[Mpi-forum] MPI_DATATYPE_NULL when count=0

Wed Jan 13 14:09:36 CST 2016

Can you provide the specifics of a tool that does this today?  I'd like to
read the source code to see what changes are actually required.

How can you piggyback an MPI collective call by hacking the datatype if
count=0?  You already have to check if count=0 and bump it to count=1 if
you are going to send a piggy for a null transfer, so how hard is it to
replace MPI_DATATYPE_NULL with MPI_BYTE?  If you are not sending a piggy
for a null transfer, then the issue is moot.

If nothing else, the overhead of implementing the trivial branch that
solves this issue will be negligible compared to firing up the datatype
engine.

Jeff

On Wed, Jan 13, 2016 at 11:41 AM, Schulz Martin <schulzm at llnl.gov> wrote:

> In general, I agree that allowing any argument for datatype if the count
> is zero makes sense. However, such a change (even if it is just allowing
> more) can cause backwards compatibility problems: e.g., any PMPI tool or
> other kind of internal wrapper library can currently have the assumption
> that it is only passed valid datatypes and hence can use the datatype
> without having to check. We, for example, use this to implement
> piggybacking (by creating a new struct datatype that contains the original
> one and adds a piggyback field). If we allowed arbitrary datatypes, we
> would brake such codes, unless we then also allowed it for datatype
> creation.
>
> If we wanted to change this, we probably need to be very careful and make
> sure we are consistent across the standard and touch all routines that take
> datatype arguments. Just adding the exception for P2P, or even for all
> communication routines, seems insufficient.
>
> Martin
>
>
> ________________________________________________________________________
>
> ________________________________________________________________________
> Martin Schulz, schulzm at llnl.gov, http://scalability.llnl.gov/
> CASC @ Lawrence Livermore National Laboratory, Livermore, USA
>
>
> From: mpi-forum <mpi-forum-bounces at lists.mpi-forum.org> on behalf of Jeff
> Hammond <jeff.science at gmail.com>
> Reply-To: Main mailing list <mpi-forum at lists.mpi-forum.org>
> Date: Wednesday, January 13, 2016 at 6:24 AM
> To: Main mailing list <mpi-forum at lists.mpi-forum.org>
>
> Subject: Re: [Mpi-forum] MPI_DATATYPE_NULL when count=0
>
> That is precisely what I want: when count=0, the buffer and the datatype
> arguments are ignored.  There may be a good reason for MPI_DATATYPE_NULL to
> be invalid when it is actually relevant, but when count=0, the datatype
> argument is not relevant, because there are zero of them.
>
> We already state explicitly that arguments are ignored for MPI_IN_PLACE
> and MPI_NO_OP:
>
> * The “in place” option for intracommunicators is specified by passing
> MPI_IN_PLACE as the value of recvbuf at the root. In such a case, recvcount
> and recvtype are ignored..."
>
> * "When MPI_NO_OP is specified as the operation, the origin_addr,
> origin_count, and origin_datatype arguments are ignored."
>
> I see no reason why count=0 is different from these cases.
>
> Jeff
>
> On Wed, Jan 13, 2016 at 5:47 AM, William Gropp <wgropp at illinois.edu>
> wrote:
> >
> > True, MPI_BOTTOM need not be null, but it might be, so the standard
> doesn’t prohibit it.
> >
> > A carefully worded exception to the general rule for NULL handles would
> be ok.  I would still make it narrow; for example, just for the use of
> MPI_DATATYPE_NULL in communication operations with a zero count.
> >
> > Bill
> >
> > William Gropp
> > Director, Parallel Computing Institute
> > Thomas M. Siebel Chair in Computer Science
> > Chief Scientist, NCSA
> > University of Illinois Urbana-Champaign
> >
> >
> >
> >
> >
> > On Jan 12, 2016, at 11:22 PM, Fab Tillier <ftillier at microsoft.com>
> wrote:
> >
> > Don’t datatypes with absolute addresses rely on MPI_BOTTOM, and a
