[mpi-21] Ballot 4 - Why no MPI_INPLACE for MPI_EXSCAN?

[Rolf Rabenseifner]

This is a proposal for MPI 2.1, Ballot 4.

This is a follow up to:
  Why no MPI_INPLACE for MPI_EXSCAN? 
  in http://www.cs.uiuc.edu/homes/wgropp/projects/parallel/MPI/mpi-errata/index.html
with mail discussion in
  http://www.cs.uiuc.edu/homes/wgropp/projects/parallel/MPI/mpi-errata/discuss/exscan/

Bill Gropp has already put a proposal on the web page.
I would chang the location:
Instead of putting the clarification at the end of the Advice to users,
I would add at the beginning of the 2nd paragraph of the Rationale.

Proposal for MPI 2.1, Ballot 4:
-------------------------------

MPI-2, Sect. 7.3.6, page 167. lines 6-8 read:
     The reason that MPI-1 chose the inclusive scan is that the definition
  of behavior on processes zero and one was thought to offer too many
  complexities in definition, particularly for user-defined operations.
  (End of rationale.)
but should read:
     No in-place version is specified for MPI_EXSCAN because it isn't 
  clear what this means for the process for rank zero.
  The reason that MPI-1 chose the inclusive scan is that the definition
  of behavior on processes zero and one was thought to offer too many
  complexities in definition, particularly for user-defined operations.
  (End of rationale.)
-------------------------------

Background information:
The total rationale On MPI-2 page 167 would be then

     Rationale. The exclusive scan is more general than the inclusive scan
  provided in MPI-1 as MPI SCAN. Any inclusive scan operation can be
  achieved by using the exclusive scan and then locally combining the
  local contribution. Note that for noninvertable operations such as
  MPI MAX, the exclusive scan cannot be computed with the inclusive
  scan.
     No in-place version is specified for MPI_EXSCAN because it isn't clear 
  what this means for the process for rank zero.
  The reason that MPI-1 chose the inclusive scan is that the definition
  of behavior on processes zero and one was thought to offer too many
  complexities in definition, particularly for user-defined operations.
  (End of rationale.)

Best regards
Rolf

Dr. Rolf Rabenseifner . . . . . . . . . .. email rabenseifner_at_[hidden]
High Performance Computing Center (HLRS) . phone ++49(0)711/685-65530
University of Stuttgart . . . . . . . . .. fax ++49(0)711 / 685-65832
Head of Dpmt Parallel Computing . . . www.hlrs.de/people/rabenseifner
Nobelstr. 19, D-70550 Stuttgart, Germany . (Office: Allmandring 30)