Error in amgx_mpi_poisson5pt.c example

[fiyanshu7]

There are two errors in the 5 point 2D discretization of Poisson's equation. The A matrix is generated in the example code that comes along with AmgX library as follows:

1.         for (int i = 0; i < n; i ++)
2.         {
3.             row_ptrs[i] = nnz;
4. 
5.             if (rank > 0 || i > ny)
6.             {
7.                 col_indices[nnz] = (i + start_idx - ny);
8. 
9.                 if (sizeof_m_val == 4)
10.                 {
11.                     ((float *)values)[nnz] = -1.f;
12.                 }
13.                 else if (sizeof_m_val == 8)
14.                 {
15.                     ((double *)values)[nnz] = -1.;
16.                 }
17. 
18.                 nnz++;
19.             }
20. 
21.             if (i % ny != 0)
22.             {
23.                 col_indices[nnz] = (i + start_idx - 1);
24. 
25.                 if (sizeof_m_val == 4)
26.                 {
27.                     ((float *)values)[nnz] = -1.f;
28.                 }
29.                 else if (sizeof_m_val == 8)
30.                 {
31.                     ((double *)values)[nnz] = -1.;
32.                 }
33. 
34.                 nnz++;
35.             }
36. 
37.             {
38.                 col_indices[nnz] = (i + start_idx);
39. 
40.                 if (sizeof_m_val == 4)
41.                 {
42.                     ((float *)values)[nnz] = 4.f;
43.                 }
44.                 else if (sizeof_m_val == 8)
45.                 {
46.                     ((double *)values)[nnz] = 4.;
47.                 }
48. 
49.                 nnz++;
50.             }
51. 
52.             if ((i + 1) % ny == 0)
53.             {
54.                 col_indices[nnz] = (i + start_idx + 1);
55. 
56.                 if (sizeof_m_val == 4)
57.                 {
58.                     ((float *)values)[nnz] = -1.f;
59.                 }
60.                 else if (sizeof_m_val == 8)
61.                 {
62.                     ((double *)values)[nnz] = -1.;
63.                 }
64. 
65.                 nnz++;
66.             }
67. 
68.             if ( (rank != nranks - 1) || (i / ny != (nx - 1)) )
69.             {
70.                 col_indices[nnz] = (i + start_idx + ny);
71. 
72.                 if (sizeof_m_val == 4)
73.                 {
74.                     ((float *)values)[nnz] = -1.f;
75.                 }
76.                 else if (sizeof_m_val == 8)
77.                 {
78.                     ((double *)values)[nnz] = -1.;
79.                 }
80. 
81.                 nnz++;
82.             }
83.         }

The two errors are in the following conditions: Line 5. if (rank > 0 || i > ny) Line 52. if ((i + 1) % ny == 0)

The correct statement should be: Line 5. if (rank > 0 || i >= ny) Line 52. if ((i + 1) % ny != 0)

After the above mentioned correction, the output for 4x4 matrix is attached. The solution has been verified against matlab solution.

output.log

[marsaev]

Thanks for the report! Actually it seems this fix is also slightly incorrect with some input parameters. Will track this internally and fix: AMGX-55