[ruby-core:118357] [Ruby master Bug#20462] Native threads are no longer reused

Issue #20462 has been updated by maciej.mensfeld (Maciej Mensfeld).


byroot (Jean Boussier) wrote in #note-5:
> > Not sure if related
> 
> It is. The thread cache was to reuse short lived thread and avoid all these allocations etc. Note however that it wasn't enabled on all platforms, so relying on it never really was a good idea.

I myself do not rely on this explicitly. It came up during a deeper investigation related to high memory usage / potential memory leaks of 3.3.x. We've been postponing the deployment of 3.3 due to much higher memory usage than 3.2 and this is one of the things that came up. As of now I am trying to identify short-lived thread usage amongst other Ruby OSS gems.

----------------------------------------
Bug #20462: Native threads are no longer reused
https://bugs.ruby-lang.org/issues/20462#change-108863

* 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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