[Mpi-21] [mpi-21] problem with MPI_Get_count and MPI_Probe

Richard Treumann treumann at [hidden]
Wed Mar 5 15:31:34 CST 2008


I did not notice this until I was going over the Ballot 4 slides.  I would
prefer not to vote in the currently proposed wording

The correction is for page 22, not 222

I propose:

MPI 1.1, page 22, line 48 reads
  used after a call to MPI_PROBE. (End of rationale.)
but should read
  used after a call to MPI_PROBE or MPI_IPROBE.
  In this case the call to MPI_GET_COUNT should use the same
  datatype as will be used in the MPI_RECV.
  (End of rationale.)

  (Advice to users.)
  The buffer size required for the receive can be affected by
  data conversions and by the stride of the receive
  datatype. Calling MP_GET_COUNT with MPI_BYTE to calculate the
  receive buffer size may not be portable unless the MPI_RECV
  uses MPI_BYTE.
  (End of advice to users.)

Dick Treumann  -  MPI Team/TCEM
IBM Systems & Technology Group
Dept 0lva / MS P963 -- 2455 South Road -- Poughkeepsie, NY 12601
Tele (845) 433-7846         Fax (845) 433-8363

                                                                           
             "Rolf                                                         
             Rabenseifner"                                                 
             <rabenseifner_at_hlr                                          To 
             s.de>                     mpi-21_at_[hidden]                  
             Sent by:                                                   cc 
             mpi-21-bounces_at_cs         mpi-21_at_[hidden]                
             .uiuc.edu                                             Subject 
                                       Re: [mpi-21] problem with           
                                       MPI_Get_count and MPI_Probe         
             01/18/2008 12:55                                              
             PM                                                            
                                                                           
                                                                           
             Please respond to                                             
             "Mailing list for                                             
             discussion of MPI                                             
                   2.1"                                                    
             <mpi-21_at_cs.uiuc.e                                             
                    du>                                                    
                                                                           
                                                                           

This is a proposal for MPI 2.1, Ballot 4.

This is a follow up to:
  Datatypes and MPI_PROBE
  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/probedatatype/

Proposal for MPI 2.1, Ballot 4:
-------------------------------
MPI 1.1, page 222, line 48 reads
  used after a call to MPI_PROBE. (End of rationale.)
but should read
  used after a call to MPI_PROBE or MPI_IPROBE.
  With a status returned from MPI_PROBE or MPI_IPROBE, the same
  dataypes are allowed as in a call to MPI_RECV to receive this message.
  (End of rationale.)

  Advice to users. To allocate the appropriate amount of memory as receive
  buffer, the same datatype as in the following receive call should be used
  to determine the needed space. In portable programs due to possible data
  conversions, it is not guaranteed that the count returned by
MPI_GET_COUNT
  with datatype MPI_BYTE is the correct amount of needed memory space in
  the receive buffer (although MPI_BYTE is matching every datatype).
  (End of advice to users.)
-------------------------------

Reason for the first part: The current MPI-1.1 text says "The datatype
argument
should match the argument provided by the receive call that set the status
variable." With MPI_PROBE, there isn't such a receive call.

Reason for the advice to users: It helps to write portable code.
Because malloc needs a byte count, users may write wrong programs
by using MPI_BYTE.

-------------------------------

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)
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