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Subject: [OMPI users] MPI_Intercomm_create: peer_leader process
From: Jalel Chergui (LIMSI-CNRS) <Jalel.Chergui_at_[hidden] (Jalel.Chergui_at_[hidden])
Date: 2009-07-10 11:40:12


Hello,

   Two questions from an old MPI user who's still learning.

   Let's assume mpirun's spawned N copies of program A and M (/=N) copies of
   program B. This execution option leads A and B processes to belong to the
   same MPI_COMM_WORLD and ranks are ordered consequently from 0 to N-1 for A
   and from N to (N+M)-1 for B.

   Now, since A and B are two independent programs, let's consider that
   MPI_COMM_WORLD has to be splitted into two communicator groups using
   MPI_Comm_split. Then, as we know, the color argument must be set so that
   its value is different for A and B.

   Q1: A part from setting it as a constant in A and B or reading it from a
       file, is there a more elegant way to set it automatically so that
       color(A) is always different from color(B)?

   Furthermore, let's consider that MPI_COMM_WORLD has been splitted into A
   and B groups anyway. We want to create an inter-communicator between them
   using MPI_Intercomm_create. Group B can always set the peer_leader rank
   process of A as 0.

   Q2: Is there a MPI-1 solution that can return to A any peer_leader (or first)
       rank of the remote group B?

   Both questions Q1 and Q2 can be bypassed using MPI_Comm_spawn or
   server/client base processing or a combination of both. But this is not my
   aim since the constraint is to stick to a MPI-1 implementation to execute
   such MPMD applications on some special architecture like BlueGene machine.

Thanks a lot for your help.
Regards,
Jalel

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