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

[Richard Treumann]

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

mpi-21-bounces_at_[hidden] wrote on 01/18/2008 12:55:09 PM:

> 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)
> _______________________________________________
> mpi-21 mailing list
> mpi-21_at_[hidden]
> http://lists.cs.uiuc.edu/mailman/listinfo/mpi-21





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