On Mon, May 3, 2021 at 6:34 PM Øyvind Teig <oyvind.t...@teigfam.net> wrote:
> meaning that there is not any state where random select is ever used. >>> >> It is. >> > Trouble A: If random select is never used […] > I was unclear: When I said "It is", I meant "it is used". Your understanding of what select does is correct. There will be cases, where both channels are ready when the inner `select` is entered and thus, there will be a pseudorandom choice over which will proceed. The argument put forward is that that's exactly how a priority select would have to behave as well. As least as I imagine it and as well as I understand the implementation of `select` (which is, admittedly, not *super* well). If so, maybe a constructive point is to try to write some runnable Go code > that uses this pri select pattern. I have thought about it, but I don't > know how to check whether it would ever enter one of the selects > This is precisely what my question was getting at. FWIW, it's ultimately pretty straight forward to demonstrate this behavior: https://play.golang.org/p/UUA7nRFdyJE This program exits if and only if the inner select choses `lo`. *Given that we know* that `hi` is being closed before `lo` ("Within a single goroutine, the happens-before order is the order expressed by the program. <https://golang.org/ref/mem#tmp_2>"), `lo` being ready implies `hi` being ready as well. Thus, by seeing that the program exits, we can show that the inner select sometimes chooses the `lo` case, even though the `hi` is ready as well. Crucially, however, we needed to *know* that `close(hi)` happens before `close(lo)` to prove this case was taken - thus, the communications are not concurrent. That's what I meant by "external synchronization primitives". I think my question was flawed, because really, the issue isn't about how the `select` with `default` construct we showed works - the question is how a priority `select` could work. That is, could we implement a priority `select` such that this code terminates: https://play.golang.org/p/4G8CY36L0Qy I don't *think* we can - and based on that assumption I extrapolated how a priority select would actually behave - but I have to admit that I really don't understand `select` or the underlying hardware primitives enough to make a solid case either way here. Maybe you can provide an equivalent program in a language of your choice that terminates - that would certainly prove that it's at least possible (though to be clear: I don't understand your xC code, so I can't promise that I'd understand whatever you send here, personally :) ). All of that being said: I really think that in the cases where a priority select is needed, this construct is good enough to hold up. and in fact need to do a random select, and select a lower when a higher is > present (or became present). Plus, if I try to synchronise clients and send > over sequence counts, the scheduling pattern could become so repetitive > that no such situation would occur. > > Is there a way to inspect the built code and do it from code inspection? > (I guess so?) > > But for all this, I would need even more help... > > (Maybe I'll try to trigger a student since I have so much xC ahead of me..) > > Øyvind > > >> *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/cda2055a-8024-4ab1-87ca-18a177aa1cb2n%40googlegroups.com > <https://groups.google.com/d/msgid/golang-nuts/cda2055a-8024-4ab1-87ca-18a177aa1cb2n%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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