[J3] FORM TEAM statement NEW_INDEX= specifier & failed images

Nathan Weeks weeks at iastate.edu
Sun May 12 10:20:25 EDT 2019


Hi John,

I think there's still a problem with the FORM TEAM statement in the
program from C.6.8. Suppose the program is executed by 11 images, so 1
is intended to be a spare. If image 9 in the initial team fails
immediately before it executes the first FORM TEAM statement, then
image 10 in the initial team, which executes FORM TEAM with a
team-number == 1 and NEW_INDEX == 10 (== me), will have specified a
NEW_INDEX= value greater than the number of possible images in the new
team. (In general, it appears that if an image whose image index in
the initial team is > 1 and < images_used fails in the "setup" DO
construct before the FORM TEAM statement, a similar situation can
occur)

Additionally, if this is an error condition for FORM TEAM, per 11.6.9
p5 ("If an error condition other than detection of a failed image
occurs, the team variable becomes undefined"), the simulation_team
team variable would be undefined---and I assume execution of
subsequent CHANGE TEAM statement would result in undefined behavior?

Best,

--
Nathan

On Sun, May 12, 2019 at 7:38 AM John Reid <John.Reid at stfc.ac.uk> wrote:
>
> Nathan,
>
> Nathan Weeks via J3 wrote:
> > Hi all,
> >
> > Thanks for the helpful clarification (and identifying where the standard
> > is unclear). I'll note that this issue impacts the first failed-images
> > example in section C.6.8 of the Fortran 2018 standard, so there is
> > motivation for clarification in the standard itself.
>
> I think we were a bit hasty in choosing to assign failed images to new
> teams in a processor-dependent manner. We definitely want the C.6.8
> example to work. It was always a design objective that following an
> image failure, it would be possible to form a new team of active images
> and continue the calculation there. We don't want any failed images in
> the team because we want to be able to test for newly failed images.
>
> Anyway, 11.1.5.2, para 5 says
>
> 5 Successful execution of a CHANGE TEAM statement performs an implicit
> synchronization of all images of the new team that is identified by
> team-value. All active images of the new team shall execute the same
> CHANGE TEAM statement. On each image of the new team, execution of the
> segment following the CHANGE TEAM statement is delayed until all other
> images of that team have executed the same statement the same number of
> times in the original team.
>
> It is clearly expected that all images of the team are active. The
> adjective "active" is not used in the first and third sentences. It
> should be deleted from the second, for consistency.
>
> To go back to your question:
>
> "What happens in the case where an image specifies both NEW_INDEX= and
> STAT= in a FORM TEAM statement, and the image index specified for
> NEW_INDEX= turns out to be greater than the number of images in the
> new team due to image failure during the execution of FORM TEAM?",
>
> I think this is an error condition. Note that in C.6.8 the NEW_INDEX
> values are carefully set.
>
> Cheers,
>
> John.


More information about the J3 mailing list