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

Steidel, Jon L jon.l.steidel at intel.com
Mon May 13 15:24:38 EDT 2019


We didn't look far enough.  A specification inquiry function is defined in 10.1.11 p3 as

A specification inquiry is a reference to 
    1) an intrinsic inquiry function other than PRESENT
    2) a type parameter inquiry
    3) an inquiry function from the intrinsic modules IEEE_ARITHMETIC and IEEE_EXCEPTIONS
    4) the function C_SIZEOF from the intrinsic module ISO_C_BINDING, or
    5) the COMPILER_VERSION or COMPILER_OPTIONS from the intrinsic module ISO_FORTRAN_ENV.


-----Original Message-----
From: J3 [mailto:j3-bounces at mailman.j3-fortran.org] On Behalf Of John Reid via J3
Sent: Sunday, May 12, 2019 2:51 PM
To: Nathan Weeks <weeks at iastate.edu>
Cc: John Reid <John.Reid at stfc.ac.uk>; General J3 interest list <j3 at mailman.j3-fortran.org>
Subject: Re: [J3] FORM TEAM statement NEW_INDEX= specifier & failed images

> 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).

Yes, this has not been allowed for but it is a low-probability event. 
Image 9 was active when image 1 referenced FAILED_IMAGES. Nevertheless, we should cover the case. We seem to need to test status after the FORM TEAM statement.
> 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?

Yes, we need a test after the FORM TEAM statement.

It looks as we need to set up an interp request.


> 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,, 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.

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