

With Ruby 3.2.0, this would return 1.  With Ruby 3.3.x, it returns 1000.  It means we cannot amortize the cost of a pthread for short lived threads.

I was able to bisect this to commit be1bbd5b7d40ad863ab35097765d3754726bbd54.  But the change is big so I don't know how to fix it.



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