Question

I have 3 function and 4 cores. I want execute each function in new thread using MPI and C++
I write this

int rank, size;
MPI_Init(&argc, &argv);
MPI_Comm_rank(MPI_COMM_WORLD,&rank);
MPI_Comm_size(MPI_COMM_WORLD,&size);
size--;
if (rank == 0)
{
    Thread1();
}
else 
{
    if(rank == 1)
    {
        Thread2();
    }
    else
    {
        Thread3();
    }
}
MPI_Finalize();

But it execute just Thread1(). How i must change code?

Thanks!

Answer 1

Print to screen the current value of variable size (possibly without decrementing it) and you will find 1. That is: “there is 1 process running”.

You are likely running your compiled code the wrong way. Consider to use mpirun (or mpiexec, depending on your MPI implementation) to execute it, i.e.

 mpirun -np 4 ./MyCompiledCode

the -np parameter specifies the number of processes you will start (doing so, your MPI_Comm_size will be 4 as you expect).

---

Currently, though, you are not using anything explicitly owing to C++. You can consider some C++ binding of MPI such as Boost.MPI.

---

I worked a little bit on the code you provided. I changed it a little bit producing this working mpi code (I provided some needed correction in capital letters).

FYI:

• compilation (under gcc, mpich):

   $ mpicxx -c mpi1.cpp 
   $ mpicxx -o mpi1 mpi1.o
  

• execution

  $ mpirun -np 4 ./mpi1
  

• output

  size is 4
  size is 4
  size is 4
  2 function started.
  thread2
  3 function started.
  thread3
  3 function ended.
  2 function ended.
  size is 4
  1 function started.
  thread1
  1 function ended.
  

be aware that stdout is likely messed out.

Are you sure you are compiling your code the right way?