Context Identifier

A context identifier of type IMPI_Uint4 is associated with every MPI communicator (intra- and inter-communicators). It has the following properties:

• It uniquely identifies a communicator within a process.
• All processes within a communicator group use the same context identifier for that communicator.

Discussion: The need to negotiate a value for the MPI_COMM_WORLD cid was discussed at the SC98 IMPI meeting. This document previously mandated a cid of 0 for MPI_COMM_WORLD. Arguments for or against changing this to a negotiated value are requested.

The proposed negotiation could be accomplished with an additional per-client COLL label, e.g. (IMPI_C_CID as shown in section 2.3.5. The clients would agree to use the maximum of all the submitted cids for MPI_COMM_WORLD.

Advice to implementors.Mandating a collectively unique context ID may be a burden on some implementations that use memory addresses to segregate message contexts. Such implementations may choose to let the agent handle the mapping between context IDs and memory addresses and not impact the performance of the intra-implementation communication protocols.(End of advice to implementors.)