[ruby-core:118352] [Ruby master Bug#20462] Native threads are no longer reused

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


Not sure if related but here's a benchmark of the impact (written by Gordon Chan):

```ruby
#!/usr/bin/env ruby

# Disable GC during the benchmark
GC.disable

# Get the current memory usage
mem_before = GC.stat[:malloc_increase_bytes]
obj_before = GC.stat[:total_allocated_objects]

# Perform the operation you want to benchmark
result = 100_000.times { Thread.new{}.join }

# Get the new memory usage and object count
mem_after = GC.stat[:malloc_increase_bytes]
obj_after = GC.stat[:total_allocated_objects]

# Re-enable GC
GC.enable
GC.stress
sleep(5)

# Calculate the memory and object allocation deltas
mem_delta = mem_after - mem_before
obj_delta = obj_after - obj_before

puts "Memory allocated: #{mem_delta} bytes"
puts "Objects allocated: #{obj_delta}"
```

and the difference:

```
# 3.1.3
Memory allocated: 46402080 bytes
Objects allocated: 700009

# 3.2.3
Memory allocated: 49603216 bytes
Objects allocated: 700009

# 3.3.3
Memory allocated: 104926404048 bytes
Objects allocated: 700010
```

3.1 and 3.2 are consistent, while 3.3 is as you can see above (2000x).

----------------------------------------
Bug #20462: Native threads are no longer reused
https://bugs.ruby-lang.org/issues/20462#change-108857

* 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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