[Mpi-forum] Question about MPI_Info set on communicators

Thu Feb 18 11:56:55 CST 2016

Jim:

Is there a WG session in Chicago where we can discuss this stuff?

I ask because this issue has stalled the Open MPI implementation effort to actually start doing interesting/useful things with MPI_COMM_SET_INFO.

-----Original Message-----
From: Jim Dinan <james.dinan at gmail.com>
Reply: Main MPI Forum mailing list <mpi-forum at lists.mpi-forum.org>
Date: February 18, 2016 at 11:04:02 AM
To: Main MPI Forum mailing list <mpi-forum at lists.mpi-forum.org>
Subject:  Re: [Mpi-forum] Question about MPI_Info set on communicators

> This is the semantic that we have been working with in the info assertions
> proposal. But, I do see the ambiguity.
>  
> Several things have come up that would impact the info assertions proposal:
>  
> 1. If comm_get_info + dup_with_info is not equivalent to comm_dup, then we
> are losing functionality by removing info propagation in dup. We may need
> a new get_info routine to support both types of queries.
>  
> 2. If comm_get_info does not return values indicating the current
> configuration/behavior of the MPI library then we may need to adjust the
> language we added to comm_set_info indicating that users should check
> whether changes to info key values were accepted by the MPI library.
>  
> ~Jim.
>  
> On Wed, Feb 17, 2016 at 8:50 PM, Thakur, Rajeev wrote:
>  
> > In the I/O chapter, an implementation is allowed to ignore the hints
> > provided by the user. MPI_File_get_info returns the current value of hints
> > actually used in the implementation. So suppose a user passes the info key
> > “striping_unit" with value 10, but the system on which the file is created
> > doesn’t support user control of striping unit, the implementation can
> > ignore the user-provided value and, in MPI_File_get_info, return the
> > default striping unit used for the file (which could be 8, say).
> >
> > Rajeev
> >
> >
> >
> > > On Feb 17, 2016, at 5:57 PM, Schulz Martin wrote:
> > >
> > >>
> > >>>> See my reply to Jim: if we're supposed to provide the value of *the
> > >>>> hint*, that to me sounds like the *user's input* (as opposed to what
> > >>> the
> > >>>> system chose to do with that hint).
> > >>>
> > >>> I agree, hints are given by the user to express what an application
> > >>> does.
> > >>> Those should never be changed by the MPI library (or no are no longer
> > >>> (user) hints).
> > >>
> > >> Even if the MPI implementation should never change the value of these
> > >> hints, my original question still stands, because MPI_COMM_GET_INFO's
> > use
> > >> of the phrase "actually used by the system" is still ambiguous (MPI-3.1
> > >> p250):
> > >>
> > >> -----
> > >> // Assume that the MPI implementation understands the "foo" info key
> > >> hint, but
> > >> // is unable to apply it after a communicator is created.
> > >> MPI_Info_set(myinfo, "foo", "yes");
> > >> MPI_Comm_set_info(comm, myinfo);
> > >> MPI_Comm_get_info(comm, myinfo_returned);
> > >> ------
> > >>
> > >> So I still don't have a definitive answer as to what is in
> > >> myinfo_returned:
> > >>
> > >> 1. "foo" = "yes", because that's what the user set, and the MPI
> > >> implementation understood the "foo" hint key (regardless of what it
> > chose
> > >> to do with that hint)
> > >> --> This is what I think should happen
> > >>
> > >> 2. "foo" = "no", because the MPI implementation wasn't able to use the
> > >> "foo" hint
> > >> --> There's at least 2 explicit votes against this behavior (JeffSq,
> > >> Martin)
> > >>
> > >> 3. no "foo" key is set, because the MPI implementation wasn't able to
> > use
> > >> the "foo" hint
> > >> --> I'm not a big fan of this, because you can't tell the difference
> > >> between "foo is not understood by the implementation" and "foo is
> > >> understood by the implementation, but this hint can't be used right
> > now".
> > >
> > > I would say, #3 is correct - the system doesnąt use/apply the hint, I.e.,
> > > the behavior is the same as if the hint wasnąt supplied. I agree, this is
> > > not ideal since knowing what an implementation supports would be good,
> > but
> > > the standard never makes that distinction.
> > >
> > >>
> > >> =========
> > >>
> > >> Regardless of what is returned by MPI_COMM_GET_INFO, what happens in
> > >> MPI_COMM_DUP?
> > >>
> > >> The "actually used by the system" ambiguous phrase only applies to
> > >> MPI_COMM_GET_INFO. MPI_COMM_DUP is defined in MPI-3.1 p238:
> > >>
> > >> "MPI_COMM_DUP duplicates the existing communicator comm with ... info
> > >> hints."
> > >>
> > >> It doesn't have any scoping language about *which* hints are copied --
> > it
> > >> pretty much implies that *all* hints are copied.
> > >>
> > >> As such, I think that "foo"="yes" should be propagated to the new
> > >> communicator.
> > >
> > > Yes, I agree - even if the hint is not used and hence not returned by
> > > MPI_COMM_GET_INFO, itąs still set on the communicator. The user set it
> > and
> > > expects it to be set and so it should be part of the duplication.
> > >
> > >> Which is another reason that I think that MPI_COMM_GET_INFO should also
> > >> return "foo"="yes" in myinfo_returned (i.e., so that MPI_COMM_GET_INFO
> > >> and MPI_COMM_DUP both behave the same way).
> > >
> > > I agree, thatąs what it should do - GET and SET should be matching, but
> > > thatąs not how the standard is written (sadly).
> > >
> > > The idea behavior would be three calls, IMHO:
> > >
> > > MPI_COMM_GET_INFO - return all hints set
> > > MPI_COMM_GET_INFO_UNDERSTOOD - return all hints that could be used by an
> > > implementation
> > > MPI_COMM_GET_INFO_USED - return all hints used (thatąs the current
> > > behavior of MPI_COMM_GET_INFO)
> > >
> > > doing this, though, throws into the usual backwards compatibility
> > > problems, though.
> > >
> > > Martin
> > >
> > >
> > >
> > >>
> > >> Thoughts?
> > >>
> > >> --
> > >> 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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--  
Jeff Squyres
jsquyres at cisco.com  
For corporate legal information go to: http://www.cisco.com/web/about/doing_business/legal/cri/