From: "jhawthorn (John Hawthorn) via ruby-core" <ruby-core@...>
Date: 2024-05-01T22:54:48+00:00
Subject: [ruby-core:117747] [Ruby master Bug#20462] Native threads are no longer reused

Issue #20462 has been updated by jhawthorn (John Hawthorn).


In a benchmark of thread creation we see that 3.3 is slower than 3.2

```
jhawthorn@zergling:~   [ruby 3.2.2]
$ time ruby --disable-gems -e '100_000.times { Thread.new{}.join }'
ruby --disable-gems -e '100_000.times { Thread.new{}.join }'  0.73s user 1.16s system 107% cpu 1.751 total

jhawthorn@zergling:~   [ruby 3.3.1]
$ time ruby --disable-gems -e '100_000.times { Thread.new{}.join }'
ruby --disable-gems -e '100_000.times { Thread.new{}.join }'  0.55s user 1.67s system 70% cpu 3.126 total
```

----------------------------------------
Bug #20462: Native threads are no longer reused
https://bugs.ruby-lang.org/issues/20462#change-108157

* Author: tenderlovemaking (Aaron Patterson)
* Status: Open
* Backport: 3.1: UNKNOWN, 3.2: UNKNOWN, 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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