[ruby-core:120510] [Ruby master Bug#21007] Ractor scheduler issue when multiple threads in a ractor

6 Jan 2025

      Issue #21007 has been reported by luke-gru (Luke Gruber).

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Bug #21007: Ractor scheduler issue when multiple threads in a ractor
https://bugs.ruby-lang.org/issues/21007

* Author: luke-gru (Luke Gruber)
* Status: Open
* Backport: 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: UNKNOWN
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When there are multiple threads in a ractor, these threads can get in a state where they are yielding every 10ms instead of every 100ms.

This occurs because in `thread_sched_switch0`, which is called by `thread_sched_switch`, `ruby_thread_set_native` is called. This function calls
`rb_ractor_set_current_ec` for the next thread to run, but then when the next thread sets itself up before it runs, it calls `rb_ractor_thread_switch`,
but since the ec has already been changed, it never sets back `th->running_time_us` to `0`.

The yielding happens every 10ms because a very large value in `th->running_time_us` is always compared to `100ms` so it always yields.

This script takes a very long time due to this issue:

```ruby
ractors = 5.times.map do |i|
  Ractor.new(i) do |i0|
    ts = 4.times.map do
      Thread.new do
        counter = 0
        while counter < 30_000_000
          counter += 1
        end
      end
    end
    until ts.none? { |t| t.alive? }
      $stderr.puts "Ractor #{i0} main thread sleeping"
      sleep 1
    end
    ts.each(&:join)
    $stderr.puts "Ractor #{i0} done"
  end
end

while ractors.any?
  r, obj = Ractor.select *ractors
  ractors.delete(r)
end
```

The fix is to set `next_th->running_time_us` back to 0 in `thread_sched_switch0`.

-- 
https://bugs.ruby-lang.org/