[petsc-users] Help initializing matrix to a constant

[Rohan Yadav]

Thanks Mark, I will try that -- that seems to be what I want.

Junchao, that excerpt seems like it runs into the same problem as above
right? If every rank tries to get the whole matrix then the process will
surely OOM.

Rohan

On Tue, Dec 14, 2021 at 3:27 PM Junchao Zhang <junchao.zhang at gmail.com>
wrote:

> From https://petsc.org/release/src/ksp/ksp/tutorials/ex77.c.html
>
> 114:       MatDenseGetArrayWrite <https://petsc.org/release/docs/manualpages/Mat/MatDenseGetArrayWrite.html#MatDenseGetArrayWrite>(B,&x);115:       for (i=0; i<m*N; ++i) x[i] = 1.0;116:       MatDenseRestoreArrayWrite <https://petsc.org/release/docs/manualpages/Mat/MatDenseRestoreArrayWrite.html#MatDenseRestoreArrayWrite>(B,&x);
>
> --Junchao Zhang
>
>
> On Tue, Dec 14, 2021 at 1:05 PM Rohan Yadav <rohany at alumni.cmu.edu> wrote:
>
>> 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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