CODEWITHSUNDEEP
cmpi
Arjun J Rao
Arjun J Rao

Reputation: 935

Why is this MPI program stalling

//#define SIZE 3
void master(int n,int size)
{
for(int j=1;j<size;j++){
    MPI_Send(&n,1,MPI_INT,j,1,MPI_COMM_WORLD);
    printf("\n Sent %d to process %d",n,j);
    fflush(stdout);
    }
}

void slave(int size)
{
for(int j=1;j<size;j++){
    int k=0;
    MPI_Status status;
    MPI_Recv(&k,1,MPI_INT,0,1,MPI_COMM_WORLD,&status);
    printf("\n Process %d has received %d",j,k);
    fflush(stdout);
    }
}

int main(int argc,char** argv)
{

MPI_Init(&argc,&argv);
int la_size;
int rank;
MPI_Comm_size(MPI_COMM_WORLD,&la_size);

MPI_Comm_rank(MPI_COMM_WORLD,&rank);

for(int i=0;i<3;i++){
    if(rank==0)
        master(i,la_size);
    else
        slave(la_size);

}
MPI_Finalize();
printf("\nprogram finished...");
fflush(stdout);
return 0;

}

The program above seems simple enough but its stalling. Is it a deadlock ? The output is : 

     Sent 0 to process 1  
 Sent 0 to process 2  
 Sent 1 to process 1  
 Sent 1 to process 2  
 Process 1 has received 0  
 Process 2 has received 1  
 Process 1 has received 2  
 Sent 2 to process 1  
 Sent 2 to process 2  
 Process 1 has received 0  
 Process 2 has received 1  
 Process 1 has received 2

Upvotes: 0

Views: 492

Answers (2)

Arjun J Rao
Arjun J Rao

Reputation: 935

Thanks to Novelocrat's answer, the correct code is actually this : 

#include <stdio.h>
#include <mpi.h>
#include <stdlib.h>
//#define SIZE 3

void master(int n,int size)
{
    for(int j=1;j<size;j++){
    MPI_Send(&n,1,MPI_INT,j,1,MPI_COMM_WORLD);
    printf("\n Sent %d to process %d",n,j);
    fflush(stdout);
    }
}

void slave(int size)
{
    int k=0,rank=0;
    MPI_Comm_rank(MPI_COMM_WORLD,&rank);
    MPI_Status status;
    MPI_Recv(&k,1,MPI_INT,0,1,MPI_COMM_WORLD,&status);
    printf("\n Process %d has received %d",rank,k);
    fflush(stdout);

}

int main(int argc,char** argv)
{

    MPI_Init(&argc,&argv);
    int la_size;
    int rank;
    MPI_Comm_size(MPI_COMM_WORLD,&la_size);

    MPI_Comm_rank(MPI_COMM_WORLD,&rank);

    for(int i=0;i<3;i++){
    if(rank==0)
        master(i,la_size);
    else
        slave(la_size);

    }
    MPI_Finalize();
    printf("\nprogram finished...");
    fflush(stdout);
    return 0;

}

Upvotes: 0

Phil Miller
Phil Miller

Reputation: 38158

In each iteration of the loop in main, rank 0 is doing a single send to each of the many slave ranks, but each of the slaves is posting as many receives as there are slave ranks in total. Since there are no posted sends to match the later receives, the receivers block indefinitely, and the program hangs.

Upvotes: 2

Related Questions