Assignment

Your assignment is to use TotalView to locate an error in the following program. This is a 4-processor MPI example. It uses gather to collect the results from the individual process. It does not produce correct results.

The correct output is:
0 0 204 204 408 408 612 612

#include <stdio.h> #include "mpi.h" int main( int argc, char **argv ) { int a[2][8], b[8], cpart[2], ctotal[8]; int rank, size, i, k, root; MPI_Init( &argc, &argv ); MPI_Comm_rank( MPI_COMM_WORLD, &rank ); MPI_Comm_size( MPI_COMM_WORLD, &size ); root = 1; if (size != 4) { printf("Error!:# of processors must be equal to 4\n"); printf("Programm aborting....\n"); MPI_Abort(MPI_COMM_WORLD, 1); } for (i=0;i<2;i++){ for (k=0;k<8;k++) a[i][k] = rank*(k+1); } /* for (i=0;i<2;i++){ for (k=0;k<8;k++) { if(k<7) printf("%d ",a[i][k]); if(k==7) printf("%d\n",a[i][k]); } } */ for (k=0;k<8;k++) b[k] = k+1; for (i=0;i<2;i++){ cpart[i] = 0; for (k=0;k<8;k++) cpart[i]=cpart[i]+a[i][k]*b[k]; } /* printf("%d %d\n",cpart[0],cpart[1]); */ MPI_Gather(&cpart,2,MPI_INT,&ctotal,8,MPI_INT,root,MPI_COMM_WORLD); if (rank == root) { for (k=0;k<8;k++) { if(k<7) printf("%d ",ctotal[k]); if(k==7) printf("%d\n",ctotal[k]); } } MPI_Finalize(); return 0; }

Details

cut and paste the code into the file gather.c

icc -g -lmpi -o gather gather.c

mpirun -np 4 ./gather

Tips

• Launch Totalview with:
  totalview mpirun -a -np 4 ./gather

A solution

It has been a common mistake that in the use of MPI_gather function one sets the receive account is n times of the send count, where n is the number of processes (n=4 in this example) when no derived datatype is used. For the simple datatype used in this example, rcound should be equal to scount.

#include <stdio.h> #include "mpi.h" int main( int argc, char **argv ) { int a[2][8], b[8], cpart[2], ctotal[8]; int rank, size, i, k, root; MPI_Init( &argc, &argv ); MPI_Comm_rank( MPI_COMM_WORLD, &rank ); MPI_Comm_size( MPI_COMM_WORLD, &size ); root = 1; if (size != 4) { printf("Error!:# of processors must be equal to 4\n"); printf("Programm aborting....\n"); MPI_Abort(MPI_COMM_WORLD, 1); } for (i=0;i<2;i++){ for (k=0;k<8;k++) a[i][k] = rank*(k+1); } /* for (i=0;i<2;i++){ for (k=0;k<8;k++) { if(k<7) printf("%d ",a[i][k]); if(k==7) printf("%d\n",a[i][k]); } } */ for (k=0;k<8;k++) b[k] = k+1; for (i=0;i<2;i++){ cpart[i] = 0; for (k=0;k<8;k++) cpart[i] = cpart[i] + a[i][k] * b[k]; } /* printf("%d %d\n",cpart[0],cpart[1]); */ MPI_Gather(&cpart, 2, MPI_INT, &ctotal, 2, MPI_INT, root, MPI_COMM_WORLD); if (rank == root) { for (k=0;k<8;k++) { if(k<7) printf("%d ",ctotal[k]); if(k==7) printf("%d\n",ctotal[k]); } } MPI_Finalize(); return 0; }