C/C++ Trouble with OpenMPI Bcast in C++

[FunkyDwarf]

Hey gang,

I'm trying to use MPIBcast to distribute a user entered value across all nodes, but it seems like only the root node is working (i.e. no broadcast).

I'm using OpenMPI 1.5.4 with the following code:

  A=0;
  std::clock_t start;
  double duration;
  start = std::clock();
  MPI::Status status;
  MPI::Init(argc, argv);
  N=0;
  numnodes = MPI::COMM_WORLD.Get_size();
  mynode = MPI::COMM_WORLD.Get_rank();
  
  if(mynode==0){
    cout<<"Enter number of sections: "<<endl;
    cin>>N;
    MPI::COMM_WORLD.Bcast(&N, 1, MPI::INT, 0);
  }
  MPI::COMM_WORLD.Barrier();
  
  if(mynode!=0) {
    
    while(N==0) {
     
     //Loop until Bcast received
  
    }
    
  }
  
 if(mynode==0) {
  cout<<"root node: "<<N<<endl;
 } else {
   cout<<"other nodes: "<<N<<endl;
   
}

Unfortunately the other node never gets a non-zero value of N and just keeps looping :\ I have a feeling my Bcast syntax or usage is wrong but I don't know why/how. Any help would be great! (I know I could use the wait function instead of a loop but this was quicker :) )

Cheers,
-FD

 

[chiro]

Hey FunkyDwarf.

I noticed that your Bcast procedure is in the if statement and only gets executed if mynodes is 0. Is it meant to be outside the if block just below it?

 

[Pinčiukas]

colective communication routines must be called by all nodes on your commutator. Also theese routines sync up all nodes so you don't need any kind of "Loop until Bcast received".

 

[FunkyDwarf]

What if I only want to input the value for N once? If it is outside the if statement the cin will be called by all nodes. Am i forced to do some send/receive stuff?

(thanks for replys btw)