Question

i have an issue with synchronizing Master/slaves processes using MPI.
I wish that the master control the order of the execution of the slaves. Eeach slave have to do : 1- read 2-process.
Here is my code :

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

int rank,numprocess;

MPI_Init( &argc, &argv );
MPI_Comm_rank( MPI_COMM_WORLD, &rank );
MPI_Comm_size( MPI_COMM_WORLD, &numprocess );


if(rank==0){ //master

    MPI_Status s;int Sstate=1;int Rstate;

    int p;
    for(p=1;p<numprocess;p++){

        fflush(stdout);printf("master : order P%d to start reading\n",p);
        MPI_Send(&Sstate, sizeof(int), MPI_INT, p, 20, MPI_COMM_WORLD);

        MPI_Recv(&Rstate,sizeof(int),MPI_INT,p,21,MPI_COMM_WORLD,&s);
        fflush(stdout);printf("master : P%d finished reading\n",p);
    }

}
else{ //workers

    int state; MPI_Status s;
    MPI_Recv(&state,sizeof(int),MPI_INT,0,20,MPI_COMM_WORLD,&s);

    //read here
    Sleep(1000);

    //send to master : finish reading
    state=2;
    MPI_Send(&state, sizeof(int), MPI_INT, 0, 21, MPI_COMM_WORLD);

    //processing
    Sleep(3000);
    fflush(stdout);printf("worker %d ended processing\n",rank);
}

MPI_Finalize();
return 0;
}

My probleme is with the last process. In fact it didn’t act like the others. here is my output :

mpiexec -n 4 master.exe
master : order P1 to start reading 
master : p1 finished reading
master : order P2 to start reading 
master : p2 finished reading
master : order P3 to start reading 
worker 1 ended processing 
worker 2 ended processing 
master : p3 finished reading
worker 3 ended processing 

Why the third process is not synchronized?
Thank you for precious help!

Answer 1

What you’ve shown is Normal for a multi-threaded or multi-process architecture. Basically, after your processes call MPI_Send to the master process, they are released to function on their own as they have no more blocking MPI calls. Thus after they sleep they print their end statement, independently from one another and the master itself!

Below I have a diagram which I hope will illustrate what you’re seeing: