[mpich-discuss] a question about MPI_Send & MPI_Recv

[chong tan]

I will try to answer this before the real MPICH gurus jump in.

First, MPI_Send() may not be truely blocked, it could be buffered.  So, we can't count on
it being truely blocked for our algorithm to work.

secondly, when calling MPI_Send, the 2nd param is the size of the message to be sent.  THis
should be the intented size, or anything if your algorithm has a bug.  The MPI_Recv() call 
actually do not know the size of the incoming message, the size you specify here is the maximum
size your buffer allow.  In your case, size is the size of the var 'message'.

One experiment you can try is to create a second var like :

char    recvBuf[20] ;

and have the sender send in long messages, say 100 bytes.  Then have the reciever do:

  MPI_Recv( recvBuf, 100, ... )

and see what happen.

tan

 



________________________________
From: samantha lin <wl8150 at googlemail.com>
To: mpich-discuss at mcs.anl.gov
Sent: Tuesday, August 18, 2009 4:10:30 PM
Subject: [mpich-discuss] a question about MPI_Send & MPI_Recv

Hi, 

I got a simple example program from a book, PPMPI. The purpose of the program:
Send a message from all processes with rank != 0 to process 0. Process 0 prints the messages received.

The code is basically like the following:

int main(int argc, char* argv[])
{
....
char        message[100];  /* storage for message  */
...
    if (my_rank != 0) {
        /* Create message */
        sprintf(message, "Greetings from process %d!",
            my_rank);
        printf("len = %d\n", strlen(message));
        dest = 0;
        /* Use strlen+1 so that '\0' gets transmitted */
        MPI_Send(message, strlen(message)+1, MPI_CHAR, dest, tag, MPI_COMM_WORLD);
    } else { /* my_rank == 0 */
        for (source = 1; source < p; source++) {
            MPI_Recv(message, 100, MPI_CHAR, source, tag, MPI_COMM_WORLD, &status);
            printf("%s\n", message);
        }
    }
...
}

The program can compile and run successfully. 
But count size of sent message, strlen(message)+1, is somehow different from that of received message,100.
As this is synchronous send and receive, the receiver is supposed to wait until it receives enough data.
Does anyone know how this is implemented in MPI? Or I misunderstand the functions?

Sam


      
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