Still "missing" priority or ordered select in go?

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Øyvind Teig

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Apr 29, 2021, 4:51:43 AM4/29/21
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I know from some years ago that go did not have any priority or ordered select construct [1].

The idea is that select always is a nondeterministic choice operator, if this is still so. Implemented with a random, I assume.

Programming user defined "fair" algorithms or patterns is then not possible, I believe.

Is this still so in Go? No prioritised or ordered select in go?

But is there still a way to code this by f.ex. combining several selects? I saw a different take at this at [2], where the select is replaced a single chan containing a chan bundle, thus becoming "deterministic".

[1] Blog note Nondeterminism (disclaimer: no ads, no gifts, only fun and expenses)

Jan Mercl

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Apr 29, 2021, 5:15:14 AM4/29/21
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On Thu, Apr 29, 2021 at 10:52 AM Øyvind Teig <oyvin...@teigfam.net> wrote:

> I know from some years ago that go did not have any priority or ordered select construct [1].

Not sure what does "ordered" wrt select mean, but the default clause
of the select statement provides exactly that - a priority mechanism:
first try something, otherwise do something else. The most elementary
one, sure, but all the more complex ones can be built with this basic
building block.

Øyvind Teig

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Apr 29, 2021, 5:23:54 AM4/29/21
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Thanks! A construct like "pri select" or "alt select" or "ordered select", so it's not a "wrt", it's a tagging of select changing ets sematics from being nondeterminstic to deterministic. 

CSP has both: external choice (nondeterminsitic), internal choice (deterministic).

The occam programming language has both "ALT" (=select) and "PRI ALT".

The xC programming language has both "select" and "[[ordered]] select".

This is not solved with a default clause, which transforms the selective choice waiting for some event to happen into busy polling. It's nice yo have some times, but that is something orthogonal to pri/ordered.

Jan Mercl

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Apr 29, 2021, 5:36:45 AM4/29/21
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On Thu, Apr 29, 2021 at 11:24 AM Øyvind Teig <oyvin...@teigfam.net> wrote:

> This is not solved with a default clause, which transforms the selective choice waiting for some event to happen into busy polling. It's nice yo have some times, but that is something orthogonal to pri/ordered.

Not sure if I would call it busy polling, but I meant schematically this:

select {
case x := <-highPriority:
handle(x)
default:
select {
case x := <-highPriority:
handle(x)
case x := <-lowPriority:
handle(x)
}
}

Øyvind Teig

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Apr 29, 2021, 8:26:15 AM4/29/21
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Interesting! 

Your suggestion would in fact do pri select in the special case 1. below:
  1. Poll highPri first (take it if it's ready), if highPri not ready then take lowPri (provided highPri has not become ready since the first poll)
  2. However, if highPri has become ready between the first and the second, then it would be taken (provided lowPri is not also ready)
  3. If both have become ready when the second select is entered they would be taken 50% of the time on the average
I fail to see that this is the general pri select that I am quering about whether it has "appeared" in go over the last years. I have a stomach feeling that it can not be implemented by polling. In the semantics of a select the whole select is evaluated before it is entered to se if there is/are any guard(s) ready. If not, pick randomly. If not, set alle guards up in some wait state.

The default case I have always used like "since no event ready (polling) then do something else than listening again on the same events". occam has deafult (although it's called TRUE & SKIP), xC does not.

Øyvind Teig

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Apr 29, 2021, 8:27:47 AM4/29/21
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If not, pick randomly -> If so, pick randomly

Axel Wagner

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Apr 29, 2021, 8:35:31 AM4/29/21
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On Thu, Apr 29, 2021 at 2:26 PM Øyvind Teig <oyvin...@teigfam.net> wrote:
Interesting! 

Your suggestion would in fact do pri select in the special case 1. below:
  1. Poll highPri first (take it if it's ready), if highPri not ready then take lowPri (provided highPri has not become ready since the first poll)
  2. However, if highPri has become ready between the first and the second, then it would be taken (provided lowPri is not also ready)
  3. If both have become ready when the second select is entered they would be taken 50% of the time on the average

One thing to point out is that since we are talking about concurrency, where you can really only talk about something happening "after" something else happened, if there is a causal edge between them, this really *is* equivalent to a simple priority select. You can't observe the difference, unless you add more synchronization - in which case we aren't really talking about the code under discussion anymore.

I fail to see that this is the general pri select that I am quering about whether it has "appeared" in go over the last years.

I tend to agree with you on this though. So to answer that question plainly: No, there is no prioritized select in Go and I find it personally unlikely that we'll get one. It might be possible to address some use cases when we get generics, though.
 
