torsdag 6. mai 2021 kl. 20:30:59 UTC+2 skrev axel.wa...@googlemail.com:
> On Thu, May 6, 2021 at 8:22 PM Robert Engels <ren...@ix.netcom.com> wrote:
>
>> “If lo” means that if the lo channel was read.
>>
>
> Exactly. To fulfill the requirements and answering the question, it must
> not be read.
>
> This code will prevent a lo from being processed if a hi is available at
>> the happens before moment of a value being ready.
>>
>
> What the receiver does with the value is immaterial. Point is, that the
> receiver has already read the value, thus the communication has happened,
> thus the sender was unblocked. The question is about a select that wouldn't
> do that.
>
>
>> Btw using indents rather than brackets in the above - maybe that is
>> causing the confusion.
>>
>
> I'm not confused. Your code is simply not answering the question posed.
> Which is about a select which always lets the high priority communication
> happen, if it is ready before the low priority communication - and
> consequently *doesn't* let the low priority communication happen.
>
This one of the difficult things with implementing a select. I have
implemented several, for CSP-based run-time systems that we used for small
controllers. I guess that's why they, in the world I come from talk about
(1) set-up of select (to see if any is ready at that time, if so pick out
one, pri or not pri. Several may have "their bits" set at that time), if
not then (2) wait in the select until one event happes (input, port,
timeout and for some output) and (3) tear-down the select when it's being
triggered, at when any other side that is also waiting is put back on the
select as a precond for the next time. It's easy to forget to put the
others back on the select (ie. don't touch them). It we don't we'll just
deadlock or the other part believes that the action (input or output) did
happen when it didn't. That would depend on how it's coded I gues. It is
unsafe to let the user control any queue, because much of the idea with CSP
is that we have "WYSIWYG semantics" (A term by Peter Welch, UofKent, UK
(emeritus)). In order to *understand* the code in a task, all we look at is
the channel usage and the protocol. Not what the other task(s) do.
Øyvind
>
>
>>
>>
>>
>> On May 6, 2021, at 12:37 PM, 'Axel Wagner' via golang-nuts <
>> golan...@googlegroups.com> wrote:
>>
>>
>> No, it is not. Your "if lo" branch implies that the communication
>> happened - e.g. the sender was already unblocked. A `select` would not
>> unblock the other side unless that's the actual branch taken.
>>
>> On Thu, May 6, 2021 at 7:32 PM Robert Engels <ren...@ix.netcom.com>
>> wrote:
>>
>>> I already showed you - just change it to
>>>
>>> Select hi
>>> Default:
>>> Select hi,lo
>>> If lo:
>>> Select hi
>>> Default :
>>> Pass
>>>
>>> And enqueue the lo if a hi and lo are read.
>>>
>>> That is all that is needed.
>>>
>>>
>>>
>>> On May 6, 2021, at 10:28 AM, 'Axel Wagner' via golang-nuts <
>>> golan...@googlegroups.com> wrote:
>>>
>>>
>>>
>>>
>>> On Thu, May 6, 2021 at 4:43 PM roger peppe <rogp...@gmail.com> wrote:
>>>
>>>>
>>>> On Thu, 6 May 2021 at 14:41, 'Axel Wagner' via golang-nuts <
>>>> golan...@googlegroups.com> wrote:
>>>>
>>>>> PS: And I'm not saying there is no argument. Maybe "select is not
>>>>> atomic" is such an argument. But if there is an argument and/or if this
>>>>> is
>>>>> that argument, I don't fully understand it myself.
>>>>>
>>>>
>>>> One reason is that the semantics can conflict. Consider this code, for
>>>> example (assuming a hypothetical "pri select" statement that chooses the
>>>> first ready arm of the select) - the priorities conflict. I suspect Occam
>>>> doesn't encounter that issue because it only allows (or at least, it did
>>>> back when I used Occam) select on input, not output. I believe that
>>>> restriction was due to the difficulty of implementing bidirectional select
>>>> between actual distributed hardware processors, but I'm sure Øyvind knows
>>>> better.
>>>>
>>>> func main() {
>>>> c1, c2, c3 := make(chan int), make(chan int), make(chan int)
>>>>
>>>> go func() {
>>>> pri select {
>>>> case c1 <- 1:
>>>> case v := <-c2:
>>>> c3 <- v
>>>> }
>>>> }()
>>>> go func() {
>>>> pri select {
>>>> case c2 <- 2:
>>>> case v := <-c1:
>>>> c3 <- v
>>>> }
>>>> }()
>>>> fmt.Println(<-c3)
>>>> }
>>>>
>>>
>>> Interesting case. I would argue, though, that there is no happens-before
>>> edge here to order the cases and I was only considering providing a
>>> guarantee if there is one.
>>>
>>>
>>>> That said, I suspect that the semantics could be ironed out, and the
>>>> real reason for Go's lack is that it's not actually that useful; that it
>>>> would be one more feature; and that in practice a random choice makes
>>>> sense
>>>> almost all the time.
>>>>
>>>
>>> As I said, this would certainly satisfy me as an answer :)
>>>
>>>
>>>>
>>>>
>>>> On Thu, May 6, 2021 at 3:40 PM Axel Wagner <axel.wa...@googlemail.com>
>>>>> wrote:
>>>>>
>>>>>> FWIW after all this discussion I *am* curious about a more detailed
>>>>>> argument for why we can't have a priority select that guarantees that
>>>>>> *if* the high-priority case becomes ready before the low-priority
>>>>>> one (in the sense of "there exists a happens-before edge according to
>>>>>> the
>>>>>> memory model"), the high-priority will always be chosen.
>>>>>>
>>>>>> That is, in the example I posted above
>>>>>> <https://play.golang.org/p/UUA7nRFdyJE>, we *do* know that `hi`
>>>>>> becoming readable happens-before `lo` becoming readable, so a true
>>>>>> prioritized select would always choose `hi` and never return. The
>>>>>> construct
>>>>>> we presented *does* return.
>>>>>>
>>>>>> Now, I do 100% agree that it's not possible to have a select that
>>>>>> guarantees that `hi` will be read if both *become readable
>>>>>> concurrently*. But I don't see a *fundamental* issue with having a
>>>>>> select that always chooses `hi` if `*hi` becoming readable
>>>>>> happens-before `lo` becoming readable*.
>>>>>>
>>>>>> And to be clear, I also kinda like that we don't have that - I think
>>>>>> the value provided by the pseudo-random choice in preventing starvation
>>>>>> is
>>>>>> worth not having an "ideal" priority select construct in the language.
>>>>>> But
>>>>>> I couldn't really make a good case why we *can't* have it.
>>>>>>
>>>>> --
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>>>>>
>>>>> <https://groups.google.com/d/msgid/golang-nuts/CAEkBMfEJNtu1i1RyZxW5FNYkD0TB73nq0WyVCCW_E9_JOAVJmw%40mail.gmail.com?utm_medium=email&utm_source=footer>
>>>>> .
>>>>>
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