Sending and Receiving Messages Simultaneously

• The above communication pattern appears frequently in many message-passing programs,
• For this reason MPI provides the MPI_Sendrecv function that both sends and receives a message.
• MPI_Sendrecv does not suffer from the circular deadlock problems of MPI_Send and MPI_Recv.
• You can think of MPI_Sendrecv as allowing data to travel for both send and receive simultaneously.
• The calling sequence of MPI_Sendrecv is as the following:

  int MPI_Sendrecv(void *sendbuf, int sendcount, MPI_Datatype senddatatype, int dest, int sendtag, 
      void *recvbuf, int recvcount, MPI_Datatype recvdatatype, int source, int recvtag,
      MPI_Comm comm, MPI_Status *status)
  

• The arguments of MPI_Sendrecv are essentially the combination of the arguments of MPI_Send and MPI_Recv.

• The safe version of our previous example using MPI_Sendrecv is as the following;

      1 int a[10], b[10], npes, myrank; 
      2 MPI_Status status; 
      3 ... 
      4 MPI_Comm_size(MPI_COMM_WORLD, &npes); 
      5 MPI_Comm_rank(MPI_COMM_WORLD, &myrank); 
      6 MPI_SendRecv(a, 10, MPI_INT, (myrank+1)%npes, 1, b, 10, MPI_INT, (myrank-1+npes)%npes, 1, MPI_COMM_WORLD, &status); 
      7 ...