Is there a way of getting an unique communicator identifier in MPI?

Question

I would like to be able to get a unique id for a certain communicator but this seems an impossible task when you get this communicator by a MPI_Comm_split() call. I know when MPI_Comm_split() is called collectively each of the resulting disjoint communicators has the same context ID yet different group information. I expected by calling MPI_Comm_group(), the group handler associated to a certain communicator would be different for each communicator created by the split, but is the same in all of them!

[CODE]

#include <mpi.h>
#include <stdio.h>

int main() {
    MPI_Init(NULL, NULL);
    int rank;
    MPI_Comm_rank(MPI_COMM_WORLD, &rank);
    MPI_Comm split_comm;
    MPI_Comm_split(MPI_COMM_WORLD, rank / 3, rank, &split_comm);
    int split_rank;
    MPI_Group split_group;
    MPI_Comm_group(split_comm, &split_group);
    MPI_Comm_rank(split_comm, &split_rank);
    printf("rank: %d| comm: %u, group: %u\n", split_rank, split_comm, split_group);
}

If one runs the code above with mpirun -np 6 ./exec the result is:

rank: 0| comm: 2214592516, group: 2281701376
rank: 1| comm: 2214592514, group: 2281701376
rank: 2| comm: 2214592514, group: 2281701376
rank: 0| comm: 2214592514, group: 2281701376
rank: 1| comm: 2214592514, group: 2281701376
rank: 2| comm: 2214592514, group: 2281701376

So it makes impossible to identify which of the two communicators a process belongs to.

Is there any way of getting an id which identifies uniquely a communicator?

I have added a piece of code that hopefully clarifies my question. — Eduardo R.
Are you sure the output corresponds to the code and the execution command line? I wouldn't expect split_rank to go from 0 to 5... — Gilles
Yes, sorry. I had one line wrong. Now it should be alright but the situation is the same with groups and communicator handles. — Eduardo R.
MPI uses opaque objects - they are intentionally hidden from the application. What you have is only a handle, you should not rely on any semantics of that handle except using it in API calls. What do you actually intend to do with that information. How would you define a 'unique communicator' among all ranks in the program? — Zulan

Ed Smith Ed Smith · Accepted Answer · 2016-02-22T19:19:25

As noted in the comments, you have no control over the value MPI sets for its COMMs. You can however set the comm name as follows,

#include <mpi.h>
#include <stdio.h>
#include <iostream>

int main() {

    MPI_Init(NULL, NULL);
    int rank;
    MPI_Comm_rank(MPI_COMM_WORLD, &rank);
    MPI_Comm split_comm;
    int colour = rank/3;
    MPI_Comm_split(MPI_COMM_WORLD, colour, rank, &split_comm);

    // Define string from split colour and use to set comm name
    std::string name = "Comm-";
    name += std::to_string(colour);
    const char * commname = name.c_str();
    MPI_Comm_set_name(split_comm, commname);

    int split_rank;
    MPI_Group split_group;
    MPI_Comm_group(split_comm, &split_group);
    MPI_Comm_rank(split_comm, &split_rank);

    //Retrieve commname and print
    int rlen;
    char nameout[MPI_MAX_OBJECT_NAME];
    MPI_Comm_get_name(split_comm, nameout, &rlen);
    printf("rank: %d| comm: %u, comm_name: %s, group: %u\n", split_rank, split_comm, nameout, split_group);
}

where the output for your mpirun -np 6 ./exec example is,

rank: 0| comm: 2214592516, comm_name: Comm-0, group: 2281701376
rank: 1| comm: 2214592514, comm_name: Comm-0, group: 2281701376
rank: 2| comm: 2214592514, comm_name: Comm-0, group: 2281701376
rank: 0| comm: 2214592514, comm_name: Comm-1, group: 2281701376
rank: 1| comm: 2214592514, comm_name: Comm-1, group: 2281701376
rank: 2| comm: 2214592514, comm_name: Comm-1, group: 2281701376

This may solve the problem?

Is there a way of getting an unique communicator identifier in MPI?

2 Answers