On Thu, Apr 29, 2021 at 10:47 PM Øyvind Teig <oyvind.t...@teigfam.net>
wrote:
>
> 1. I'm sorry I didn't follow up on your answer where you had got to
> the length of spelling out some code right before my eyes. I got lost in
> the other points (I actually started a response..)
> 2. I trust you
> 3. But to do more than trusting, understanding would be much better.
> I'm sorry: to understand I would need more than pseudo code
> 4. But I was really after whether the "pri" keyword (or whatever) in
> front of "select" had been introduced over the last years. The answer seems
> to be "no"
>
>
FWIW I did answer this explicitly above :) No, there is no way to manually
specify priorities of `select` and no, I subjectively wouldn't predict it
happening at any point in the near or far future. Unless someone comes up
with an extremely convincing, so far unheard of argument in its favor :)
>
> 1. Although I appreciate that Turing proved that any language can code
> anything, I some times have had a hard time believing it. Maybe that's not
> exactly what he meant. But then, since I trust you, I do find that solution
> as surprising as as I would be happy to run it on The Go Playground
>
> torsdag 29. april 2021 kl. 22:02:23 UTC+2 skrev ren...@ix.netcom.com:
>
>> I already gave you the solution to this. Trust me it works. I did HFT for
>> 7+ years - the algo is well known.
>>
>> On Apr 29, 2021, at 2:05 PM, Øyvind Teig <oyvin...@teigfam.net> wrote:
>>
>>
>>
>> torsdag 29. april 2021 kl. 20:22:32 UTC+2 skrev rog:
>>
>>> I agree with Axel's take here. It seems, Øyvind, that you are concerned
>>> more with principle than practice here. Can you give an example of a real
>>> world case where you think that this might actually matter?
>>>
>>
>> Thanks, yes. I have written some about that in the *Nondeterminsim* blog
>> note, referred to at the top. I admit I indicated that seeing some code
>> might be interesting, but it was the principle I was after. In the end a
>> "yes" or "no".
>>
>> Some from the chapter "*-Nondeterministic selective choice in
>> implementations is not good*": (Preceeding the quote I have been telling
>> about CSP's *external* nondeterministic choice in the *specfications*
>> ("implement this any way you want") but in the *implementation* part we
>> have to take decisions (deterministic, inner choice: "we do it *this*
>> way"). I was thinking this is relevant because Why build concurrency on
>> the ideas of CSP? <https://golang.org/doc/faq#csp> Here's the quote:
>>
>> *"The statement was that with the non-deterministic guarded choice in Go,
>> what happens is up to the run-time, which is “not good”. This
>> is implementation, not specification. With occam there is ALT or PRI ALT,
>> always coded as PRI ALT. For a server to be “fair” I have to code it
>> myself, it’s up to me, at the application level to find the best algorithm.
>> Which, during my years as occam programmer was “new starting channel index
>> in the ALT-set is the channel index of the served channel + 1
>> modulo-divided by number of channels”. Channels are clients[0..4]
>> (five) ALT‘ed in set [4,0,1,2,3] served index 4, then 4+1 rem 5 == 0 yields
>> next ALT set [0,1,2,3,4]. Just served 4 and you’re at the back of the set."*
>>
>> The example here is a server with N clients where it is essential that
>> none of clients will starve and none jam the server. I have needed to do
>> this coding several times. Go has random select which in theory may mean
>> starving and jamming. I worked with safety critical fire detection, and it
>> was necessary to ensure this. Or at least we didn't dare to take the
>> chance. We could not just add another machine.
>>
>> To use select when that's fair enough (pun 1) - "fair enough" (pun 2).
>> But If I want to be certain of no starving or jamming I need to code the
>> fairness algorithm. I can then promise a client that may have been ready
>> but wasn't served to come in before I take the previous clients that were
>> allowed. This is at best very difficult if all we have is select. Having
>> pri select as the starting point is, in this case, easier.
>>
>> Øyvind
>>
>>
>>>
>>> On Thu, 29 Apr 2021, 15:44 'Axel Wagner' via golang-nuts, <
>>> golan...@googlegroups.com> wrote:
>>>
>>>> FWIW, maybe this helps:
>>>>
>>>> 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?
>>>>
>>>> On Thu, Apr 29, 2021 at 4:39 PM Axel Wagner <axel.wa...@googlemail.com>
>>>> wrote:
>>>>
>>>>> On Thu, Apr 29, 2021 at 3:54 PM Øyvind Teig <oyvin...@teigfam.net>
>>>>> wrote:
>>>>>
>>>>>> They could still both have become ready (not in the same "cycle")
>>>>>> between the two selects. Even if that probability is low, it would need
>>>>>> knowledge like yours to show that this may in fact be zero. There could
>>>>>> be
>>>>>> a descheduling in between, one of those in my opinion, not relevant
>>>>>> arguments.
>>>>>
>>>>>
>>>>> FTR, again: Yes, it's definitely possible, but it's irrelevant. It
>>>>> makes no observable difference. Even if we had a prioritized select, it
>>>>> would still be *de facto* implemented as a multi-step process and even
>>>>> then, you might run into exactly the same situation - you could have both
>>>>> channels becoming ready while the runtime does setup, or you could have a
>>>>> random scheduling event delaying one of the goroutines infinitesimally, or
>>>>> you could have…
>>>>>
>>>>> This is why we *don't* talk about the behavior of concurrent programs
>>>>> in terms of cycles and time, but instead based on causal order. We don't
>>>>> know how long it takes to park or unpark a goroutine, so all we can say is
>>>>> that a read from a channel happens after the corresponding write. In terms
>>>>> of time, between entering the `select` statement and between parking the
>>>>> goroutine might lie a nanosecond, or a million years - we don't know, so
>>>>> we
>>>>> don't talk about it.
>>>>>
>>>>> The memory model is exactly there to abstract away these differences
>>>>> and to not get caught up in scheduling and cycle discussions - so, FWIW,
>>>>> if
>>>>> these arguments are not relevant, you shouldn't bring them up. Logically,
>>>>> between the first `select` statement and the second `select` statement,
>>>>> there is zero time happening. Arguing that there is, is using exactly
>>>>> those
>>>>> irrelevant arguments about schedulers and processing time.
>>>>>
>>>>>
>>>>>> torsdag 29. april 2021 kl. 15:47:42 UTC+2 skrev Jan Mercl:
>>>>>>
>>>>>>> On Thu, Apr 29, 2021 at 3:23 PM Øyvind Teig <oyvin...@teigfam.net>
>>>>>>> wrote:
>>>>>>>
>>>>>>> > 4c is not "correct" as I want it. In the pri select case, if more
>>>>>>> than one is ready, then they shall not be randomly chosen. Never. They
>>>>>>> should be selected according to priority.
>>>>>>>
>>>>>>> That's not what 4c says. Instead of "more than one ready" it says
>>>>>>> "both high and low _get ready at the same time_".
>>>>>>>
>>>>>>> Note that in the first approximation the probability of 4c happening
>>>>>>> is approaching zero. If we consider time "ticks" in discrete quanta,
>>>>>>> the probability is proportional to the size of the quantum. And
>>>>>>> depending on a particular implementation of the scheduler the
>>>>>>> probability of 4c can still be exactly zero. For example, the OS
>>>>>>> kernel may deliver only one signal at a time to the process etc.
>>>>>>>
>>>>>>> So the "Never" case may quite well never happen at all.
>>>>>>>
>>>>>> --
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>>>>>>
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