One more thing, if (by presumably faulty reasoning above) random select is never done, would we need the two guards in the second select?
https://go2goplay.golang.org/p/EJ2J1sQXVEk søndag 2. mai 2021 kl. 21:23:13 UTC+2 skrev Øyvind Teig: > *Axel*, it is impolite not to try to comment and discuss each and every > point above. I actually started to do exactly that when I discovered that > it all boils down to this (FWIW?): > > 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..) > > *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. > 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.* > > 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. > > *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. 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