Point-to-Point Communication

Overview

Teaching: 10 min
Exercises: 10 min

Questions

• How do I send data between processes?

Objectives

• Describe what is meant by point-to-point communication.

• Learn how to send and receive data between ranks.

In the previous episode we introduced the various types of communication in MPI. In this section we will use the MPI library functions MPI_Send and MPI_Recv, which employ point-to-point communication, to send data from one rank to another.

Let’s look at how MPI_Send and MPI_Recvare typically used:

• Rank A decides to send data to rank B. It first packs the data to send into a buffer, from which it will be taken.
• Rank A then calls MPI_Send to create a message for rank B. The underlying MPI communication is then given the responsibility of routing the message to the correct destination.
• Rank B must know that it is about to receive a message and acknowledge this by calling MPI_Recv. This sets up a buffer for writing the incoming data when it arrives and instructs the communication device to listen for the message.

As mentioned in the previous episode, MPI_Send and MPI_Recv are synchronous operations, and will not return until the communication on both sides is complete.

Sending a Message: MPI_Send

The MPI_Send function is defined as follows:

int MPI_Send(
    const void* data,
    int count,
    MPI_Datatype datatype,
    int destination,
    int tag,
    MPI_Comm communicator)

The arguments to

For example, if we wanted to send a message that contains "Hello, world!\n" from rank 0 to rank 1, we could state (assuming we were rank 0):

char *message = "Hello, world!\n";
MPI_Send(message, 14, MPI_CHAR, 1, 0, MPI_COMM_WORLD);

So we are sending 14 elements of MPI_CHAR one time, and specified 0 for our message tag since we don’t anticipate having to send more than one type of message. This call is synchronous, and will block until the corresponding MPI_Recv operation receives and acknowledges receipt of the message.

MPI_Ssend: an Alternative to MPI_Send

MPI_Send represents the “standard mode” of sending messages to other ranks, but some aspects of its behaviour are dependent on both the implementation of MPI being used, and the circumstances of its use: there are three scenarios to consider:

1. The message is directly passed to the receive buffer, in which case the communication has completed
2. The send message is buffered within some internal MPI buffer but hasn’t yet been received
3. The function call waits for a corresponding receiving process

In scenarios 1 & 2, the call is able to return immediately, but with 3 it may block until the recipient is ready to receive. It is dependent on the MPI implementation as to what scenario is selected, based on performance, memory, and other considerations.

A very similar alternative to MPI_Send is to use MPI_Ssend - synchronous send - which ensures the communication is both synchronous and blocking. This function guarantees that when it returns, the destination has categorically started receiving the message.

Receiving a Message: MPI_Recv

Conversely, the MPI_Recv function looks like the following:

int MPI_Recv(
    void* data,
    int count,
    MPI_Datatype datatype,
    int source,
    int tag,
    MPI_Comm communicator,
    MPI_Status* status)

Continuing our example, to receive our message we could write:

char message[15];
MPI_Status status;
MPI_Recv(message, 14, MPI_CHAR, 0, 0, MPI_COMM_WORLD, &status);

Here, we create our buffer to receive the data, as well as a variable to hold the exit status of the receive operation. We then call MPI_Recv, specifying our returned data buffer, the number of elements we will receive (14) which will be of type MPI_CHAR and sent by rank 0, with a message tag of 0. As with MPI_Send, this call will block - in this case until the message is received and acknowledgement is sent to rank 0, at which case both ranks will proceed.

Let’s put this together with what we’ve learned so far. Here’s an example program that uses MPI_Send and MPI_Recv to send the string "Hello World!" from rank 0 to rank 1:

#include <stdio.h>
#include <mpi.h>

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

  // First call MPI_Init
  MPI_Init(&argc, &argv);

  // Check that there are two ranks
  MPI_Comm_size(MPI_COMM_WORLD,&n_ranks);
  if( n_ranks != 2 ){
    printf("This example requires exactly two ranks\n");
    MPI_Finalize();
    return(1);
  }

  // Get my rank
  MPI_Comm_rank(MPI_COMM_WORLD,&rank);

  if( rank == 0 ){
     char *message = "Hello, world!\n";
     MPI_Send(message, 14, MPI_CHAR, 1, 0, MPI_COMM_WORLD);
  }

  if( rank == 1 ){
     char message[14];
     MPI_Status  status;
     MPI_Recv(message, 14, MPI_CHAR, 0, 0, MPI_COMM_WORLD, &status);
     printf("%s",message);
  }

  // Call finalize at the end
  return MPI_Finalize();
}

MPI Data Types in C

In the above example we send a string of characters and therefore specify the type MPI_CHAR. For a complete list of types, see the MPICH documentation.

