[Mpi-forum] Question about MPI_Info set on communicators

Jeff Hammond jeff.science at gmail.com
Mon Feb 15 16:58:23 CST 2016

Well, this program will not compile, since MPI_Comm_dup_with_info takes
three arguments.  Furthermore, the info is NOT duplicated and instead the
second argument is the info the user provides for the new communicator.


On Mon, Feb 15, 2016 at 2:34 PM, Jeff Squyres (jsquyres) <jsquyres at cisco.com>
> 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:
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Jeff Hammond
jeff.science at gmail.com
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