On Thu, 29 Apr 2021, 20:05 Øyvind Teig, <oyvind.t...@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.
>
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.
You can do it with reflection though. It's not too hard to code something
quite similar to your algorithm above.
For example: https://go2goplay.golang.org/p/S_5WFkpqMP_H
I think the above code should fit your definition of fairness, and it seems
to me that it's a reasonably general approach.
cheers,
rog.
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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