> portable program can’t assume that MPI_BOTTOM == NULL?
> >
> > I think from an ease of use perspective, if I have a count of zero,
> having to pick an arbitrary committed datatype is non-intuitive.  If count
> is zero, there is no buffer or datatype, and there should be no requirement
> such non-existent datatype be committed.
> >
> > Cheers,
> > -Fab
> >
> > From: mpi-forum [mailto:mpi-forum-bounces at lists.mpi-forum.org] On
> Behalf Of William Gropp
> > Sent: Wednesday, 13 January 2016 4:52 p.m.
> > To: Main MPI Forum mailing list <mpi-forum at lists.mpi-forum.org>
> > Subject: Re: [Mpi-forum] MPI_DATATYPE_NULL when count=0
> >
> > Yes, NULL buffers are valid - they aren’t an MPI object.  There is no
> prohibition about passing them; only if they are used to form an invalid
> address (note that a datatype with absolute addresses relies on NULL as the
> buffer address).
> >
> > Bill
> >
> >
> > William Gropp
> > Director, Parallel Computing Institute
> > Thomas M. Siebel Chair in Computer Science
> > Chief Scientist, NCSA
> > University of Illinois Urbana-Champaign
> >
> >
> >
> >
> > On Jan 12, 2016, at 9:49 PM, Jeff Hammond <jeff.science at gmail.com>
> wrote:
> >
> >
> > Who wrote Example 4.23 of MPI 3.1?
> >
> > I propose to add the exception rather than delete that example. I do not
> want to have to use real datatypes when sending nothing.
> >
> > Is NULL a valid buffer handle? Can I pass null pointers when count=0?
> >
> >
> > Jeff
> >
> >
> > On Tuesday, January 12, 2016, William Gropp <wgropp at illinois.edu> wrote:
> >
> > The standard says that the null handles are invalid for input unless
> explicitly permitted.  There’s no exemption for a datatype argument where
> the associated count is zero.
> >
> > Bill
> >
> >
> > William Gropp
> > Director, Parallel Computing Institute
> > Thomas M. Siebel Chair in Computer Science
> > Chief Scientist, NCSA
> > University of Illinois Urbana-Champaign
> >
> >
> >
> >
> > On Jan 12, 2016, at 9:26 PM, Jeff Hammond <jeff.science at gmail.com>
> wrote:
> >
> >
> > Is it allowed to communicate messages with
> (buffer=NULL,count=0,type=MPI_DATATYPE_NULL)?
> >
> > George thinks MPI_DATATYPE_NULL cannot be used even when count=0, which
> I think is ridiculous, particularly when one makes the analogy to
> buffer=NULL.
> >
> > See http://www.open-mpi.org/community/lists/users/2016/01/28255.php for
> the thread in question.
> >
> > There is example code in MPI that uses this behavior.  George and I
> agree that example text is non-normative, but I cannot imagine how this
> example came to exist without the Forum believing it to be valid.
> >
> > Jeff
> >
> > --
> > Jeff Hammond
> > jeff.science at gmail.com
> > http://jeffhammond.github.io/
> > _______________________________________________
> > mpi-forum mailing list
> > mpi-forum at lists.mpi-forum.org
> > http://lists.mpi-forum.org/mailman/listinfo.cgi/mpi-forum
> >
> >
> >
> >
> >
> > --
> > Jeff Hammond
> > jeff.science at gmail.com
> > http://jeffhammond.github.io/
> > _______________________________________________
> > mpi-forum mailing list
> > mpi-forum at lists.mpi-forum.org
> > http://lists.mpi-forum.org/mailman/listinfo.cgi/mpi-forum
> >
> >
> > _______________________________________________
> > mpi-forum mailing list
> > mpi-forum at lists.mpi-forum.org
> > http://lists.mpi-forum.org/mailman/listinfo.cgi/mpi-forum
> >
> >
> >
> > _______________________________________________
> > mpi-forum mailing list
> > mpi-forum at lists.mpi-forum.org
> > http://lists.mpi-forum.org/mailman/listinfo.cgi/mpi-forum
>
>
>
>
> --
> Jeff Hammond
> jeff.science at gmail.com
> http://jeffhammond.github.io/
>
> _______________________________________________
> mpi-forum mailing list
> mpi-forum at lists.mpi-forum.org
> http://lists.mpi-forum.org/mailman/listinfo.cgi/mpi-forum
>

-- 
Jeff Hammond
jeff.science at gmail.com
http://jeffhammond.github.io/
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