Bug #21007
closed
Ractor scheduler issue when multiple threads in a ractor
Description
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:
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.
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