[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: Programming prioritisation


On 1 Oct 2012, at 19:03, Larry Dickson wrote:

The problem arises when you compose PRI constructions in parallel with conflicting priority, which might arise through the pattern of communication across a network of processes.  For example:

PAR
PRI PAR
a! w
b! x
PRI ALT
b? y
... respond to b
a? z
... respond to a

which deadlocks.  (Excuse the tabs.  I too favour 2-space indents, but not with variable width font.)
This still seems to involve a misunderstanding. (a) As written, the PRI PAR forces b!x to be low priority, the same as the PRI ALT, which I do not think is what you intended.
My recollection of PRI ALT is that priority decreases downward.  I just wanted to show how simple it is to program a conflict in priority :  here, the PRI PAR prioritises channel 'a', and the PRI ALT channel 'b'.  No loop required for a problem to arise.

True, the two processes should not deadlock, but they will have to engage in some sort of negotiation, which is likely to be inefficient and will require some ancillary protocol for resolution.  My point is just that this is best avoided.

All I'm after is a simple and transparent way to express pretty simple behaviour :  ideally with one construction.

Ian

Ian East
Open Channel Publishing Ltd.
(Reg. in England, Company Number 6818450)