Delay GVL handoff on blocking operation#15529
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When a thread invokes the blocking operation such as I/O,
the interpreter currently tries to pass the GVL to another
thread immediately. However, if the blocking operation completes
in a sufficiently short amount of time, the cost of context switching,
especially on multi-core machines outweigh the benefit.
This patch introduces delayed GVL passing for blocking operations.
Scenario1: Short blocking operation
(1) `thread_sched_to_waiting_common()` does not immediately
schedule another ready thread, but instead set the `waiting` flag.
(2) When the blocking operation finishes,
`thread_sched_to_running_common()` is called without a context switching
and `sched->running` is not changed.
Clear the `waiting` flag and keep going.
Scenario2: Long blocking operation
(1) Same as above.
(2) The timer thread checks the running threads to provide
timeslice every 10ms and finds the thread (th) performing
blocking operation.
(3) The timer thread switches the running thread from th to another
ready thread.
(4) When the blocking operation (on th) finishes,
`thread_sched_to_running_common()` is called and `sched->running`
should be another thread (or be NULL) GVL.
fix https://bugs.ruby-lang.org/issues/21685 (and the idea is from discussions)
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luke-gruber
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Dec 16, 2025
| struct rb_thread_sched *sched = TH_SCHED(th); | ||
| thread_sched_lock(sched, th); | ||
| { | ||
| if (th->sched.waiting && sched->running == th && sched->readyq_cnt > 0) { |
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Here, I think it would be good to check an event_serial from the thread to make sure the waiting you are seeing is the right one. The thread could have already context switched to another thread, setting th->sched.waiting to false and then the next thread could have context switched back to th. So, th could only be waiting for a very small amount of time.
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When a thread invokes the blocking operation such as I/O, the interpreter currently tries to pass the GVL to another thread immediately. However, if the blocking operation completes in a sufficiently short amount of time, the cost of context switching, especially on multi-core machines outweigh the benefit.
This patch introduces delayed GVL passing for blocking operations.
Scenario1: Short blocking operation
(1)
thread_sched_to_waiting_common()does not immediatelyschedule another ready thread, but instead set the
waitingflag.(2) When the blocking operation finishes,
thread_sched_to_running_common()is called without a context switchingand
sched->runningis not changed.Clear the
waitingflag and keep going.Scenario2: Long blocking operation
(1) Same as above.
(2) The timer thread checks the running threads to provide
timeslice every 10ms and finds the thread (th) performing
blocking operation.
(3) The timer thread switches the running thread from th to another
ready thread.
(4) When the blocking operation (on th) finishes,
thread_sched_to_running_common()is called andsched->runningshould be another thread (or be NULL) GVL.
fix https://bugs.ruby-lang.org/issues/21685 (and the idea is from discussions)