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Re: Programming prioritisation


I find this thread surprising. PRI ALT arises naturally because that is by far the easiest way to program an ALT. The order of disabling determines the priority. This seems to hold for other languages' approximations to ALT (i.e. select) and as far as I can see must hold for everyone. Fair ALTs can only be built on top of this natural PRI ALT by randomizing or rotating the order.

Does anyone know any other way of doing it?

PRI PAR, on the other hand, is harder to program than plain PAR.


On Oct 4, 2012, at 2:04 AM, "Teig, Oyvind CCS" <Oyvind.Teig@xxxxxxxxxxxxxxxx> wrote:

| where he rejects prioritised choice (PRI ALT) because it presents problems "when the priority orders of different parallel processes cannot be resolved".
1.     But if these priority orders are known and may be resolved, then PRI ALT may be useul? (I think so!)
2.     So, does a too encapsulated or out of scope PRI PAR rule out PRI ALT because the components in the ALT have UNKNOWN PAR?
3.     But then, how can PRI PAR be out of scope for a component? Hmm..
Fra: Ian East [mailto:ian.east@openchannelpublishing.com] 
Sendt: 4. oktober 2012 10:51
Til: Teig, Oyvind CCS
Kopi: occam-com@xxxxxxxxxx
Emne: Re: SV: Programming prioritisation
Hi Øyvind
I certainly referred to Bill Roscoe's second book on CSP, which he describes as part sequel, part successor, to his original Theory and Practice of Concurrency :
            Roscoe, A. W. :  2010, Understanding Concurrent Systems, Springer ISBN 978-1-84882-257-3
I mentioned the one section on priority in the book (#20.2 Priority, pp. 486-496 (though there is a brief mention earlier in the context of timed systems), where he rejects prioritised choice (PRI ALT) because it presents problems "when the priority orders of different parallel processes cannot be resolved".
I managed a brief chat with Gavin Lowe, whose thesis concerned prioritisation, and suggested I really ought perhaps to read his thesis first.  He recommended instead starting with his more recent papers, as they were more up-to-date.
From these contributions, this discussion (thread), and my own thoughts, I think there are two clear conclusions thus far :
            1          in the presence of many low-cost processors ('cores'), we can usually forget prioritisation in practice
            2          where a programming language does incorporate prioritisation, it is better to prioritise events (and their response), in which case
                        there must be an opportunity to declare a partial order over the events concerned over the scope affected.
I remain concerned that the first approach may sometimes result in a more complicated communication pattern which may be more difficult to program (or, conversely, invite more error).
I have yet to fully understand the consensus (if there is one) in the CSP community over programming prioritisation.
On 4 Oct 2012, at 08:13, Teig, Oyvind CCS wrote:

UK mainlanders and others.
I wonder where this thread started? At CPA-2012 I think it was said that in Bill Roscoe’s “latest book” he shows that PRI ALT is
… not wanted? ... not needed? … plain wrong? … ok, provided?
I do see that Roscoe has published quite a lot even this year http://www.cs.ox.ac.uk/people/publications/personal/Bill.Roscoe.html. (Or was it Steve Schneider, but on Amazon I see nothing very new from him). I have seen one of these names in a mail comment lately, but I can’t find it! Anyway, I’d appreciate more precise statements:
1.      What was the source of this?
2.      And a brief explanation of what was stated, and the context?
I did start a little blog note yesterday, it’s somewhat off-topic to the core of this discussion, but I would like to put in a little about the (pros and) cons of PRI ALT. See http://www.teigfam.net/oyvind/home/047-priority-select-in-go/ (I’d like comments, but it’s very crude at the moment, my suggestion is probably wild or wrong, but there is a point, at least to trash, there). If I do quote this thread, how should I do it?
Med vennlig hilsen / sincerely
Øyvind Teig
Ian East
Open Channel Publishing Ltd.
(Reg. in England, Company Number 6818450)