On Thu, May 6, 2021 at 11:38 PM Robert Engels <reng...@ix.netcom.com> wrote:
> Yes, but barring the nanosecs of a window this is simply implemented as > Ian wrote with > > Select hi > Default: > Select hi,lo > No, it is not. "Barring a nanosecond" is not a guarantee. And that this is not actually doing what I'm asking is the entire start of my inquiry. Again: "It can't be implemented" is a valid answer to the question, though I'm specifically asking for details why. Posting code which doesn't do it, is not an answer. If both channels are ready when the code is entered the high will always be > taken. > But not if they are not both ready when the code is entered. Again, I was very specific about the guarantee I'm asking about: "If `hi` becomes ready before `lo` (in the sense of the memory model happens-before relationship), then the `hi` case should always be taken". > There is no such thing as simultaneous source events > I am specifically talking about *non* simultaneous events. I am specifically talking about causally ordered events with a well-defined order, according to the memory model. And I specifically acknowledged (and originally pointed out) the impossibility to even assign meaning to the question in the context of concurrent events. I am specifically *not* asking about any guarantees in the case where the two events are concurrent. *Neither* about the low-priority communication becoming ready before the high priority one. I am exclusively talking about the situation where it can be proven that the high-priority event happens before the low priority one - as is the case in the example I posted <https://play.golang.org/p/LcWgDcX5ojl>. Again: If you don't have an answer, it is fine to not respond. If you don't know if it's possible to implement a `select` statement with these semantics - or why it isn't - you don't have an answer. The point of my question is pure curiosity. I'm not suggesting that we add a construct like this or arguing it is needed or saying that I want it. I'm only curious if it could be done. So there is absolutely nothing lost by not getting an answer and there is absolutely nothing gained by trying to convince me a non-answer actually is one. Sorry to be so blunt, but this interaction is very frustrating. - even the first that occurred in external observed time might arrive at > the controller later than the second. You need full hardware and OS support > to even begin to minimize this possibility. Unless you have a full RTS this > is the best you will do with any software based solution. > > On May 6, 2021, at 3:15 PM, 'Axel Wagner' via golang-nuts < > golang-nuts@googlegroups.com> wrote: > > > To clarify again: As a litmus test, a `select` construct like the one I'm > talking about would mean that this code blocks forever: > https://play.golang.org/p/LcWgDcX5ojl > With the current `select`, it doesn't. With a different `select`, which > uses source order to express priority and under the semantics I'm asking > about, this would always block, because `lo` would never be read. > > On Thu, May 6, 2021 at 10:05 PM Axel Wagner <axel.wagner...@googlemail.com> > wrote: > >> >> >> On Thu, May 6, 2021 at 9:40 PM Robert Engels <reng...@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 < >>> golang-nuts@googlegroups.com> wrote: >>> >>> >>> On Thu, May 6, 2021 at 8:22 PM Robert Engels <reng...@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 < >>>> golang-nuts@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 <reng...@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 < >>>>> golang-nuts@googlegroups.com> wrote: >>>>> >>>>> >>>>> >>>>> >>>>> On Thu, May 6, 2021 at 4:43 PM roger peppe <rogpe...@gmail.com> wrote: >>>>> >>>>>> >>>>>> On Thu, 6 May 2021 at 14:41, 'Axel Wagner' via golang-nuts < >>>>>> golang-nuts@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.wagner...@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+unsubscr...@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+unsubscr...@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+unsubscr...@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+unsubscr...@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/CAEkBMfHL_G5bGs6tGXO6U8H%3DYMNf6f3%2B4V1JDxALEfOpGhQjvA%40mail.gmail.com > <https://groups.google.com/d/msgid/golang-nuts/CAEkBMfHL_G5bGs6tGXO6U8H%3DYMNf6f3%2B4V1JDxALEfOpGhQjvA%40mail.gmail.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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