When to use tags when sending and receiving messages in MPI?

Question

I'm not sure when I have to use different numbers for the tag field in MPI send, receive calls. I've read this, but I can't understand it.

Sometimes there are cases when A might have to send many different types of messages to B. Instead of B having to go through extra measures to differentiate all these messages, MPI allows senders and receivers to also specify message IDs with the message (known as tags). When process B only requests a message with a certain tag number, messages with different tags will be buffered by the network until B is ready for them.

Do I have to use tags, for example, when I have multiple calls "isend" (with different tags) from process A and only 1 call to "ireceive" in process B?

Answer 1

Message tags are optional. You can use arbitrary integer values for them and use whichever semantics you like and seem useful to you.

Like you suggested, tags can be used to differentiate between messages that consist of different types (MPI_INTEGER, MPI_REAL, MPI_BYTE, etc.). You could also use tags to add some information about what the data actually represents (if you have an nxn matrix, a message to send a row of this matrix will consist of n values, as will a message to send a column of that matrix; nevertheless, you may want to treat row and column data differently).

Note that the receive operation has to match the tag of a message it wants to receive. This, however, does not mean that you have to specify the same tag, you can also use the wildcard MPI_ANY_TAG as message tag; the receive operation will then match arbitrary message tags. You can find out which tag the sender used with the help of MPI_Probe.

Answer 2

In general, I tend to avoid them. There is no requirement that you use tags. If you need to get the message size before parsing the message, you can use MPI_Probe. That way you can send different messages rather than specifying Tags. I typically use tags because MPI_Recv requires that you know the message size before fetching the data. If you have different sizes and types, tags can help you differentiate between them by having multiple threads or processes listening over a different subset. Tag 1 can mean messages of type X and tag 2 will be messages of type Y. Also, it enables you to have multiple "channels" of communication without having to do the work of creating unique communicators and groups.

#include <mpi.h> #include <iostream>  using namespace std;  int main( int argc, char* argv[] ) {     // Init MPI     MPI_Init( &argc, &argv);      // Get the rank and size     int rank, size;     MPI_Comm_rank( MPI_COMM_WORLD, &rank );     MPI_Comm_size( MPI_COMM_WORLD, &size );      // If Master     if( rank == 0 ){          char* message_r1 = "Hello Rank 1";         char* message_r2 = "Hello Rank 2";          // Send a message over tag 0         MPI_Send( message_r1, 13, MPI_CHAR, 1, 0, MPI_COMM_WORLD );          // Send a message over tag 1         MPI_Send( message_r2, 13, MPI_CHAR, 2, 1, MPI_COMM_WORLD );       }     else{          // Buffer         char buffer[256];         MPI_Status status;          // Wait for your own message         MPI_Recv( buffer, 13, MPI_CHAR, 0, rank-1, MPI_COMM_WORLD, &status );          cout << "Rank: " << rank << ", Message: " << buffer << std::endl;      }      // Finalize MPI     MPI_Finalize();  }