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Date: Wed, 24 Oct 2001 16:57:18 -0500
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From: William Gropp <gropp@XXXXXXXXXXX>
Subject: Re: next ballot
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At 10:05 PM 10/24/2001 +0200, Jesper Larsson Traeff wrote:
On Wed, Oct 24, 2001 at 01:40:45PM -0500, William Gropp wrote:
> >
> >Item 7:
> >
> >Drop rationale (or at least the last sentence, "Extentding it...")
>
> I'd rather leave in the rationale (more information is better than less),
> though I'm open to alternate wordings.
>
Considering what you say below, I don't agree with the rationale.

MPI_REPLACE could be very useful with user-defined datatypes to
implement simultaneous put's by many processes to the same locations
in target window. (this is also why I thought the last sentence should be
dropped)

>
> >Drop the preceeding lines 6-10 on p. 120 (they are confusing, and seem
> >to say that with MPI_Accumulate the predefined op's can also be used with
> >certain user-defined datatypes. This is probably not intended!?)
>
> It was intended that MPI_Accumulate be used with predefined ops on derived
> datatypes.  For example, an indexed datatype with MPI_REPLACE allows
> MPI_Accumulate to perform a scatter operation (with different atomicity
> than MPI_PUT).  Dropping lines 6-10 would significantly change the standard
> as written.
>
I thought/feared that it was intended that MPI_Accumulate can be
used with special derived datatypes. But this is quite a significant
extension of the predefined op's - and why should this then be limited
only to MPI_Accumulate?

Because MPI_Accumulate allows only predefined ops.  This already introduces an exception: not all MPI routines that accept MPI_Op accept user-defined MPI_Op.  Saying the MPI_Accumulate accepts only predefined MPI_Op but allows conforming user-defined types is just a different restriction.

The more that I think about it, the more that I want to vote against 7.  MPI_REPLACE should behave like the other predefined operations (your option 3).  For MPI-1 collective, it should apply only to predefined datatypes.  For MPI_Accumulate, it should be allowed for derived datatypes whose basic elements are a single predefined type, just as for the other predefined ops.

It is true that if we had thought about it, we might have chosen to extend predefined MPI_Op's to the same datatypes allowed for MPI_Accumulate in MPI_Reduce etc.  I propose that we leave that for MPI-3 :)

Bill