From: rusrabe@awsrr.rus.uni-stuttgart.de (Rolf Rabenseifner)
Message-Id: <199906180724.JAA16277@awsrr.rus.uni-stuttgart.de>
Subject: Re: MPI_PROC_NULL as target rank in RMA
To: gropp@mcs.anl.gov (William Gropp)
Date: Fri, 18 Jun 1999 09:24:22 +0200 (DST)
Cc: mpi-core@mcs.anl.gov
In-Reply-To: <4.1.19990617130027.00bede50@localhost> from "William Gropp" at Jun 17, 99 01:04:44 pm
Content-Type: text

Bill Gropp wrote: 
> Is it invalid to use MPI_PROC_NULL as the target rank in MPI_Put, MPI_Get,
> and MPI_Accumulate?  I can make the case both for and against this, based
> on the text in the standard.  It would match the spirit of the standard
> (which says that MPI_PUT is like a send at the origin and a receive at the
> target) to allow MPI_PROC_NULL.
> 
> Bill


My vote is to allow MPI_PROC_NULL in MPI_PUT, MPI_GET 
and MPI_ACCUMULATE.

My reasons are:
 
- MPI 2.0 page 115:1-4 "The data transfer [of MPI_PUT] is the same 
  as that which would occur if the origin process executed a send
  operation with arguments ... target_rank, ... and the target 
  process executed a receive operation with arguments ... source, ..."
 
  in combination with
 
  MPI 1.1 page 60:47-48 "The special value MPI_PROC_NULL can be 
  used instead of a rank whereever a source or destination argument
  is required."

  For me, this implies that the same rules are valid for the
  "target_rank" of MPI_PUT as for the "dest" in MPI_SEND
  and this means that MPI_PROC_NULL is allowed. 

  (Bill, have you seen these lines on page 115 before writing
  "I can make the case both for and against this, based
  on the text in the standard." ?) 

- Same argument for MPI_GET with MPI 2.0, page 116:35 "Similar to 
  MPI_PUT, except that the direction of data transfer is reversed." 

- Same argument for MPI_ACCUMULATE based of MPI 2.0, page 120:1-2
  "This is like MPI_PUT except that data is combined into the target
  area instead of overwriting it."

- I didn't find any wording in the standard against usage of 
  MPI_PROC_NULL in the "one-sided" chapter.
 
- spirit of MPI_PROC_NULL 

Rolf


Rolf Rabenseifner                          High Performance Computing
Parallel Computing                         Center Stuttgart    (HLRS)
Rechenzentrum Universitaet Stuttgart (RUS) Phone:    ++49 711 6855530
Allmandring 30                             FAX:      ++49 711 6787626
D-70550 Stuttgart                   rabenseifner@rus.uni-stuttgart.de
Germany                        http://www.hlrs.de/people/rabenseifner