MPI_Rank return same process number for all process
mpi, openmpi
Solution
Make sure that both `mpicc` and `mpirun` come from the same MPI implementation. When `mpirun` fails to provide the necessary universe information to the launched processes, with the most common reason for that being that the executable was build against a different MPI implementation (or even a different version of the same implementation), `MPI_Init()` falls back to the so-called singleton MPI initialisation and creates an `MPI_COMM_WORLD` that only contains the calling process. Thus the result is many MPI processes within their own separate `MPI_COMM_WORLD` instances.
Usually commands like `mpicc --showme`, `which mpicc` and `which mpirun` could help you find out if that is the case indeed.
Problem
I'm trying to run this sample hello world program with openmpi and mpirun on debian 7. ``` #include <stdio.h> #include <mpi/mpi.h> int main (int argc, char **argv) { int nProcId, nProcNo; int nNameLen; char szMachineName[MPI_MAX_PROCESSOR_NAME]; MPI_Init (&argc, &argv); // Start up MPI MPI_Comm_size (MPI_COMM_WORLD,&nProcNo); // Find out number of processes MPI_Comm_rank (MPI_COMM_WORLD, &nProcId); // Find out process rank MPI_Get_processor_name (szMachineName, &nNameLen); // Get machine name printf ("Hello World from process %d on %s\r\n", nProcId, szMachineName); if (nProcId == 0) printf ("Number of Processes: %d\r\n", nProcNo); MPI_Finalize (); // Shut down MPI return 0; } ``` My problem is `MPI_Comm_Rank` returns 0 for all copies of the process. When I run this command on the shell: ``` mpirun -np 4 helloWorld ``` It produces this output: ``` Hello World from process 0 on debian Number of Processes: 1 Hello World from process 0 on debian Number of Processes: 1 Hello World from process 0 on debian Number of Processes: 1 Hello World from process 0 on debian Number of Processes: 1 ``` Why is the number of processes still 1?