[Mpi-comments] MPI_Get_count_x

[George Bosilca]

Hubert,

This function belong to ticket #265 (https://svn.mpi-forum.org/trac/mpi-forum-web/ticket/265) the one adding support for MPI_Count. I think the rationale was the following. If the "compatible" number of top-level elements is greater than an int (this is what MPI_Get_count can safely returns), then MPI_Get_count is supposed to return MPI_UNDEFINED, and the application should fallback on using MPI_Get_elements_X instead.

I thought I could provide an implementation for MPI_Get_count_X, but without knowledge about the MPI implementation, or an impressively costly MPI_Type_get_envelope/MPI_Type_get_contents I couldn't figure out how. Moreover, I don't think there is an easy way to distinguish if the MPI_UNDEFINED returned by MPI_Get_count is due to a lack of space for the return type or a mismatch in the size of the data.

This should be added to the growing list of things to be fixed post 3.0. Meanwhile, implement your own [expensive] way of parsing datatypes using MPI_Type_get_envelope/MPI_Type_get_contents if you want to implement a really portable library (aka supporting MPI_Count).

  george.

PS: MPI_Type_create_keyval can be helpful here in order to speed things up when a datatype is used multiple times.

On Aug 31, 2012, at 16:20 , Hubert Ritzdorf <Hubert.Ritzdorf at EMEA.NEC.COM> wrote:

> Hi George,
> 
> I know MPI_Get_elements_x (). But Page 483 claims:
> 
> These functions modify the opaque part of status so that a call to
> MPI_GET_ELEMENTS or MPI_GET_ELEMENTS_X will return count. MPI_GET_COUNT
> will return a compatible value.
> 
> This will not work for large MPI_Count values provided by MPI_SET_ELEMENTS_X
> (i.e. MPI_UNDEFINED is returned by MPI_Get_count() in significant use of MPI_SET_ELEMENT_X).
> And Page 114 says that function MPI_Get_count has a different behavior than
> MPI_GET_ELEMENTS/MPI_GET_ELEMENTS_X. Thus, MPI_GET_COUNT is not obsolete
> and application has to develop a separate version of MPI_Get_count_x() if MPI_SET_ELEMENT_X
> might be used in the application or a library.
> 
> Hubert
> 
> From: mpi-comments-bounces at lists.mpi-forum.org [mpi-comments-bounces at lists.mpi-forum.org] on behalf of George Bosilca [bosilca at eecs.utk.edu]
> Sent: Friday, August 31, 2012 2:26 PM
> To: Comments on the MPI Standard
> Subject: Re: [Mpi-comments] MPI_Get_count_x
> 
> MPI_Get_elements_x, chap Datatypes page 114 (current make cleandoc).
> 
>   george.
> 
> On Aug 31, 2012, at 13:45 , Hubert Ritzdorf <Hubert.Ritzdorf at EMEA.NEC.COM> wrote:
> 
>> Hi,
>> 
>> I think, that function 
>> 
>> MPI_Get_count_x (const MPI_Status *status, MPI_Datatype datatype, MPI_Count *count)
>> 
>> is missing. After use of function MPI_Status_set_elements_x, MPI_Get_count is possibly not able
>> to return a compatible value. This is contradictory to Page 483, Lines 22-33.
>> 
>> Hubert
>> 
>> 
>> 
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