C++ for Scientists - Technische Universität Dresden

260 CHAPTER 13. PARALLELISM
}
MPI Finalize();
return 0 ;
13.2.2 Generic Message Passing
Everybody sends to process number 0.
#include
#include
#include
int main (int argc, char∗ argv[])
{
MPI Init(&argc, &argv);
}
int myrank, nprocs;
MPI Comm rank(MPI COMM WORLD, &myrank);
MPI Comm size(MPI COMM WORLD, &nprocs);
float vec[2];
vec[0]= 2∗myrank; vec[1]= vec[0]+1;
// Local accumulation
float local= std::abs(vec[0]) + std::abs(vec[1]);
// Global accumulation
float global= 0.0f;
MPI Status st;
// Receive from predecessor
if (myrank > 0)
MPI Recv(&global, 1, MPI FLOAT, myrank−1, 387, MPI COMM WORLD, &st);
// Increment
global+= local;
// Send to successor
if (myrank+1 < nprocs)
MPI Send(&global, 1, MPI FLOAT, myrank+1, 387, MPI COMM WORLD);
else
std::cout ≪ ”Hello, I am the last process and I know that |v| 1 is ” ≪ global ≪ ”.\n”;
MPI Finalize();
return 0 ;
low abstraction level
The library performs the reduction.
#include
#include
#include