[petsc-users] Help initializing matrix to a constant

[Rohan Yadav]

Hi,

I'm having trouble setting all entries of a matrix to a constant value,
similar to the `VecSet` method on vectors. I have a dense matrix that I
want to initialize all entries to 1. The only related method I see on the
`Mat` interface is `MatZeroEntries`, which sets all entries to 0. The
obvious first attempt is to use the `MatSetValue` function to set all
entries to the constant.

```
  Mat C;

  MatCreateDense(PETSC_COMM_WORLD, PETSC_DECIDE, PETSC_DECIDE, k, j, NULL,
&C);

  for (int kk = 0; kk < k; kk++) {

    for (int jj = 0; jj < j; jj++) {

      MatSetValue(C, kk, jj, 1, INSERT_VALUES);

    }

  }

  MatAssemblyBegin(C, MAT_FINAL_ASSEMBLY);

  MatAssemblyEnd(C, MAT_FINAL_ASSEMBLY);
```

However, when run with a relatively large matrix C (5GB) and a
rank-per-core on my 40-core machine this code OOMs and crashes. It does not
OOM with only 1 and 10 rank, leading me to believe that this API call is
somehow causing the entire matrix to be replicated on each rank.

Despite looking through the documentation, I could not find another API
call that would allow me to set all the values in the matrix to a constant.
What should I do here?

Thanks,

Rohan
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