MPI datatype alignment

Question

I'm working on a sample coding but just couldn't get send and receive the same value.

the datatype i create is

typedef struct customData{
    double iv;
    double dv[5];
    char cv[10];
} customData;

I create datatype here

MPI_Datatype type;
MPI_Datatype ctype[3] = {MPI_DOUBLE, MPI_DOUBLE, MPI_CHAR};
int blocklen[3] = {1, 5, 10};
MPI_Aint disp[3] = { 0, sizeof(double), sizeof(double)*6 };

MPI_Type_create_struct(1, blocklen, disp, ctype, &type);
MPI_Type_commit(&type);

send and receive

if(rank == 0){
    customData data = {5, 
            {1,2,3,4,5}, 
            {'h','d','h','a','q','w','e','s','l','z'}};
    printf("%f %.2f %c\n", data.iv, data.dv[0], data.cv[0]);

    MPI_Send(&data, 1, type, 1, 0, MPI_COMM_WORLD);     
} else {
    customData recv;
    MPI_Status status;
    MPI_Recv(&recv, 1, type, 0, 0, MPI_COMM_WORLD, &status);
    printf("%f %.2f %c\n", recv.iv, recv.dv[0], recv.cv[0]);
}

The output is
5.000000 1.00 h
5.000000 0.00 �

Generally speaking, you should use `offsetof()` to populate the displacements array (since the compiler might add some padding here and there) — Gilles Gouaillardet, Jun 20 '18 at 22:03

score 1 · Accepted Answer · answered Jun 20 '18 at 21:33

1

You should put the number of element in the structure in your MPI_Type_create_struct, 3 instead of 1.

MPI_Type_create_struct(3, blocklen, disp, ctype, &type);

I tested it and I got:

5.000000 1.00 h
5.000000 1.00 h

Some info here:

Trouble Understanding MPI_Type_create_struct

struct serialization in C and transfer over MPI

answered Jun 20 '18 at 21:33

supercheval

357
3
8

so the "count" is not the number of ctype but the number of entries? – WhoAmIWhereAmIWhatIAmDoing Jun 21 '18 at 01:12
you have 3 variables in ctype corresponding to the 3 variables of your structure. If your structure was made of n variables, you would use count=n. I hope it helps! – supercheval Jun 21 '18 at 12:32

MPI datatype alignment

1 Answers1