[Mpi-forum] Question about MPI_Info set on communicators

[Jim Dinan]

"Same" should mean that these call sequences produce the same newcomm,
right?

MPI_Comm_get_info(comm, myinfo_returned);
MPI_Comm_dup_with_info(comm, &newcomm, myinfo_returned);

and

MPI_Comm_dup(comm, &newcomm);

I think this is an argument for "no".

I'm not sure if the question is asking whether the MPI implementation can
change communicator info values dynamically.  I'm not sure the spec
actually addresses that possibility.  If this is allowed, newcomm would
still only get "yes" if the implementation had also toggled the info key on
the parent communicator.

 ~Jim.

On Mon, Feb 15, 2016 at 5:34 PM, Jeff Squyres (jsquyres) <jsquyres at cisco.com
> wrote:

> This question has come up in the Open MPI community:
>
> -----
> MPI_Info_set(myinfo, "use_shared_memory", "yes");
> MPI_Comm_set_info(comm, myinfo);
> MPI_Comm_get_info(comm, myinfo_returned);
> // Assume here that there was not enough shared memory available to use
> shared-memory for
> // communication on this comm, so MPI_Info_get() tells us that
> use_shared_memory is "no".
>
> // ...time passes, and now assume there is more shared-memory available
> MPI_Comm_dup_with_info(comm, &newcom);
> MPI_comm_get_info(newcom, myinfo_returned);
> -----
>
> Should newcom have "use_shared_memory" set to "yes" or "no"?
>
> Arguments for "yes":
> - the user originally asked for "yes"
> - now there is shared memory available
>
> Arguments for "no":
> - the current value for "used_shared_memory" on the parent communicator is
> "no"
>
> Which should it be?
>
> MPI-3.1 6.4.2 p238 says:
>
> "...Returns in newcomer a new communicator with the same group or groups,
> same topology, same info hints, any copied cached information, but a new
> context (see Section 6.7.1)."
>
> So the specific question here is: what exactly does "Same info hints"
> mean?  The user-specific hint, or the current value of the hint?
>
> --
> Jeff Squyres
> jsquyres at cisco.com
> For corporate legal information go to:
> http://www.cisco.com/web/about/doing_business/legal/cri/
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