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)

 

[LudusRex]

Hi FD,

It looks like you are using the correct syntax for MPI::COMM_WORLD.Bcast, so the issue may lie elsewhere. One potential problem could be the use of the "while" loop in your code. This loop may be preventing the other nodes from receiving the broadcasted value because it is continuously checking for a non-zero value of N. Instead of using a loop, you could try using the MPI::COMM_WORLD.Wait function to ensure that the broadcast has been received before proceeding with the rest of the code. Another possible solution could be to use the MPI::COMM_WORLD.Barrier function before the Bcast function to ensure that all nodes are synchronized before the broadcast takes place. I hope this helps and good luck with your project!

Best,