[Mpi-forum] MPI_DATATYPE_NULL when count=0

Wed Jan 13 13:41:11 CST 2016

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 ³in place² 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 doesn¹t
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:
>
> Don¹t datatypes with absolute addresses rely on MPI_BOTTOM, and a portable
program can¹t 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 aren¹t 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.  There¹s 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/
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>
>  
>
>
>
> --
> Jeff Hammond
> jeff.science at gmail.com
> http://jeffhammond.github.io/
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--
Jeff Hammond
jeff.science at gmail.com
http://jeffhammond.github.io/

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