Re: [OMPI devel] Datasize confusion in MPI_Write can lead to data los!

[George Bosilca]

MPI_Type_size is supposed to return only the size of useful data, which apparently it does (MPI_SHORT_INT is 6 bytes). What I think it happens is that the MPI_SHORT_INT type is a predefined one, but it's a really strange predefined type. It's one of the few that are not contiguous. The problem seems to come from the fact that the MPI_File_write do a contiguous write for the predefined data types, making the assumption that they are all contiguous.

  Thanks,
    george.

On Feb 8, 2008, at 12:50 PM, Christoph Niethammer wrote:

I tracked the problem down in the romio/adio/common/is_contig.c file. For Open MPI the last #else branch is used. The first case in the switch check for the MPI_COMBINER_NAMED (which is what an MPI is supposed to return for predefined data types) and set the flag to 1 (which means contiguous). This is obviously wrong for MPI_SHORT_INT. It really look like a ROMIO problem, so I guess this email should be redirected to their mailing list.

Hello!


lead to data los:

When creating a struct for a mixed datatype like

struct {
 short a;
 int b;
}


type due to the 4byte alignment of the integer on 32bit systems.


MPI_SHORT_INT as mpi_datatype this leads to a data los.


MPI_File_write.

of the complete data size to be written into the file.

This problem occures also in the other io functions.

data types.

The question is now how to "fix":

MPI_SHORT_INT, MPI_DOUBLE_INT, ... should be forbidden to be used with
structs of these types,

return the value of eg. true_ub which contains the correct value

the "continues" way on the data and should take care of the gaps.

Regards

Christoph Niethammer
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I tested openMPI at HLRS for some time without detecting new problems in the implementation but now I recognized some awful ones with MPI_Write which can the C-compiler introduce a gap of 2 bytes in the data representation for this If I now try to use MPI_File_write to write these data to a file and use I located the problem at the combined use of "write" and MPI_Type_size in So MPI_Type_size(MPI_SHORT_INT) returns 6 bytes where the struct uses 8 bytes in memory as there is a gap of 2 bytes. The write function in ad_write.c now leads to the los of the data because the gaps are not within the calculation As far as I could find out the problem seems not to be present with derived i) Either the MPI_Standard is not clear in this point and the data types ii) Or the implementation of the MPI_Type_size function has to be modified to iii) Or the MPI_File_write function has not to use the write function in

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