I've added https://go-review.googlesource.com/c/go/+/412237 to try to make this obvious to more people.
On Sun, Jun 12, 2022 at 2:47 AM Brian Candler <b.cand...@pobox.com> wrote: > And just as an aside, I think you would be interested in the talk > "Rethinking Classical Concurrency Patterns" by Bryan C. Mills > https://www.youtube.com/watch?v=5zXAHh5tJqQ > > On Sunday, 12 June 2022 at 05:06:47 UTC+1 ke...@burke.dev wrote: > >> That's a very clear explanation, it's obvious what the problem is now. >> Thank you! >> >> On Saturday, June 11, 2022 at 9:36:59 AM UTC-7 se...@liao.dev wrote: >> >>> sync.Cond does not affect goroutine scheduling priority, and Signal only >>> makes the waiting goroutine available to be scheduled, but not force it to >>> be. >>> After a Signal() (and Unlock()), every other waiting worker and the >>> flusher then contends (fairly) for the lock. >>> What you want appears a better fit for either channels (send everything >>> to the flusher) or just inlining the check+flush logic into writeEvent, >>> essentially a proper serialization of events >>> >>> See also: https://github.com/golang/go/issues/21165 >>> > On top of that, condition variables are fiendishly difficult to use: >>> they are prone to either missed or spurious signals [citation needed] >>> >>> - sean >>> >>> >>> On Sat, Jun 11, 2022 at 3:27 PM Kevin Burke <ke...@burke.dev> wrote: >>> >>>> Hi, >>>> Recently I inherited some code in production that was hitting an error >>>> case that I didn't think should be possible to hit. I reduced it down to >>>> this test case. To be clear, there are several different ways to improve >>>> the code here, but I'd like to understand why it's behaving the way it does >>>> first. >>>> >>>> You should be able to just do "go test ." here to reproduce the error: >>>> https://github.com/kevinburke/sync-cond-experiment >>>> >>>> What the code is doing: >>>> >>>> - Multiple different goroutines are taking a sync.Cond lock and >>>> then appending data to a shared buffer. >>>> - A "flush" goroutine calls sync.Cond.Wait() to wait for an >>>> incoming signal that data has been appended >>>> - Each goroutine that appends to the buffer calls Signal() after >>>> the write, to try to wake up the "flush" goroutine >>>> >>>> I *expect* that the flush goroutine will wake up after each call to >>>> Signal(), check whether the batch is ready to be flushed, and if not go >>>> back to sleep. >>>> >>>> What I see instead is that lots of other goroutines are taking out the >>>> lock before the flush goroutine can get to it, and as a result we're >>>> dropping data. >>>> >>>> I didn't expect that to happen based on my reading of the docs for >>>> sync.Cond, which (to me) indicate that Signal() will prioritize a goroutine >>>> that calls Wait() (instead of any other goroutines that are waiting on >>>> sync.Cond.L). Instead it looks like it's just unlocking any goroutine? >>>> Maybe this is because the thread that is calling Signal() holds the lock? >>>> >>>> Thanks for your help, >>>> Kevin >>>> >>>> -- >>>> You received this message because you are subscribed to the Google >>>> Groups "golang-nuts" group. >>>> To unsubscribe from this group and stop receiving emails from it, send >>>> an email to golang-nuts...@googlegroups.com. >>>> To view this discussion on the web visit >>>> https://groups.google.com/d/msgid/golang-nuts/d7e17b2f-159c-4124-a023-eb2cdb8ba423n%40googlegroups.com >>>> <https://groups.google.com/d/msgid/golang-nuts/d7e17b2f-159c-4124-a023-eb2cdb8ba423n%40googlegroups.com?utm_medium=email&utm_source=footer> >>>> . >>>> >>> -- > You received this message because you are subscribed to a topic in the > Google Groups "golang-nuts" group. > To unsubscribe from this topic, visit > https://groups.google.com/d/topic/golang-nuts/J-E3VR0UEYA/unsubscribe. > To unsubscribe from this group and all its topics, send an email to > golang-nuts+unsubscr...@googlegroups.com. > To view this discussion on the web visit > https://groups.google.com/d/msgid/golang-nuts/8d7ff74a-a623-4cce-b5fc-870ea8c1495en%40googlegroups.com > <https://groups.google.com/d/msgid/golang-nuts/8d7ff74a-a623-4cce-b5fc-870ea8c1495en%40googlegroups.com?utm_medium=email&utm_source=footer> > . > -- You received this message because you are subscribed to the Google Groups "golang-nuts" group. To unsubscribe from this group and stop receiving emails from it, send an email to golang-nuts+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/golang-nuts/CAKcy5eje4VN_ByT_226U7%3DLiq56HkHjtTc344oTz_YAH83k5OQ%40mail.gmail.com.
