Re: [OMPI users] data types and alignment to word boundary

[Gus Correa]

jody wrote:

Hi

I have noticed on my machine that a struct which i have defined as

typedef struct {
    short  iSpeciesID;
    char   sCapacityFile[SHORT_INPUT];
    double adGParams[NUM_GPARAMS];
} tVStruct;

(where SHORT_INPUT=64 and NUM_GPARAMS=4)

has size 104 (instead of 98) whereas the corresponding MPI Datatype i created

    int aiLengthsT5[3]       = {1, SHORT_INPUT, NUM_GPARAMS};
    MPI_Aint aiDispsT5[3]    = {0, iShortSize, iShortSize+SHORT_INPUT};
    MPI_Datatype aTypesT5[3] = {MPI_UNSIGNED_SHORT, MPI_CHAR, MPI_DOUBLE};
    MPI_Type_create_struct(3, aiLengthsT5, aiDispsT5, aTypesT5,
&m_dtVegetationData3);
    MPI_Type_commit(&m_dtVegetationData3);

only has length 98 (as expected). The size differences resulted in an
error when doing

    tVegetationData3 VD;
    MPI_Send(&VD, 1, m_dtVegetationData3, 1, TAG_STEP_CMD, MPI_COMM_WORLD);

and the corresponding

    tVegetationData3 VD;
    MPI_Recv(&VD, 1, m_dtVegetationData3, MPI_ANY_SOURCE,
TAG_STEP_CMD, MPI_COMM_WORLD, &st);

(in fact, the last double in my array was not transmitted correctly)

It seems that on my machine the struct was padded to a multiple of 8.
By manually adding some padding bytes to my MPI Datatype in order
to fill it up to the next multiple of 8 i could work around this problem.
(not very nice, and very probably not portable)


My question: is there a way to tell MPI to automatically use the
required padding?


Thank You
  Jody
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Hi Jody

My naive guesses:

I think when you create the MPI structure you can pass the
byte displacement of each structure component.
You would need to modify your aiDispsT5[3], to match the
actual memory alignment, I guess.
Yes, indeed portability may be sacrificed.

There is some clarification in "MPI, The Complete Reference, Vol 1,
2nd Ed, Marc Snir et al.".
Section 3.2 and 3.3 (general on type map & type signature).
Section 3.4.8 MPI_Type_create_struct (examples, specially 3.13).
Section 3.10, on portability, doesn't seem to guarantee portability of
MPI_Type_Struct.

I hope this helps,
Gus Correa