No worries, I was not upset :) But I did misunderstand you, thanks for clearing that up.
On Sun, May 2, 2021 at 11:56 PM Øyvind Teig <oyvind.t...@teigfam.net> wrote: > Axel, I was saying (at least *meaning*) that it was impolite *of me* not > to answer *your* exhaustive comments! The other way around. But I now see > why you got upset. It may read that way too! I read things over many times, > did spell checking, but I didn't see this coming! I will read the rest of > the comments tomorrow! Please excuse me for my Bad English! I have always > been impressed by the golang-nuts group, how well everything has been > responded to, and how much people have taken the time to answer my > questions, coming from the other side of the field. I don't take this for > granted at all! Øyvind > > søndag 2. mai 2021 kl. 21:42:10 UTC+2 skrev axel.wa...@googlemail.com: > >> On Sun, May 2, 2021 at 9:23 PM Øyvind Teig <oyvin...@teigfam.net> wrote: >> >>> *Axel*, it is impolite not to try to comment and discuss each and every >>> point above. >> >> >> I wasn't trying to be impolite. But I also won't go through your messages >> sentence by sentence, trying to say something even if I have nothing to >> say. Sorry. If that doesn't suit you, I'll step out of the conversation. >> >> >>> I have tried to expand on Jan's code ( >>> https://go2goplay.golang.org/p/7xDzP6Jvyl8), here: >>> https://go2goplay.golang.org/p/vhmo_Vw6OQy. I have added a >>> mediumPriority channel. (Hope it's right..) >>> >> >> That code is missing a case in the innermost select. This one seems >> correct: >> https://go2goplay.golang.org/p/ZEy752iEOb9 >> >> >>> *Ian* said that select is not an atomic operation. I assume (but >>> everyone here seems to tell me the opposite), that at each default >>> there are starts of new, unique selects? >>> >>> Here is one of the comments I wrote to one of Axel's points above, and >>> it could be iterated over three priorities as well: >>> >>> I think this is where I need to understand Go a little better, because >>> it might be different from occam's default (TRUE & SKIP). Actually, this >>> may be the reason why this thread is still not closed. To me it is very >>> strange that between the first polling of the highPri and the default, why >>> that outer select is not torn down. Then enter a new select, which would >>> have two guards: high and low pri. In my head when setting up the new >>> select there would be a decision of which one to select. It would select >>> from the set of ready guards right there. They could both have become ready. >>> >> >> This description sounds correct. This is how Go behaves. >> >> >>> Remember in my head these two may be hw pins. (If the first high pri >>> poll was done at 0 ns and the second select's decision could be 10 ns >>> later, then both hw pins could have become ready at 5 ns). If so the >>> decision needs to be on one of them. With "only random" (yes, I think think >>> this is so, on a general basis, but I accept that Go doesn't have the other >>> option) to chose from, then it *may* chose the low pri, even *if the >>> high pri also was, hw wise, ready.* >>> >> >> This is fundamentally correct (though I'm not sure what you mean by "hw >> pin"). >> >> >>> If these two (or three) cannot be hardware pins (as in Go), then I >>> reason (by induction(?)) that all of the code must be atomic with no >>> descheduling in between, for me to understand that the scheme is 100% as >>> intended: meaning that there is not any state where random select is ever >>> used. >>> >> >> It is. >> >> Again: Your understanding is correct. But the resulting situation is >> still equivalent to a priority select. There is no observable behavior in >> difference between the two. >> So let me repeat my question: >> >> Assume a read happened from lowPriority, even though highPriority was >> ready to read as well. That's, AIUI, the outcome you are concerned about. >> In that situation, how would you know that highPriority was ready to read >> as well? >> >> I believe you'll find that the answer is "you can't". >> >> >>> >> >>> *rog* wrote above (where I had indicated that occam (and also xC, said >>> here) has a looping channel construct): "To start with, if you've got N >>> clients where N isn't known in advance, it's not possible to use Go's >>> select statement directly because it doesn't provide support for reading >>> from a slice." Does this mean that aside from reflection ( >>> https://go2goplay.golang.org/p/S_5WFkpqMP_H - which still does not >>> serve "client 2", shouldn't it?) then idiomatic Go for a small number of >>> priorities is the one with default case(s), and it works 100% as intended, >>> with no cognitive (?) reliance on Go's inner working under the hood? (I >>> mean: "WYSIWYG semantics" kind of.) >>> >>> I am at a point now that if the answer to the above is *yes*, I'll just >>> say thank you for your help, and I will be a Go-wise wiser person. With my >>> cognitive bias I will then have to accept that this is Go, nothing more to >>> say. Just accept it. Anyhow, in case, thank you! >>> >>> Øyvind >>> >>> fredag 30. april 2021 kl. 10:42:47 UTC+2 skrev axel.wa...@googlemail.com >>> : >>> >>>> On Fri, Apr 30, 2021 at 9:53 AM Øyvind Teig <oyvin...