[mpi-21] Ballot 4 - Re: MPI Process Topologies - discussion?

[Rolf Rabenseifner]

This is a proposal for MPI 2.1, Ballot 4.

This is a follow up to:
  Questions about Graph_create, Cart_crate, and Cart_coords 
  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/topo/

Based on the questions for clarification, I propose:

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

MPI-1.1, Sect. 6.5.3, page 181, line 1-3 read:
   If the size, nnodes, of the graph is smaller than the size of the 
   group of comm, then some processes are returned MPI_COMM_NULL, in 
   analogy to MPI_CART_CREATE and MPI_COMM_SPLIT. 
but should read
   If the size, nnodes, of the graph is smaller than the size of the 
   group of comm, then some processes are returned MPI_COMM_NULL, in 
   analogy to MPI_CART_CREATE and MPI_COMM_SPLIT. If the graph is empty,
   i.e., nnodes == 0, then MPI_COMM_NULL is returned in all processes.  
------
Rationale for this clarification:
   As in MPI_COMM_CREATE, empty groups are allowed, but empty groups are
   described here in a different way, and should be mentioned explicitly 
   therefore.
-------------------------------

After MPI-1.1, Sect. 6.5.3, page 181, line 35, the following paragraph 
should be added:
   It is allowed that at a process, a neighbor process is defined 
   multiple in the list of neighbors (i.e., multiple edges). It is 
   also allowed that a process is neighbor to itself (i.e., a self loop
   in the graph). It is allowed, that the adjacency matrix is not 
   symmetric.

       Advice to users.  Whether using of multiple edges or a 
       non-symmetric adjacency matrix may have possible performance 
       implications is not defined by this standard. The definition 
       of a node-neighbor edge does not imply a direction of the 
       communication. (End of advice to users.) 
------
Rationale for this clarification:
   The Example 6.3, MPI-1.1, page 15, line 29 - page 186, line 13,
   clearly shows multiple edges between nodes and self loops:
   the two (multiple) self-loops of node 0 and of node 7.
   It is nowhere forbidden, that the graph has edges only in 
   one direction.
-------------------------------

After MPI-1.1, Sect. 6.4, page 178, end of the sentence on lines 6-7,
the following sentence should be added:
   All input arguments must have identical values on all processes of the group of comm_old.
------
Rationale for this clarification:
   This statement is missing.  
-------------------------------

I have split this discussion track into this track on graph topologies
and an additional track (will come) on 0-dim MPI_CART_CREATE and MPI_CART_SUB.

Discussion should be done through the new mailing list
mpi-21_at_cs.uiuc.edu.

I have sent out this mail with CC through the old general list
mpi-21_at_[hidden]

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)