From: Dave Goodell <goodell@XXXXXXXXXXX>
Date: January 22, 2008 4:55:00 PM CST
To: mpi-21@XXXXXXXXXXXXX
Cc: Dave Goodell <goodell@XXXXXXXXXXX>
Subject: Re: [mpi-21] Proposal EH2 & MPI_INPLACE
Reply-To: mpi-21@XXXXXXXXXXXXX

Erez,

Bronis and Dries are probably referring to collectives like  
MPI_SCATTER that *do* use MPI_IN_PLACE in the recv buffer.  See the  
MPI-2 spec, chapter 7, page 154, line 22:

     The ``in place'' option for intracommunicators is specified by  
passing MPI IN PLACE as
     the value of recvbuf at the root. In such case, recvcount and  
recvtype are ignored, and root
     ``sends'' no data to itself. The scattered vector is still  
assumed to contain n segments, where
     n is the group size; the root-th segment, which root should  
``send to itself,'' is not moved.

-Dave

On Jan 22, 2008, at 4:45 PM, Erez Haba wrote:

> :) ... and I though I answered the right question (:
>
> Operations that use the MPI_IN_PLACE, specify it as the send-buffer  
> argument. These collectives write into the receive-buffer and not  
> the send-buffer (MPI_IN_PLACE value is used as a flag).
>
> Which is why I don't see the problem. Having the send buffer as  
> const is perfectly alright.
>
> .Erez
>
> -----Original Message-----
> From: owner-mpi-21@mpi-forum.org [mailto:owner-mpi-21@mpi- 
> forum.org] On Behalf Of Bronis R. de Supinski
> Sent: Tuesday, January 22, 2008 2:32 PM
> To: mpi-21@mpi-forum.org
> Subject: RE: [mpi-21] Proposal EH2 & MPI_INPLACE
>
>
> Erez:
>
> The issue at hand was not whether MPI_INPLACE should be const,
> it's whether the send buffer should be const for operations that
> use MPI_INPLACE for the receive buffer. The problem is that the
> send buffer is modified.
>
> Bronis
>
> On Tue, 22 Jan 2008, Erez Haba wrote:
>
>> I don't see the problem, would you care to elaborate?
>>
>> As far as I know the users specifies MPI_IN_PLACE as the sendbuf  
>> and the real read/write buff to the recvbuff. Thus the  
>> implementation read/writes to the recvbuf and not to the  
>> MPI_IN_PLACE value.
>>
>> As a complement to the const proposal we should actually define  
>> MPI_IN_PLACE and a pointer to const (e.g., '#define MPI_IN_PLACE   
>> (const void*)-1'), which will help people avoid making the mistake  
>> of specifying MPI_IN_PLACE to the recv buffer.
>>
>> Thanks,
>> .Erez
>>
>> -----Original Message-----
>> From: owner-mpi-21@mpi-forum.org [mailto:owner-mpi-21@mpi- 
>> forum.org] On Behalf Of Dries Kimpe
>> Sent: Tuesday, January 22, 2008 12:40 PM
>> To: mpi-21@mpi-forum.org
>> Subject: [mpi-21] Proposal EH2 & MPI_INPLACE
>>
>> There is a problem with adding const to the send buffer of some of  
>> the
>> collective operations:
>>
>> Because of MPI_IN_PLACE, in some cases, the send buffer will not be
>> read-only but read-write.
>>
>> Example:
>> MPI_Allreduce
>>
>> Affected:
>> All collectives that support MPI_IN_PLACE --AND-- modify the data  
>> in the send
>> buffer.
>>
>> Solution:
>> Drop the const from the send buffer argument of the affected  
>> functions
>>
>>   Dries
>>
>>
>