[Mpi-forum] use of communicator in MPI_Pack and MPI_Unpack

[Michael Rezny]

Hi Rajeev,
thank you for the clarification. It is most helpful.

kindest regards
Mike


On 18 February 2016 at 13:55, Thakur, Rajeev <thakur at mcs.anl.gov> wrote:

> Your understanding is correct in that the communicator you pass needs to
> contain the processes to which the message is sent or received from. The
> standard is saying it in a simple way.
>
> But I would suggest reconsidering whether you really need pack and unpack
> in the first place. They are rarely used functions, and there is an
> associated performance penalty. You could try sending the messages
> directly, e.g., by using derived datatypes if the data is noncontiguous.
>
> Rajeev
>
>
> > On Feb 17, 2016, at 8:38 PM, Michael Rezny <michael.rezny at monash.edu>
> wrote:
> >
> > 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 <thakur at mcs.anl.gov> 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 <michael.rezny at monash.edu>
> 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
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