[ruby-core:123193] [Ruby Bug#21560] RUBY_MN_THREADS=1 causes large performance regression in Puma 7

Issue #21560 has been updated by jhawthorn (John Hawthorn).


I suspect this might be the issue: it seems like under M:N threads sleeps < 10ms are rounded up.

``` ruby
Thread.new do
  5.times do
    before = Process.clock_gettime(Process::CLOCK_MONOTONIC)
    sleep 0.0001
    after = Process.clock_gettime(Process::CLOCK_MONOTONIC)
    puts(after - before)
  end
end.join
```

```
$ RUBY_MN_THREADS=1 ruby test.rb
0.010995999909937382
0.011000000406056643
0.011068999767303467
0.0102849998511374
0.011036999989300966

$ ruby test.rb
0.0001360001042485237
0.00013200007379055023
0.00013400008901953697
0.00013200007379055023
0.00013299984857439995
```

----------------------------------------
Bug #21560: RUBY_MN_THREADS=1 causes large performance regression in Puma 7
https://bugs.ruby-lang.org/issues/21560#change-114524

* Author: schneems (Richard Schneeman)
* Status: Open
* Assignee: ractor
* Backport: 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: UNKNOWN
----------------------------------------
## 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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