On Fri, May 7, 2021 at 8:49 PM Øyvind Teig <oyvind.t...@teigfam.net> wrote:
> I don't know if one would use a pri select to pick up channels from the > same task. Here's a mod that seems to behave wildly differently when the > channels are signalled from one task each and those two tasks are swapped > in order. How may that be explained? I would have suggested the same log > trail for both: https://play.golang.org/p/lJA0XPWtj-w > It does seem to make a difference, but that's mainly due to coincidences of the implementation. I assume it has something to do with how the newly created goroutines are scheduled - you do start one after the other and maybe, for some implementation specific detail, it tends to schedule the last one first. Either way, you can't rely on that behavior. From a pure spec and memory model POV, both programs provide you the same behavioral guarantees. > > Øyvind > > >> >> On Thu, May 6, 2021 at 10:05 PM Axel Wagner <axel.wa...@googlemail.com> >> wrote: >> >>> >>> >>> On Thu, May 6, 2021 at 9:40 PM Robert Engels <ren...@ix.netcom.com> >>> wrote: >>> >>>> But that is not really true because there are no constraints on if the >>>> source channels are buffered - if they are then my code operates similarly. >>>> >>> >>> I was very clear. I was asking if it is possible to implement a priority >>> select with the semantics "if the high priority case becomes ready before >>> the low priority case, it always takes the high priority case". Saying "but >>> what if we try to ensure that both of them are always ready" is not >>> answering the question. >>> Please stop insisting that the code you provide solves this. It simply >>> doesn't. Given that I phrased the question, I feel justified in claiming >>> the authority if it does or not. >>> >>> >>>> Even if using unbuffered channels there is buffering being done at a >>>> lower level (hardware buffers, network stack buffers, etc) - so not >>>> “unblocking a sender” is a dubious endeavor. >>>> >>> >>> That is how select behaves, though. It chooses a communication to >>> proceed (currently, uniformly at random, under the premise of the question, >>> the highest priority one) and lets that proceed. >>> >>> If you don't have an answer to the question I posed, it is okay to just >>> not answer it. If there is no answer, that's okay too. But arguing about >>> code which clearly does not answer it is frustrating. >>> >>> >>>> >>>> >>> >>>> On May 6, 2021, at 1:30 PM, 'Axel Wagner' via golang-nuts < >>>> golan...@googlegroups.com> wrote: >>>> >>>> >>>> On Thu, May 6, 2021 at 8:22 PM Robert Engels <ren...@ix.netcom.com> >>>> wrote: >>>> >>>>> “If lo” means that if the lo channel was read. >>>>> >>>> >>>> Exactly. To fulfill the requirements and answering the question, it >>>> must not be read. >>>> >>>> This code will prevent a lo from being processed if a hi is available >>>>> at the happens before moment of a value being ready. >>>>> >>>> >>>> What the receiver does with the value is immaterial. Point is, that the >>>> receiver has already read the value, thus the communication has happened, >>>> thus the sender was unblocked. The question is about a select that wouldn't >>>> do that. >>>> >>>> >>>>> Btw using indents rather than brackets in the above - maybe that is >>>>> causing the confusion. >>>>> >>>> >>>> I'm not confused. Your code is simply not answering the question posed. >>>> Which is about a select which always lets the high priority communication >>>> happen, if it is ready before the low priority communication - and >>>> consequently *doesn't* let the low priority communication happen. >>>> >>>> >>>>> >>>>> >>>>> >>>>> On May 6, 2021, at 12:37 PM, 'Axel Wagner' via golang-nuts < >>>>> golan...@googlegroups.com> wrote: >>>>> >>>>> >>>>> No, it is not. Your "if lo" branch implies that the communication >>>>> happened - e.g. the sender was already unblocked. A `select` would not >>>>> unblock the other side unless that's the actual branch taken. >>>>> >>>>> On Thu, May 6, 2021 at 7:32 PM Robert Engels <ren...@ix.netcom.com> >>>>> wrote: >>>>> >>>>>> I already showed you - just change it to >>>>>> >>>>>> Select hi >>>>>> Default: >>>>>> Select hi,lo >>>>>> If lo: >>>>>> Select hi >>>>>> Default : >>>>>> Pass >>>>>> >>>>>> And enqueue the lo if a hi and lo are read. >>>>>> >>>>>> That is all that is needed. >>>>>> >>>>>> >>>>>> >>>>>> On May 6, 2021, at 10:28 AM, 'Axel Wagner' via golang-nuts < >>>>>> golan...@googlegroups.com> wrote: >>>>>> >>>>>> >>>>>> >>>>>> >>>>>> On Thu, May 6, 2021 at 4:43 PM roger peppe <rogp...@gmail.com> wrote: >>>>>> >>>>>>> >>>>>>> On Thu, 6 May 2021 at 14:41, 'Axel Wagner' via golang-nuts < >>>>>>> golan...