@teigfam.net> >>>> wrote: >>>> >>>>> If there is no notion of simultaneity why all the effort to describe >>>>> the random distribution? >>>>> >>>> >>>> While it's not possible for two cases to become ready at the same time, >>>> it's definitely possible for two cases to be ready when entering a select. >>>> That's where the random selection comes in. >>>> >>>> There's also the notable difference between a select with a default and >>>> one without. A select with a default never blocks, so which branch is taken >>>> is *only* determined by what's ready when entering the select, whereas a >>>> select without can block and then gets woken up by the first communication >>>> that's ready - and there'll always be a "first". >>>> >>>> In a sense, the nested select uses that: The outer select handles the >>>> "what's currently ready" case and the inner select handles the "what >>>> becomes ready in the future". >>>> >>>> The priority select would use the same basic logic: >>>> - Is the high priority case ready? If so, do that >>>> - If not, block until one of the cases become ready - do the first that >>>> becomes ready >>>> >>>> The crux here is exactly that we can't have two cases "becoming ready" >>>> at the same time, so we really *have* to "take the first one that becomes >>>> ready". >>>> >>>> The select is first set up, at which time the code decides on which one >>>>> to take if more than one guard is ready. If the clients were only sending, >>>>> then nowhere in the system is this noted on "the other" side of the >>>>> channel >>>>> (in the server) before it enters the select. The channel would have noted >>>>> the first contender, yes, but the servre have yet no idea. If none is >>>>> ready, then the server was first on all the ends, and when a sender >>>>> arrives >>>>> it will match the guard set in the server and tear down the select. In due >>>>> time the server is scheduled with that one event. >>>>> >>>>> This is how I have seen it in several systems. I wonder what might be >>>>> so different with go. >>>>> >>>> >>>> I don't think I understand this exposition. But on first glance, your >>>> description doesn't sound terribly different from what's happening in Go. >>>> >>>> To be clear: No one is claiming it would be impossible to implement a >>>> priority select in Go. Obviously we could replace the pseudo-random choice >>>> by something else. We are just saying that it would be equivalent to the >>>> nested select code. >>>> >>>> Ok, so this is a pattern that Go people would use if they needed to do >>>>> pri select. Then, why go to the lengths of the other code shown above? Is >>>>> it because I have kind of "pressed" you to come up with code and then of >>>>> course, one thing may be solved several ways? >>>>> >>>> >>>> I think the first code you where shown by Jan (which is the same as >>>> Ian's) is correct and I believe it's likely that your insistence that it >>>> isn't is what prompted people to come up with more and more complicated >>>> code. >>>> >>>> Will your Go code examples stand the test of formal verification? Of >>>>> course, when it's not formally verified you probaby could not answer such >>>>> a >>>>> question. But the stomach feeling? >>>>> >>>> >>>> I'm not very familiar with formal methods for this, or what the >>>> invariant is that would be verified. >>>> I do feel quite confident about the statement that the shown snippet is >>>> equivalent to how I'd think a priority select would work. >>>> >>>> Another angle: Go does not have the expression before the select that >>>>> evaluates to true or false. Nothing like >>>>> >>>>> select { >>>>> case (do_this) => val1 <-c1: >>>>> case val2 <-c2: >>>>> } >>>>> >>>>> Instead, the chan is set to nil to exclude it from the set. What might >>>>> happen if we had a set of 100 clients and they were switched on and off >>>>> internally in the server (that's their purpose) - when will the uniform >>>>> distribution be reset? What's the life span of the distribution? With a >>>>> psudorandom sequence any one value is only visited once on a round. >>>>> >>>> >>>> I'm not sure what you mean here. Is what you call a "round" the cycle >>>> of the PRNG? In that case, this statement isn't true, the cycle is likely >>>> significantly longer than the number of cases. So we definitely chose at >>>> least one case multiple times per cycle. >>>> >>>> AFAIK this is the PRNG used by the select >>>> <https://github.com/golang/go/blob/9c7207891c16951121d8b3f19f49ec72f87da9fe/src/runtime/stubs.go#L124>, >>>> FWIW. I assume it simply calls into it (or likely `fastrandn` directly >>>> below) when entering a select with multiple available cases. >>>> >>>> We still want this to be fair. Could those having been served be served >>>>> again (before the others) after a reset of the distribution, and this >>>>> introduce a notion of unfairness? >>>>> >>>> >>>> It can definitely happen, but I'm not sure that "unfairness" is a >>>> meaningful term here. AIUI the process is "if the runtime enters a select >>>> and multiple cases are ready, it chooses one uniformly at random" (within >>>> the limits of the PRNG). Yes, as an outcome this can mean that one case is >>>> hit more often than the others. But all cases are equally likely to be hit >>>> more often. And by the law of large numbers, you'd expect the distribution >>>> to flatten over time. >>>> >>>> (I gues that jamming is that only one client alone gets to the server, >>>>> whereas starving is that a client never gets to the server). >>>>> >>>> >>>> Both are statistically unlikely, if we assume the PRNG is reasonably >>>> good - which I think we can, it has been subjected to reasonable >>>> statistical tests. >>>> >>>> >>>>> >>>>> Øyvind >>>>> >>>>> >>>>>> >>>>>> Ian >>>>>> >>>>> -- >>>>> 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/ec5e5c0f-c5bf-4efb-b1c4-dc056720ba5cn%40googlegroups.com >>>>> <https://groups.google.com/d/msgid/golang-nuts/ec5e5c0f-c5bf-4efb-b1c4-dc056720ba5cn%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...@googlegroups.com. >>> >> To view this discussion on the web visit >>> https://groups.google.com/d/msgid/golang-nuts/9186c34b-1088-4ae0-8076-6c5cd0cdde38n%40googlegroups.com >>> <https://groups.google.com/d/msgid/golang-nuts/9186c34b-1088-4ae0-8076-6c5cd0cdde38n%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/dc3898ea-032e-4b28-acd4-65e1b7cef21fn%40googlegroups.com > <https://groups.google.com/d/msgid/golang-nuts/dc3898ea-032e-4b28-acd4-65e1b7cef21fn%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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