Try It Out

Compile and run the above code. Does it behave as you expect?

Solution

mpicc mpi_hello_world.c -o mpi_hello_world
mpirun -n 2 mpi_hello_world

Note above that we specified only 2 ranks, since that’s what the program requires (see line 12). You should see:

Hello, world!

What Happens If…

Try modifying, compiling, and re-running the code to see what happens if you…

1. Change the tag integer of the sent message. How could you resolve this where the message is received?
2. Modify the element count of the received message to be smaller than that of the sent message. How could you resolve this in how the message is sent?

Solution

1. The program will hang since it’s waiting for a message with a tag that will never be sent (press Ctrl-C to kill the hanging process). To resolve this, make the tag in MPI_Recv match the tag you specified in MPI_Send.
2. You will likely see a message like the following:

   [...:220695] *** An error occurred in MPI_Recv
   [...:220695] *** reported by process [2456485889,1]
   [...:220695] *** on communicator MPI_COMM_WORLD
   [...:220695] *** MPI_ERR_TRUNCATE: message truncated
   [...:220695] *** MPI_ERRORS_ARE_FATAL (processes in this communicator will now abort,
   [...:220695] ***    and potentially your MPI job)
   

   You could resolve this by sending a message of equal size, truncating the message. A related question is whether this fix makes any sense!

Many Ranks

Change the above example so that it works with any number of ranks. Pair even ranks with odd ranks and have each even rank send a message to the corresponding odd rank.

Solution

#include <stdio.h>
#include <mpi.h>

int main(int argc, char** argv) {
    int rank, n_ranks, my_pair;
  
    // First call MPI_Init
    MPI_Init(&argc, &argv);
  
    // Get the number of ranks
    MPI_Comm_size(MPI_COMM_WORLD,&n_ranks);

    // Get my rank
    MPI_Comm_rank(MPI_COMM_WORLD,&rank);

    // Figure out my pair
    if( rank%2 == 1 ){
       my_pair = rank - 1;
    } else {
       my_pair = rank + 1;
    }

    // Run only if my pair exists
    if( my_pair < n_ranks ){

       if( rank%2 == 0 ){
           char *message = "Hello, world!\n";
           MPI_Send(message, 14, MPI_CHAR, my_pair, 0, MPI_COMM_WORLD);
       }
  
       if( rank%2 == 1 ){
           char message[14];
           MPI_Status  status;
           MPI_Recv(message, 14, MPI_CHAR, my_pair, 0, MPI_COMM_WORLD, &status);
           printf("%s",message);
        }
    }

    // Call finalize at the end
    return MPI_Finalize();
}

Hello Again, World!

Modify the Hello World code below so that each rank sends its message to rank 0. Have rank 0 print each message.

#include <stdio.h>
#include <mpi.h>

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

    // First call MPI_Init
    MPI_Init(&argc, &argv);

    // Get my rank
    MPI_Comm_rank(MPI_COMM_WORLD, &rank);

    printf("Hello World, I'm rank %d\n", rank);

    // Call finalize at the end
    return MPI_Finalize();
}

Solution

#include <stdio.h>
#include <mpi.h>

int main(int argc, char** argv) {
    int rank, n_ranks, numbers_per_rank;
 
    // First call MPI_Init
    MPI_Init(&argc, &argv);
    // Get my rank and the number of ranks
    MPI_Comm_rank(MPI_COMM_WORLD, &rank);
    MPI_Comm_size(MPI_COMM_WORLD, &n_ranks);
 
    if( rank != 0 ) {
       // All ranks other than 0 should send a message
 
       char message[30];
       sprintf(message, "Hello World, I'm rank %d\n", rank);
       MPI_Send(message, 30, MPI_CHAR, 0, 0, MPI_COMM_WORLD);
 
    } else {
       // Rank 0 will receive each message and print them
 
       for( int sender = 1; sender < n_ranks; sender++ ) {
          char message[30];
          MPI_Status status;
 
          MPI_Recv(message, 30, MPI_CHAR, sender, 0, MPI_COMM_WORLD, &status);
          printf("%s",message);
       }
    }
    
    // Call finalize at the end
    return MPI_Finalize();
}

Blocking

Try the code below with two ranks and see what happens. How would you change the code to fix the problem?