I have a stomach feeling that it can not be implemented by polling. In the semantics of a select the whole select is evaluated before it is entered to se if there is/are any guard(s) ready. If not, pick randomly. If not, set alle guards up in some wait state.

The default case I have always used like "since no event ready (polling) then do something else than listening again on the same events". occam has deafult (although it's called TRUE & SKIP), xC does not.
torsdag 29. april 2021 kl. 11:36:45 UTC+2 skrev Jan Mercl:
On Thu, Apr 29, 2021 at 11:24 AM Øyvind Teig <oyvin...@teigfam.net> wrote:

> This is not solved with a default clause, which transforms the selective choice waiting for some event to happen into busy polling. It's nice yo have some times, but that is something orthogonal to pri/ordered.

Not sure if I would call it busy polling, but I meant schematically this:

select {
case x := <-highPriority:
handle(x)
default:
select {
case x := <-highPriority:
handle(x)
case x := <-lowPriority:
handle(x)
}
}

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Jan Mercl

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Apr 29, 2021, 8:58:23 AM4/29/21
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On Thu, Apr 29, 2021 at 2:26 PM Øyvind Teig <oyvin...@teigfam.net> wrote:

> Your suggestion would in fact do pri select in the special case 1. below:
>
> Poll highPri first (take it if it's ready), if highPri not ready then take lowPri (provided highPri has not become ready since the first poll)
> However, if highPri has become ready between the first and the second, then it would be taken (provided lowPri is not also ready)
> If both have become ready when the second select is entered they would be taken 50% of the time on the average

My analysis is different. There are four cases on entry of the outer
select statement:

1. high is ready and low is ready -> high handled, correct.
2. high is ready and low is not ready -> high handled, correct.
3. high is not ready and low is ready -> low handled, correct.

The "interesting" case is

4. high not ready and low not ready.

We enter the default clause of the outer select statement and enter
the inner select statement. Now there are three subcases when
eventually something gets ready:

4a. high gets ready before low gets ready -> high handled, correct.
4b. low gets ready before high gets ready -> low handled, correct.
4c. both high and low get ready at the same time -> one of them is
randomly chosen, correct.

Øyvind Teig

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Apr 29, 2021, 9:10:31 AM4/29/21
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I agree with you.

However, with the first select in the example we have a "first" and with the second we have an "after" which might be considered race condition(s9 for the purpose of seeing the example of what I was after.

Also, scheduling arguments cannot be used here: like "we know that there is no scheduling (of go goroutines) between the first and the second select." If we know, it's not an argument.

Robert Engels

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Apr 29, 2021, 9:20:40 AM4/29/21
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It is trivial to do a priority select. Check the high priority channel again after getting a low priority value. If the high priority is available enqueue the low value and handle the high. 

It is also trivial to “delay” the handling to allow more time for high events to arrive if dealing with synchronized events and you need to account for jitter. 

On Apr 29, 2021, at 8:10 AM, Øyvind Teig <oyvin...@teigfam.net> wrote:

I agree with you.

Øyvind Teig

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Apr 29, 2021, 9:23:36 AM4/29/21
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Jan, I agree on your list, it's better set up than mine. 

But you can add a point after (4.) though, call it (second.pre), dealing with what might have happened after the first select decided that none were taken until it evaluates the precondition to the second select. 

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.

As I discussed (and several with me) in the blog note, the fact that Go doesn't seem to have it, then go needs another definition of "fair" than what is normal discussing these things. The basic idea for Google was, if we have a problem with fairness (none gets starved, all get served), just add another computer (said over a beer). Plus, the need for the fairness that one can build with a pri select (most often by reordering the list of guards according to one's pleasing of what one might think of as fair), is according to Goodle (on go, I think), so tiny that we make it simple: drop it. Of course it's also easy to try to implement one's own fairness and fail with it. In a realtime system it's not that easy to test, and these things most often are best evaluated on paper. So a formal proof might do.

Axel Wagner

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Apr 29, 2021, 9:28:03 AM4/29/21
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My argument is that you can't distinguish the case "neither was ready and highPriority and lowPriority became ready simultaneously and lowPriority was selected" from "highPriority became ready after lowPriority" (as long as there is the same code in both highPriority cases). Because there is no "simultaneous" or "after" in concurrency, unless you use synchronization - the real phrasing is "highPriority and lowPriority became ready simultaneously" and if, *in real time* that means one or the other came first is indeterminate. There is no implementation or scheduling argument here - this argument is purely based on the Go memory model and the spec.

If there is no observable difference in behavior, the two are equivalent.

But either way, as I said, it's not really the same, just based on readability and clarity. It's semantically equivalent, but semantically, Go is equivalent to Assembly and yet we prefer Go :)

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Øyvind Teig

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Apr 29, 2021, 9:29:51 AM4/29/21
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I sent that to the wrong entry! Is it possible that you could make a code example?

Øyvind Teig

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Apr 29, 2021, 9:38:47 AM4/29/21
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axel, it's difficult to try to explain in words how a select behaves. On that we can agree. For me, select cases can be channels, timeouts, external events etc. If a task is in a select (none triggered yet) and there is an interrupt that detects the select entry, it's ticked as triggered, the select is torn down, and in time the select will be taken with that event only. Not like Linux select which can deliver several causes. But if the timrout came just before the pin, then.. etc. Temporal logic in this case certainly makes sense.

Jan Mercl

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Apr 29, 2021, 9:47:42 AM4/29/21
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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.

Øyvind Teig

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Apr 29, 2021, 9:53:49 AM4/29/21
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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.

robert engels

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Apr 29, 2021, 10:10:00 AM4/29/21
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I will give you the pseudo code:

{ // step #1 do select on both
   select high
   select low
}

if high read:
   return high
else:
   // we read a low so do a high poll
   {
       select high:
       default:
   }

 if high read:
      enqueue low and return high
 else:
      if queue empty:
         return low
      else:
           use queue and enqueue low from step #1 // FIFO order on low reads 

The above code will always return high values over low values (if high available), and return low values in order of events


Jan Mercl

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Apr 29, 2021, 10:18:20 AM4/29/21
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On Thu, Apr 29, 2021 at 3:54 PM Øyvind Teig <oyvin...@teigfam.net> wrote:

> 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.

The upper limit of 4c happening is 1/12 for random events. I guess in
practice it is at least an order of magnitude lower because the
"effective" time ticks are small.

Ian Davis

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Apr 29, 2021, 10:27:17 AM4/29/21
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On Thu, 29 Apr 2021, at 3:09 PM, robert engels wrote:
I will give you the pseudo code:

{ // step #1 do select on both
   select high
   select low
}

if high read:
   return high
else:
   // we read a low so do a high poll
   {
       select high:
       default:
   }

 if high read:
      enqueue low and return high
 else:
      if queue empty:
         return low
      else:
           use queue and enqueue low from step #1 // FIFO order on low reads 

The above code will always return high values over low values (if high available), and return low values in order of events

This seems likely deadlock if the low channel is being filled continually by another goroutine. In addition I usually aim to give ownership of the channel to the writer so it can close cleanly, but mixing reads and writes in a single function greatly complicates that.

Ian

robert engels

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Apr 29, 2021, 10:29:55 AM4/29/21
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It needs one other fix, and that is to have a check of the queue and poll high, in case the queue has elements when the method is entered.

But if you implement the queue using a buffered channel it is trivial to change the step #1 to handle this (the low and queue channels are mimic the high and low channel).

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robert engels

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Apr 29, 2021, 10:32:11 AM4/29/21
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It is not.

Because the low items will be read if there are no highs available. If the low isn’t read, the producer on the low will block.

Or you run out of memory if you cannot process the events fast enough - which is also easily handled by bounding the queue - but then you may need to drop items.

There are no free lunches.

But no deadlock.



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robert engels

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Apr 29, 2021, 10:34:50 AM4/29/21
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To clarify, you might never process a low item if a high is always available - but I believe that is what the OP is requesting.

It is also fairly trivial to add fairness to this ti always process a low if available every N cycles.

Axel Wagner

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Apr 29, 2021, 10:40:19 AM4/29/21
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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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Axel Wagner

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Apr 29, 2021, 10:44:40 AM4/29/21
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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?

roger peppe

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Apr 29, 2021, 2:22:32 PM4/29/21
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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?

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Øyvind Teig

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Apr 29, 2021, 3:05:04 PM4/29/21
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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? 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

Robert Engels

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Apr 29, 2021, 4:02:23 PM4/29/21
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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:



Øyvind Teig

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Apr 29, 2021, 4:46:34 PM4/29/21
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  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"
  5. 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

Axel Wagner

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Apr 29, 2021, 5:02:10 PM4/29/21
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On Thu, Apr 29, 2021 at 10:47 PM Øyvind Teig <oyvin...@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 :)
 

roger peppe

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Apr 29, 2021, 5:52:05 PM4/29/21
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On Thu, 29 Apr 2021, 20:05 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? 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."