[OMPI users] (no subject)

[Alberto Giannetti]

I have simple MPI program that sends data to processor rank 0. The  
communication works well but when I run the program on more than 2  
processors (-np 4) the extra receivers waiting for data run on > 90%  
CPU load. I understand MPI_Recv() is a blocking operation, but why  
does it consume so much CPU compared to a regular system read()?


#include <sys/types.h>
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <mpi.h>

void process_sender(int);
void process_receiver(int);


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

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

  printf("Processor %d (%d) initialized\n", rank, getpid());

  if( rank == 1 )
    process_sender(rank);
  else
    process_receiver(rank);

  MPI_Finalize();
}


void process_sender(int rank)
{
  int i, j, size;
  float data[100];
  MPI_Status status;

  printf("Processor %d initializing data...\n", rank);
  for( i = 0; i < 100; ++i )
    data[i] = i;

  MPI_Comm_size(MPI_COMM_WORLD, &size);

  printf("Processor %d sending data...\n", rank);
  MPI_Send(data, 100, MPI_FLOAT, 0, 55, MPI_COMM_WORLD);
  printf("Processor %d sent data\n", rank);
}


void process_receiver(int rank)
{
  int count;
  float value[200];
  MPI_Status status;

  printf("Processor %d waiting for data...\n", rank);
  MPI_Recv(value, 200, MPI_FLOAT, MPI_ANY_SOURCE, 55,  
MPI_COMM_WORLD, &status);
  printf("Processor %d Got data from processor %d\n", rank,  
status.MPI_SOURCE);
  MPI_Get_count(&status, MPI_FLOAT, &count);
  printf("Processor %d, Got %d elements\n", rank, count);
}