Hi Rajeev, thanks for the prompt response. I am attempting to ensure that our code conforms to the standard. Would it be correct to say that, for the MPI standard, the communicator used in MPI_Pack must be a superset of the ranks in the communicator used subsequently by MPI calls that use the packed buffer? And likewise, is it correct to state that MPI_Unpack must use a communicator that is a superset of the communicator used to receive or create the buffer that is input to MPI_Unpack? If that is the case, then would MPI_COMM_WORLD always bea valid option for use as a communicator in MPI_Pack and MPI_Unpack? Then, as you said, in a homogeneous system, there should be no disadvantage in specifying MPI_COMM_WORLD. Perhaps the standard and man pages for MPI_Pack could be restated:

The comm argument must include all (be a superset) of all the

communicator"s" that will be

subsequently used for sending the packed message.

And something similar for MPI_Unpack. kindest regards Mike On 18 February 2016 at 12:57, Thakur, Rajeev <[email protected]> wrote:

You can use MPI_COMM_WORLD as the communicator. If you are running on a homogeneous system, there shouldn’t be any performance issue in using MPI_COMM_WORLD as the communicator either.

Rajeev

...

On Feb 17, 2016, at 6:09 PM, Michael Rezny <[email protected]> wrote:

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 _______________________________________________ mpi-forum mailing list [email protected] http://lists.mpi-forum.org/mailman/listinfo.cgi/mpi-forum

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