Note: If you are using the MPICH library, this example might automagically work. With OpenMPI it shouldn’t!)

#include <mpi.h>

#define ARRAY_SIZE 3

int main(int argc, char **argv) {
    MPI_Init(&argc, &argv);

    int rank;
    MPI_Comm_rank(MPI_COMM_WORLD, &rank);

    const int comm_tag = 1;
    int numbers[ARRAY_SIZE] = {1, 2, 3};
    MPI_Status recv_status;

    if (rank == 0) {
        /* synchronous send: returns when the destination has started to
           receive the message */
        MPI_Ssend(&numbers, ARRAY_SIZE, MPI_INT, 1, comm_tag, MPI_COMM_WORLD);
        MPI_Recv(&numbers, ARRAY_SIZE, MPI_INT, 1, comm_tag, MPI_COMM_WORLD, &recv_status);
    } else {
        MPI_Ssend(&numbers, ARRAY_SIZE, MPI_INT, 0, comm_tag, MPI_COMM_WORLD);
        MPI_Recv(&numbers, ARRAY_SIZE, MPI_INT, 0, comm_tag, MPI_COMM_WORLD, &recv_status);
    }

    return MPI_Finalize();
}

Solution

MPI_Send will block execution until the receiving process has called MPI_Recv. This prevents the sender from unintentionally modifying the message buffer before the message is actually sent. Above, both ranks call MPI_Send and just wait for the other to respond. The solution is to have one of the ranks receive its message before sending.

Sometimes MPI_Send will actually make a copy of the buffer and return immediately. This generally happens only for short messages. Even when this happens, the actual transfer will not start before the receive is posted.

For this example, let’s have rank 0 send first, and rank 1 receive first. So all we need to do to fix this is to swap the send and receive in the case of rank 1 (after the else):

        MPI_Recv(&numbers, ARRAY_SIZE, MPI_INT, 0, comm_tag, MPI_COMM_WORLD, &recv_status);
        MPI_Ssend(&numbers, ARRAY_SIZE, MPI_INT, 0, comm_tag, MPI_COMM_WORLD);

Ping Pong

Write a simplified simulation of Ping Pong according to the following rules:

• Ranks 0 and 1 participate
• Rank 0 starts with the ball
• The rank with the ball sends it to the other rank
• Both ranks count the number of times they get the ball
• After counting to 1 million, the rank is bored and gives up
• There are no misses or points

Solution

#include <stdio.h>
#include <mpi.h>

int main(int argc, char** argv) {
    int rank, neighbour;
    int max_count = 1000000;
    int counter;
    int bored;
    int ball = 1; // A dummy message to simulate the ball
    MPI_Status status;

    // First call MPI_Init
    MPI_Init(&argc, &argv);

    // Get my rank
    MPI_Comm_rank(MPI_COMM_WORLD, &rank);

    // Call the other rank the neighbour
    if( rank == 0 ){
        neighbour = 1;
    } else {
        neighbour = 0;
    }

    if( rank == 0 ){
        // Rank 0 starts with the ball. Send it to rank 1
        MPI_Send(&ball, 1, MPI_INT, 1, 0, MPI_COMM_WORLD);
    }

    // Now run a send and receive in a loop until someone gets bored
    // the behaviour is the same for both ranks
    counter = 0;
    bored = 0;
    while( !bored )
    {
        // Receive the ball
        MPI_Recv(&ball, 1, MPI_INT, neighbour, 0, MPI_COMM_WORLD, &status);
      
        // Increment the counter and send the ball back
        counter += 1;
        MPI_Send(&ball, 1, MPI_INT, neighbour, 0, MPI_COMM_WORLD);

        // Check if the rank is bored
        bored = counter >= max_count;
    }
    printf("rank %d is bored and giving up \n", rank);

    // Call finalize at the end
    return MPI_Finalize();
}

Key Points

• Use MPI_Send and MPI_Recv to send and receive data between ranks

• Using MPI_SSend will always block the sending rank until the message is received

• Using MPI_Send may block the sending rank until the message is received, depending on whether the message is buffered and the buffer is available for reuse

• Using MPI_Recv will always block the receiving rank until the message is received