[ruby-core:123643] [Ruby Bug#21560] RUBY_MN_THREADS=1 causes large performance regression in Puma 7

Issue #21560 has been updated by nagachika (Tomoyuki Chikanaga).

Backport changed from 3.2: DONTNEED, 3.3: REQUIRED, 3.4: DONE to 3.2: DONTNEED, 3.3: DONE, 3.4: DONE

ruby_3_3 commit:609f957ebede1a062b1f34515382e4c306f77444 merged revision(s) commit:ffc69eec0a5746d48ef3cf649639c67631a6a609, commit:0531fa4d6fea100f69f0bac9e03973fe49ecd570.

----------------------------------------
Bug #21560: RUBY_MN_THREADS=1 causes large performance regression in Puma 7
https://bugs.ruby-lang.org/issues/21560#change-115027

* Author: schneems (Richard Schneeman)
* Status: Closed
* Assignee: ractor
* Backport: 3.2: DONTNEED, 3.3: DONE, 3.4: DONE
----------------------------------------
## Expected

Running a webserver with `RUBY_MN_THREADS=1` will increase performance or keep it the same.

## Actual

- Before: 22919.85 Requests/sec
- After:   2287.13 Requests/sec

## Reproduction

Tracking this at https://github.com/puma/puma/issues/3720. I would like to get a smaller reproduction, but until then here's how we're able to induce the failure:

Prepare puma:

```
$ git clone https://github.com/puma/puma
$ cd puma
$ git checkout v7.0.0.pre1
$ rake compile
```

Boot the server to deliver a "hello world" response:

```
$ bundle exec ruby -Ilib bin/puma -w 1 -t1 --preload test/rackup/hello.ru
```

Exercise the server with `wrk` (brew install wrk):

```
$ wrk -H 'Host: tfb-server' -H 'Accept: text/plain,text/html;q=0.9,application/xhtml+xml;q=0.9,application/xml;q=0.8,*/*;q=0.7' -H 'Connection: keep-alive' --latency -d 15 -c 16 --timeout 8 -t 12 http://localhost:9292
```

### Results

Before:

```
$ bundle exec ruby -Ilib bin/puma -w 1 -t1 --preload test/rackup/hello.ru
```

```
$ wrk -H 'Host: tfb-server' -H 'Accept: text/plain,text/html;q=0.9,application/xhtml+xml;q=0.9,application/xml;q=0.8,*/*;q=0.7' -H 'Connection: keep-alive' --latency -d 15 -c 16 --timeout 8 -t 12 http://localhost:9292
Running 15s test @ http://localhost:9292
  12 threads and 16 connections
  Thread Stats   Avg      Stdev     Max   +/- Stdev
    Latency   786.74us    1.45ms  45.57ms   90.62%
    Req/Sec     1.92k   188.55     4.20k    83.22%
  Latency Distribution
     50%  386.00us
     75%  523.00us
     90%    2.11ms
     99%    4.63ms
  344638 requests in 15.04s, 25.23MB read
Requests/sec:  22919.85
Transfer/sec:      1.68MB
```

After: 

```
$ env RUBY_MN_THREADS=1 bundle exec ruby -Ilib bin/puma -w 1 -t1 --preload test/rackup/hello.ru
```

```
$ wrk -H 'Host: tfb-server' -H 'Accept: text/plain,text/html;q=0.9,application/xhtml+xml;q=0.9,application/xml;q=0.8,*/*;q=0.7' -H 'Connection: keep-alive' --latency -d 15 -c 16 --timeout 8 -t 12 http://localhost:9292
Running 15s test @ http://localhost:9292
  12 threads and 16 connections
  Thread Stats   Avg      Stdev     Max   +/- Stdev
    Latency    28.89ms   40.82ms 129.37ms   78.94%
    Req/Sec   207.34     64.09   252.00     70.70%
  Latency Distribution
     50%  212.00us
     75%   56.77ms
     90%   99.48ms
     99%  127.01ms
  34365 requests in 15.03s, 2.52MB read
Requests/sec:   2287.13
Transfer/sec:    171.45KB
```

(Notice the dramatic drop in requests per second).



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