(j3.2006) (SC22WG5.3648) [ukfortran] A comment on John Wallin's comments on Nick MacLaren's comments

N.M. Maclaren nmm1
Fri Nov 7 13:16:47 EST 2008

On Nov 7 2008, Bill Long wrote:
>Aleksandar Donev wrote:
>> I agree with Nick that we should make a decision concerning "progress", 
>> and somehow anoint in the standard, even if we cannot say anything in 
>> standardese.
>The first sentence of Note 8.31 is "The model upon which the 
>interpretation of a program is based is that there is a permanent memory 
>location for each coarray and the all images can access it."   I don't 
>see any ambiguity here.  It does not say that only some images can 
>access a coarray, or that the access is available only some of the 
>time.   Perhaps we should change "access" to "reference or define" (an 
>early UK point) to make use of well defined terms here.  But the key 
>words near the end of the sentence, ALL and CAN have unambiguous meanings.
>The other issue related to progress is whether there is an expectation 
>that a SYNC MEMORY statement actually completes execution.  Since SYNC 
>MEMORY is implicitly part of the other image control statements, 
>assuring that one works solves the global problem.   We do already 
>allow, via the stat specifier, a way to deal with cases where the 
>statement in fact does fail - ...

Neither Aleksandar nor I are talking about forbidding some images to access
some coarrays, nor are we talking about SYNC MEMORY overtly failing.  Those
are entirely separate concepts.

The (simplified) issue is whether SYNC MEMORY is allowed to wait until the
image that owns the data it is accessing reaches the next image control
statement.  No more than that.

Yes, it would be possible to introduce a TIMEOUT value into SYNC MEMORY,
and set STAT if it were exceeded, but that wouldn't help.  All it would
do is to allow a cautious programmer to diagnose the deadlock from within
the program, rather than having it show up as a hung program.  It wouldn't
make it any easier to write portable code.

Nick Maclaren,
University of Cambridge Computing Service,
New Museums Site, Pembroke Street, Cambridge CB2 3QH, England.
Email:  nmm1 at cam.ac.uk
Tel.:  +44 1223 334761    Fax:  +44 1223 334679

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