Issue #21007 has been updated by jhawthorn (John Hawthorn). Status changed from Assigned to Closed This was fixed by resetting the running_time_us ---------------------------------------- Bug #21007: Ractor scheduler issue when multiple threads in a ractor https://bugs.ruby-lang.org/issues/21007#change-113654 * Author: luke-gru (Luke Gruber) * Status: Closed * Assignee: ractor * Backport: 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: UNKNOWN ---------------------------------------- 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/