MPITAG

MPITAG refers to the tagging mechanism used in the Message Passing Interface (MPI) to label messages. The tag is an integer parameter attached to communication operations, such as MPI_Send and MPI_Recv, and serves to differentiate multiple messages that may be in transit between the same processes or across a given communicator. The tag value helps the receiving side match incoming messages to the appropriate receive call.

In MPI, the tag space is defined per communicator. The range is typically from 0 up to MPI_TAG_UB minus one, with the special value MPI_ANY_TAG available for receives that should accept messages with any tag. Tags are local to a communicator, meaning that the same tag value can be used independently in different communicators without conflict.

Message matching in MPI uses a combination of three attributes: source, destination, and tag, along with the communicator. A posted receive with a specific tag will only match a corresponding send with that tag. This mechanism allows programs to multiplex different kinds of messages (for example, data versus control messages) over the same communication channel while preserving correctness and order guarantees defined by the MPI standard.

Usage considerations include choosing a well-documented and limited set of tag values to avoid confusion and potential deadlocks. Implementations may enforce limits or provide optimizations based on tag usage. Best practices often involve defining a small, documented tag space per communicator and avoiding tag value collisions across different message types.

MPITAG, as a concept, is central to the MPI model of asynchronous and synchronous communication, enabling flexible and scalable inter-process messaging. It is supported by common MPI implementations such as MPICH and Open MPI and is described in the MPI standard documents.