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 send and receive buffers must be disjoint, and the source and destination of the messages can be the same or different.
• 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, 
  7                b, 10, MPI_INT, (myrank-1+npes)%npes, 1, 
  8                MPI_COMM_WORLD, &status); 
  9   ...
  

• In many programs, the requirement for the send and receive buffers of MPI_Sendrecv be disjoint may force us to use a temporary buffer.
• This increases the amount of memory required by the program and also increases the overall run time due to the extra copy.
• This problem can be solved by using that MPI_Sendrecv_replace MPI function.
• This function performs a blocking send and receive, but it uses a single buffer for both the send and receive operation.
• That is, the received data replaces the data that was sent out of the buffer.