Hi,

I have a question regarding the use of a communicator in MPI_Pack and MPI_Unpack.

I have googled and search the MPI mailing lists but haven't found an answer.

The OpenMPI man page for MPI_Pack states:
The comm argument is the communicator that will be
subsequently used for sending the packed message.

And something similar for MPI_Unpack.

Which seems to reflect what is defined in the MPI standards.

Basically, what I would like to do is to have an MPI rank as a coupler.

That is, receiving a packed message from one application and

passing it to another application.

Comm_A would be the ranks of application A plus the coupler and

Comm_B would be the ranks of application B plus the coupler.

In this case, MPI_COMM_WORLD = Comm_A U Comm_B.

I would like to have a solution that does not have the coupler unpacking

the buffer received from application A and then packing it to send it to application B

I just found a very nice explanation of the need for a communicator in Pack / Unpack on page 135 of Using MPI Portable Parallel Programming with the Message-Passing Interface

second edition by Gropp, Lusk, and Skjellum.

If I understand this correctly, it would be legal to use MPI_COMM_WORLD as the communicator

for MPI_Pack and MPI_Unpack. The only downside is that one might not get the optimal performance.

The alternative we are using is to use MPI_Pack_external and MPI_Unpack_external

which requires no communicator. But the downside is that on homogeneous little-endian HPCs,

this will always entail byte swapping on packing and unpacking.

So my question is: What is the correct and optimal way to implement passing packed messages

in the coupling example described above?

kindest regards

Mike