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Subject: Re: [OMPI devel] RFC: add support for large counts using derived datatypes
From: George Bosilca (bosilca_at_[hidden])
Date: 2013-07-16 17:03:27


On Jul 16, 2013, at 22:29 , Jeff Squyres (jsquyres) <jsquyres_at_[hidden]> wrote:

> On Jul 16, 2013, at 4:22 PM, George Bosilca <bosilca_at_[hidden]> wrote:
>
>> Btw, I have a question to you fellow MPI Forum attendees. I just can't remember why the MPI forum felt there was a need for the MPI_Type_get[_true]_extent_x? MPI_Count can't be bigger than MPI_Aint,
>
> Yes, it can -- it has to be the largest integer type (i.e., it even has to be able to handle an MPI_Offset).

Technicalities! In the entire standard MPI_Offset is only used to access files, not to build datatypes. As such there is no way to have the extent of an datatype bigger than MPI_Aint. Thus, these accessors returning MPI_Count are a useless overkill, as they cannot offer more precision that what the version returning MPI_Aint is already offering.

  George.

PS: I hope nobody has the idea to define the MPI_Offset as a signed type …

>> so I don't see what is the benefit of extending the MPI_Type_get_true_extent(MPI_Datatype, MPI_Aint*, MPI_Aint*) and MPI_Type_get_extent(MPI_Datatype, MPI_Aint*, MPI_Aint*) with the corresponding _X versions?
>
>
> --
> Jeff Squyres
> jsquyres_at_[hidden]
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