[mpich-discuss] Query about MPI_Bsend

25 Apr 2013

      I am trying to implement a logical ring with MPI; where every process
receives a message from process with id one less than that of current
process and forwards to the next process in cyclic fashion. My aim is to
traffic buffer such that it loses some messages or maybe puts them in out
of order.
A cycle of communication will finish when a message dispatched by root node
comes back again to root.
here is the code that I have tried: I am just including relevant parts of
it.

  if(procId!=root)
{
  sleep(100);
  while(1)
{
  tm = MPI_Wtime();
  MPI_Irecv( &message, STR_LEN, MPI_CHAR,
         ((procId-1)>=0?(procId-1):(numProc-1)),RETURN_DATA_TAG,
         MPI_COMM_WORLD,&receiveRequest);

  MPI_Wait(&receiveRequest,&status);
  printf("%d: Received\n",procId);

  if(!strncmp(message,"STOP",4)&&(procId==(numProc-1)))
    break;

  MPI_Ssend( message, STR_LEN, MPI_CHAR,
         (procId+1)%numProc, SEND_DATA_TAG, MPI_COMM_WORLD);
  if(!strncmp(message,"STOP",4))
    break;
  printf("%d: Sent\n",procId);
}
}
else
{
  for(iter=0;iter<benchmarkSize;iter++)
{
  //Synthesize the message
  message[STR_LEN-1] = '\0';
  iErr = MPI_Bsend( message, STR_LEN, MPI_CHAR,
         (root+1)%numProc, SEND_DATA_TAG, MPI_COMM_WORLD);
  if (iErr != MPI_SUCCESS) {
    char error_string[BUFSIZ];
    int length_of_error_string;
    MPI_Error_string(iErr, error_string, &length_of_error_string);
    fprintf(stderr, "%3d: %s\n", procId, error_string);
  }

  tm = MPI_Wtime();
  while(((MPI_Wtime()-tm)*1000)<delay);
  printf("Root: Sending\n");
}

  for(iter=0;iter<benchmarkSize;iter++)
{
  MPI_Recv(message,STR_LEN,MPI_CHAR,
       (numProc-1),RETURN_DATA_TAG,MPI_COMM_WORLD,&status);

  //We should not wait for the messages to be received but wait for
certain amount of time

  //Extract the fields in the message
  if(((prevRcvdSeqNum+1)!=atoi(seqNum))&&(prevRcvdSeqNum!=0))
    outOfOrderMsgs++;
  prevRcvdSeqNum = atoi(seqNum);
  printf("Seq Num: %d\n",atoi(seqNum));
  rcvdMsgs++;
  printf("Root: Receiving\n");
}
  MPI_Isend( "STOP", 4, MPI_CHAR,
     (root+1)%numProc, SEND_DATA_TAG, MPI_COMM_WORLD,&sendRequest);
  MPI_Wait(&sendRequest,&status);

  /*This is to ask all other processes to terminate, when the work is done*/
}

Now, I have these questions:
1) Why is it that when I inject some sleep in the other processes(I mean
other than root) of the ring; NO receive is taking place?
2) Even when the buffer size is only one, how is it that root node is able
to dispatch messages through MPI_Bsend without an error? for example the
case when it needs to send total 10 messages at a rate of 1000 per second
and with buffer size of 1. MPI_Bsend is able to dispatch all the messages
without any error of "buffer full"; irrespective of the presence of sleep()
in other processes of the ring!

In short How on earth is MPI_Bsend is not giving me an error "Insufficient
space in buffer" even if my buffersize is size enough to accommodate only a
single message, MPI_Bsend did not show an error for 65 of the total 100
messages sent? I intentionally made the receive buffers to be unavailable
by making other processes to sleep before they start processing receive and
send.

Thanks a ton!
-- 
V

[mpich-discuss] Query about MPI_Bsend

Vihari Piratla