Yes I agree: Bill's solution is better, but it requires that the
MPI libraries may Need to be fixed, because then, ndimm==0 && nnodes==1
is no error.
Currently, it is undefined, i.e., a MPI library may handle it
as an erronous input and may call the error handler.
I.e., this solution would require a change log entry that
all MPI library developers check there code.

Proposal: I would add after the Example 7.1 on MPI-3.1 page 293 line 42

 If ndims is zero, then nnodes must be one, and the argument
 dims will be ignored, i.e., there will be no output.

Changelog in "Changes in MPI-4.0":

 Section 7.5.2 on page 792.
 MPI_DIMS_CREATE was extended to additionally allow a zero-dimensional
 Cartesian topology, i.e., input ndims==0 together with nnodes==1.

Bill, please can you provide the ticket/issue/... with probably
better wording ...

Rolf

----- Original Message -----

From: "wgropp" <wgropp@illinois.edu>
To: "Comments on the MPI Standard" <mpi-comments@lists.mpi-forum.org>
Sent: Wednesday, November 9, 2016 7:04:44 PM
Subject: Re: [Mpi-comments] MPI_Dims_create(..., ndims=0, ...) semantic ?

I agree with the analysis of the output but disagree with the conclusion - to
make the behavior consistent with that of MPI_Cart_create, ndim == 0 should be
permitted (with nnodes == 1) but there is, as noted, no output other than a
return value / error value set of MPI_SUCCESS. Otherwise the use of
MPI_Dims_create with MPI_Cart_create in the general case looks like

if (ndim > 0) MPI_Dims_create(…)
MPI_Cart_create(…)

while the intent of the changes for MPI_Cart_create was to allow

MPI_Dims_create(…)
MPI_Cart_create(…)

even if ndim == 0.

Bill

William Gropp
Acting Director and Chief Scientist, NCSA
Director, Parallel Computing Institute
Thomas M. Siebel Chair in Computer Science
University of Illinois Urbana-Champaign





On Nov 9, 2016, at 9:17 AM, Rolf Rabenseifner < [ mailto:rabenseifner@hlrs.de |
rabenseifner@hlrs.de ] > wrote:



We added all that the MPI_CART_... routines work fine with ndims==0.

We oversaw that MPI_DIMS_CREATE is also a candidate for ndims==0.

Reading the standard, I would say that MPI_DIMS_CREATE
- with ndims==0 can never have any output, because
the ndims Array has no element,
- with ndims==0 and nnodes != 1 is erronous
because a zero-dim-cart-topology has alwas exactly one node
(and erronous implies always undefined behavior,
i.e., the MPI library may catch or ignore this error),
- with ndims==0 and nnodes == 1 would be okay but would not
produce any output (because ndims has no entries),
- but compared to the other CART routines where ndims==0
behaviour is always explicitely specified, I would conclude
that MPI_DIMS_CREATE with ndims==0 has always undefined
behavior, i.e., that also ndims==0 and nnodes == 1 may be
treated by the MPI library as erronous.

Therefore, I would recommend to clarify the standard by
defining ndims of MPI_DIMS_CREATE as "positive integer"
instead of only "integer".

Rolf

----- Original Message -----


From: "wgropp" < [ mailto:wgropp@illinois.edu | wgropp@illinois.edu ] >
To: "Comments on the MPI Standard" < [ mailto:mpi-comments@lists.mpi-forum.org |
mpi-comments@lists.mpi-forum.org ] >
Sent: Wednesday, November 9, 2016 3:18:10 PM
Subject: Re: [Mpi-comments] MPI_Dims_create(..., ndims=0, ...) semantic ?



The standard doesn’t specify this, though I recall us discussing this and IIRC
agreeing that ndims equal to zero was well-defined and not an error. And there
is nothing in the standard that suggests that it should be an error, just
nothing that says it shouldn’t.

Bill

William Gropp
Acting Director and Chief Scientist, NCSA
Director, Parallel Computing Institute
Thomas M. Siebel Chair in Computer Science
University of Illinois Urbana-Champaign





On Nov 8, 2016, at 11:40 PM, Gilles Gouaillardet < [ [ mailto:gilles@rist.or.jp
| mailto:gilles@rist.or.jp ] |
[ mailto:gilles@rist.or.jp | gilles@rist.or.jp ] ] > wrote:



Folks,


Does the standard specifiy the behavior of MPI_Dims_create() when the ndims
argument is zero ?


the standard only says negative input values of dims[i] are erroneous, but does
not say anything about ndims.

ndims is the number of Cartesian dimensions (integer), and though a negative
number likely looks erroneous

to most of us, a zero value is incorrect from a mathematical point of view (and
i wish i remembered that ...)


Could someone tell me how a zero value should be interpreted ?

(e.g. valid/invalid/up to the implementor ?)


FWIW, currently

in MPICH, this is a no-op.

In Open MPI, this is an error by default, but if parameter check is manually
disabled, then this is a no-op too.


Cheers,


Gilles

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Dr. Rolf Rabenseifner . . . . . . . . . .. email [ mailto:rabenseifner@hlrs.de |
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High Performance Computing Center (HLRS) . phone ++49(0)711/685-65530
University of Stuttgart . . . . . . . . .. fax ++49(0)711 / 685-65832
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High Performance Computing Center (HLRS) . phone ++49(0)711/685-65530
University of Stuttgart . . . . . . . . .. fax ++49(0)711 / 685-65832
Head of Dpmt Parallel Computing . . . www.hlrs.de/people/rabenseifner
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