@googlegroups.com> wrote: >>>>>>> >>>>>>>> PS: And I'm not saying there is no argument. Maybe "select is not >>>>>>>> atomic" is such an argument. But if there is an argument and/or if >>>>>>>> this is >>>>>>>> that argument, I don't fully understand it myself. >>>>>>>> >>>>>>> >>>>>>> One reason is that the semantics can conflict. Consider this code, >>>>>>> for example (assuming a hypothetical "pri select" statement that chooses >>>>>>> the first ready arm of the select) - the priorities conflict. I suspect >>>>>>> Occam doesn't encounter that issue because it only allows (or at least, >>>>>>> it >>>>>>> did back when I used Occam) select on input, not output. I believe that >>>>>>> restriction was due to the difficulty of implementing bidirectional >>>>>>> select >>>>>>> between actual distributed hardware processors, but I'm sure Øyvind >>>>>>> knows >>>>>>> better. >>>>>>> >>>>>>> func main() { >>>>>>> c1, c2, c3 := make(chan int), make(chan int), make(chan int) >>>>>>> >>>>>>> go func() { >>>>>>> pri select { >>>>>>> case c1 <- 1: >>>>>>> case v := <-c2: >>>>>>> c3 <- v >>>>>>> } >>>>>>> }() >>>>>>> go func() { >>>>>>> pri select { >>>>>>> case c2 <- 2: >>>>>>> case v := <-c1: >>>>>>> c3 <- v >>>>>>> } >>>>>>> }() >>>>>>> fmt.Println(<-c3) >>>>>>> } >>>>>>> >>>>>> >>>>>> Interesting case. I would argue, though, that there is no >>>>>> happens-before edge here to order the cases and I was only considering >>>>>> providing a guarantee if there is one. >>>>>> >>>>>> >>>>>>> That said, I suspect that the semantics could be ironed out, and the >>>>>>> real reason for Go's lack is that it's not actually that useful; that it >>>>>>> would be one more feature; and that in practice a random choice makes >>>>>>> sense >>>>>>> almost all the time. >>>>>>> >>>>>> >>>>>> As I said, this would certainly satisfy me as an answer :) >>>>>> >>>>>> >>>>>>> >>>>>>> >>>>>>> On Thu, May 6, 2021 at 3:40 PM Axel Wagner < >>>>>>>> axel.wa...@googlemail.com> wrote: >>>>>>>> >>>>>>>>> FWIW after all this discussion I *am* curious about a more >>>>>>>>> detailed argument for why we can't have a priority select that >>>>>>>>> guarantees >>>>>>>>> that *if* the high-priority case becomes ready before the >>>>>>>>> low-priority one (in the sense of "there exists a happens-before edge >>>>>>>>> according to the memory model"), the high-priority will always be >>>>>>>>> chosen. >>>>>>>>> >>>>>>>>> That is, in the example I posted above >>>>>>>>> <https://play.golang.org/p/UUA7nRFdyJE>, we *do* know that `hi` >>>>>>>>> becoming readable happens-before `lo` becoming readable, so a true >>>>>>>>> prioritized select would always choose `hi` and never return. The >>>>>>>>> construct >>>>>>>>> we presented *does* return. >>>>>>>>> >>>>>>>>> Now, I do 100% agree that it's not possible to have a select that >>>>>>>>> guarantees that `hi` will be read if both *become readable >>>>>>>>> concurrently*. But I don't see a *fundamental* issue with having >>>>>>>>> a select that always chooses `hi` if `*hi` becoming readable >>>>>>>>> happens-before `lo` becoming readable*. >>>>>>>>> >>>>>>>>> And to be clear, I also kinda like that we don't have that - I >>>>>>>>> think the value provided by the pseudo-random choice in preventing >>>>>>>>> starvation is worth not having an "ideal" priority select construct >>>>>>>>> in the >>>>>>>>> language. But I couldn't really make a good case why we *can't* >>>>>>>>> have it. >>>>>>>>> >>>>>>>> -- >>>>>>>> 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/CAEkBMfEJNtu1i1RyZxW5FNYkD0TB73nq0WyVCCW_E9_JOAVJmw%40mail.gmail.com >>>>>>>> <https://groups.google.com/d/msgid/golang-nuts/CAEkBMfEJNtu1i1RyZxW5FNYkD0TB73nq0WyVCCW_E9_JOAVJmw%40mail.gmail.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...@googlegroups.com. >>>>>> To view this discussion on the web visit >>>>>> https://groups.google.com/d/msgid/golang-nuts/CAEkBMfHEEDdL8adBDFoqwVHswK3kr_KawePGi%3DNtbaBVTP5KWw%40mail.gmail.com >>>>>> <https://groups.google.com/d/msgid/golang-nuts/CAEkBMfHEEDdL8adBDFoqwVHswK3kr_KawePGi%3DNtbaBVTP5KWw%40mail.gmail.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...@googlegroups.com. >>>>> To view this discussion on the web visit >>>>> https://groups.google.com/d/msgid/golang-nuts/CAEkBMfHv2cKR1OLS97YN7JYKZXHu_s0a-6c0-2tW%3DS0gUU8jUA%40mail.gmail.com >>>>> <https://groups.google.com/d/msgid/golang-nuts/CAEkBMfHv2cKR1OLS97YN7JYKZXHu_s0a-6c0-2tW%3DS0gUU8jUA%40mail.gmail.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...@googlegroups.com. >>>> To view this discussion on the web visit >>>> https://groups.google.com/d/msgid/golang-nuts/CAEkBMfHQq2p60OenLMYUFz%3DK9HigpbAqj7m%3D%2BRp7BnCX%2Bp1QLA%40mail.gmail.com >>>> <https://groups.google.com/d/msgid/golang-nuts/CAEkBMfHQq2p60OenLMYUFz%3DK9HigpbAqj7m%3D%2BRp7BnCX%2Bp1QLA%40mail.gmail.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/b5dcb025-7cea-4f11-a7fc-8217b06ade59n%40googlegroups.com > <https://groups.google.com/d/msgid/golang-nuts/b5dcb025-7cea-4f11-a7fc-8217b06ade59n%40googlegroups.com?utm_medium=email&utm_source=footer> > . > -- You received this message because you are subscribed to the Google Groups "golang-nuts" group. 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