Best way to force coarray images to a well-behaved stop?

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1

I have been experimenting with coarrays in the context of the Euler conjecture tutorial (see Euler Sum of Powers Conjecture: Tutorial). One thing that is part of the sample programs is that the program stops after a certain number of solutions has been found (or else continues until all candidates have been examined). This is easy enough when using a sequential program and I have a working version that does so with coarrays. However, I have so far been forced to check the condition to stop using image 1:

   outer: do i = imax1, imax2
      !
      ! Check if we need to continue
      !
      if ( nfound[1] >= findmax ) then
         write(*,*) 'Enough solutions found ', this_image()
         exit outer
      endif
      ... search for solutions ...

The outer loop is run by all images and the bounds imax1 and imax2 differ per image. The drawback: a lot of communication between image 1 and the others and the rtimings are very erratic, but in general it is not faster than the sequential version.

I have tried setting the nfound variable in all images when a solution is found, so that the combination is very limited. But that does not seem to work. I alos tried with atomic variables and subroutines, but that is the same story.

My question is therefore condensely formulated as this:

I have a program that examines a lot of independent cases and stops once a certain number of solutions have been found. What is the best way to implement this using coarrays?

2

That is what I would try first, passing back the solutions to node 1 when they are found. But then you need each node to check periodically to see if it still needs to continue searching, and that check might require a lot of communication overhead if done too often. Also, of course, you would expect to accumulate the various solutions in an arbitrary order with this approach, not sequentially as in the original sequential code.