I tried scattering a matrix by row to all the processors but it resulted in a segmentation fault.. I don’t know what i am doing wrong.. Here is my code
if(rank == 0) {
A_row = 10;
A_col = 10;
/* calculate the strip size */
strip_size = A_row / size;
/* genarate Matrix A */
A = (double **)malloc(sizeof(double*) * 10);
int k = 0;
for(i = 0; i < 10; i++) {
A[i] = (double*)malloc(sizeof(double) * 10);
for(j = 0; j < 10; j++) {
A[i][j] = k;
k++;
printf("%lf ", A[i][j]);
}
printf("\n");
}
}
/* Broadcasting the row, column size of Matrix A as well as strip size and Matrix B*/
MPI_Bcast(&A_row, 1, MPI_INT, 0, MPI_COMM_WORLD);
MPI_Bcast(&A_col, 1, MPI_INT, 0, MPI_COMM_WORLD);
MPI_Bcast(&strip_size, 1, MPI_INT, 0, MPI_COMM_WORLD);
/* defining a datatype for sub-matrix */
MPI_Type_vector(strip_size, A_col, A_col, MPI_DOUBLE, &strip);
MPI_Type_commit(&strip);
strip_A = (double **)malloc(sizeof(double*)*strip_size);
for(i= 0; i< strip_size; i++) {
strip_A[i] = (double*)malloc(sizeof(double)*A_col);
}
MPI_Scatter(&A[0][0], 1, strip, &strip_A[0][0], 1, strip, 0, MPI_COMM_WORLD);
for(i = 0; i < strip_size; i++) {
if(i == 0) {
printf("rank = %d\n", rank);
}
for(j = 0; j < A_col; j++) {
printf("%lf ", strip_A[i][j]);
}
printf("\n");
}
Can anyone tell me what is wrong…
here is my error when I run
mpirun -np 2 ./a.out
0.000000 1.000000 2.000000 3.000000 4.000000 5.000000 6.000000 7.000000 8.000000 9.000000
10.000000 11.000000 12.000000 13.000000 14.000000 15.000000 16.000000 17.000000 18.000000 19.000000
20.000000 21.000000 22.000000 23.000000 24.000000 25.000000 26.000000 27.000000 28.000000 29.000000
30.000000 31.000000 32.000000 33.000000 34.000000 35.000000 36.000000 37.000000 38.000000 39.000000
40.000000 41.000000 42.000000 43.000000 44.000000 45.000000 46.000000 47.000000 48.000000 49.000000
50.000000 51.000000 52.000000 53.000000 54.000000 55.000000 56.000000 57.000000 58.000000 59.000000
60.000000 61.000000 62.000000 63.000000 64.000000 65.000000 66.000000 67.000000 68.000000 69.000000
70.000000 71.000000 72.000000 73.000000 74.000000 75.000000 76.000000 77.000000 78.000000 79.000000
80.000000 81.000000 82.000000 83.000000 84.000000 85.000000 86.000000 87.000000 88.000000 89.000000
90.000000 91.000000 92.000000 93.000000 94.000000 95.000000 96.000000 97.000000 98.000000 99.000000
rank = 1
42.000000 43.000000 44.000000 45.000000 46.000000 47.000000 48.000000 49.000000 0.000000 0.000000
52.000000 53.000000 54.000000 55.000000 56.000000 57.000000 58.000000 59.000000 0.000000 0.000000
0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000
0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000
0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000
[seismicmstm:07338] *** Process received signal *** [seismicmstm:07338] Signal:
Segmentation fault (11)
[seismicmstm:07338] Signal code:
(128) [seismicmstm:07338] Failing at
address: (nil)
-------------------------------------------------------------------------- mpirun noticed that process rank 1 with PID 7338 on node seismicmstm.cluster exited on signal 11 (Segmentation fault).
--------------------------------------------------------------------------
There’s a couple things going on here. The good news is that the hardest stuff — getting the mpi data type created, and the basic structure of the MPI_Scatter call — are correct.
The first issue is that the MPI_Scatter line uses &(A[0][0]) — but in all but rank zero, you haven’t set A to point to anything! So you dereference a random pointer twice, and there’s your segfault.
A more subtle issue, as suggested by suszterpatt, is that there’s no guarantee that your rows of allocated memory are contiguous, so your scatter operation may not work even if you fix the above. You are trying to send strip_size * A_col doubles from somewhere in A to strip_A, but strip_A may not consist of that many doubles contiguously – it could be A_col doubles, and then some padding, then A_col doubles — or indeed, the various rows could be scattered all over memory. The three ways of fixing that are, in order of ease (IMHO): (a) make the data contiguous in memory by creating the entire array and then creating the two-d C arrays to point to the right places; (b) to just send one row at a time; or (c) to create an MPI data type that actually reflects how your data is mapped (perhaps randomly) in memory.
An approach using (a) that seems to work (for A_row evenly divided by size, anyway) looks like below: