Unable to implement MPI_Intercomm_create

问题

I am trying to implement an MPI_intercomm in Fortran between 2 inter communicators, one which has first 2 process and the other having the rest. I need to perform send, recv operations between the newly created communicators.

The code:

program hello
include 'mpif.h'
integer tag,ierr,rank,numtasks,color,new_comm,inter1,inter2

tag = 22
call MPI_Init(ierr)
call MPI_COMM_RANK(MPI_COMM_WORLD,rank,ierr)
call MPI_COMM_SIZE(MPI_COMM_WORLD,numtasks,ierr)

if (rank < 2) then
color = 0
else 
color = 1
end if

call MPI_COMM_SPLIT(MPI_COMM_WORLD,color,rank,new_comm,ierr)

if (color .eq. 0) then

call MPI_INTERCOMM_CREATE(new_comm,0,MPI_Comm_world,1,tag,inter1,ierr)

!local_comm,local leader,peer_comm,remote leader,tag,new,ierr

else if(color .eq. 1) then      
call   MPI_INTERCOMM_CREATE(new_comm,1,MPI_COMM_WORLD,0,tag,inter2,ierr)
end if

select case (color)
case (0)
call    MPI_COMM_FREE(inter1)       
case(1)
 call mpi_comm_free(inter2) 

end select

call MPI_finalize(ierr)
end

The code compiles without any issues. But gets stuck while running and sometimes shows error.

回答1:

Short answer: the problem comes from the specification of the remote_leader.

Long answer: I am assuming that your splitting logic is what you want: process 0 and 1 in color 0 and the rest of the world in color 1, and also that you will always have more than 3 processes. You have to choose:

• the local_leader for each color. This is the rank in the local communicator (new_comm in your case) of the leader of each group. The headache free approach is to choose the process of rank 0, because this is the rank in the local communicator, all process can have the exact same value. So I am choosing rank 0.

• the remote_leader for each color; this must be the rank in the peer_comm (MPI_Comm_world in your case) of the leader of the other end of the inter-communicator. It means that, process in color 0 have to know what process 0 in color 1 correspond to in MPI_Comm_world; and process in colors 1 have to know what process 0 in color 0 correspond to in MPI_Comm_world. According to your splitting logic and my logic of choosing the local leader, remote_leader must be process 2 for the color 0, and process 0 for color 1.

And you should be good to go with this modified lines of code:

if (color .eq. 0) then
    if(rank==0) print*, ' 0 here'
    call MPI_INTERCOMM_CREATE(new_comm,0,MPI_Comm_world,2,tag,inter1,ierr)

else if(color .eq. 1) then
    if(rank==2) print*, ' 2 here'
    call   MPI_INTERCOMM_CREATE(new_comm,0,MPI_COMM_WORLD,0,tag,inter2,ierr)
end if

The most important difference with your code is that remote_leader is 2 for color 0. That is the source of the problem. The secondary difference is that local_leader is 0 for color 1. This correspond to my logic of choosing local_leader. It is not the source of the problem, however, it can be if you have only 1 process in color 1.

---

Update

Thanks to Hristo Iliev, I am adding this update. If your goal was to use process 1 of color 1 as local_leader, then the remote_leader for color 0 should be 3 and the code will be:

if (color .eq. 0) then
    if(rank==0) print*, ' 0 here'
    call MPI_INTERCOMM_CREATE(new_comm,0,MPI_Comm_world,3,tag,inter1,ierr)

else if(color .eq. 1) then
    if(rank==2) print*, ' 2 here'
    call   MPI_INTERCOMM_CREATE(new_comm,1,MPI_COMM_WORLD,0,tag,inter2,ierr)
end if

Make sure you check everything for this option as I did not pay special attention to check it. Also make sure that you always have more that 1 process in color 1.

来源：https://stackoverflow.com/questions/38356001/unable-to-implement-mpi-intercomm-create