Can I use MPI_Barrier() to synchronize data in-bet

Question

Is it good idea to use MPI_Barrier() to synchronize data in-between iteration steps. Please see below pseudo code.

While(numberIterations< MaxIterations)
{
   MPI_Iprobe() -- check for incoming data
    while(flagprobe !=0)
    {
        MPI_Recv() -- receive data
        MPI_Iprobe() -- loop if more data
    }

   updateData() -- update myData

   for(i=0;i<N;i++) MPI_Bsend_init(request[i]) -- setup request

   for(i=0;i<N;i++) MPI_Start(request[i]) -- send data to all other N processors

   if(numberIterations = MaxIterations/2) 
      MPI_Barrier() -- wait for all processors -- CAN I DO THIS

   numberIterations ++

}

Answer 1

Your code will deadlock, with or without a barrier. You receive in every rank before sending any data, so none of the ranks will ever get to a send call. Most applications will have a call such as MPI_Allreduce instead of a barrier after each iteration so all ranks can decide whether an error level is small enough, a task queue is empty, etc. and thus decide whether to terminate.

Answer 2

Barriers should only be used if the correctness of the program depends on it. From your pseudocode, I can't tell if that's the case, but one barrier halfway through a loop looks very suspect.

Answer 3

In this article http://static.msi.umn.edu/rreports/2008/87.pdf it says that you have to call MPI_Free_request() before MPI_Bsend_init().