[petsc-users] about MatTranspose

[Jed Brown]

Remember that the code has a different B on every process. You can use a
viewer on COMM_SELF, but you should send it to different files so the output
isn't all mixed together.
On Sep 1, 2011 1:33 PM, "Likun Tan" <likunt at andrew.cmu.edu> wrote:
> Thank you very much.
>
> With the command:
> for(j=0; j<n; j++)
> {
> for(i=0; i<M; i++)
> {
> MatSetValues(B, 1, &i, 1, &j, &Value, INSERT_VALUES);
> }
> }
> This only defines the columns from 0 to n-1? How exactly MPI_Scan()
> getting used here?
>
> And when i try to print out the result with
>
> PetscViewer viewer;
> PetscViewerSetFormat(viewer, PETSC_VIEWER_ASCII_MATLAB);
> MatView(B, viewer);
>
> I can only see the columns from 0 to n-1. Maybe there is still problem in
> computing B.
>
> Many thanks,
> Likun
>
> On Thu, September 1, 2011 1:54 pm, Jed Brown wrote:
>> On Thu, Sep 1, 2011 at 12:45, Likun Tan <likunt at andrew.cmu.edu> wrote:
>>
>>
>>> I still have some confusions. When computing B before MPI_Scan(), could
>>> i compute the values in parallel? After using MPI_Scan(), does that mean
>>> the columns of B will be gathered in one processor?
>>>
>>
>> No, the scan is just computing the start column for a given process.
>>
>>
>>
>>>
>>> Here are main steps i took based on your suggestion,
>>>
>>>
>>> PetscSplitOwnership(PETSC_COMM_WORLD, &n, &N);
>>>
>>>
>>
>> Add these two lines here:
>>
>>
>> MPI_Scan(&n, &cstart, 1, MPIU_INT, MPI_SUM, PETSC_COMM_WORLD);
>> cstart -= n;
>>
>>
>>> MatCreateSeqDense(PETSC_COMM_SELF, M, n, PETSC_NULL, &B);
>>> MatCreateSeqDense(PETSC_COMM_SELF, M, M, PETSC_NULL, &A);
>>>
>>>
>>
>> Good
>>
>>
>>
>>> MatCreateSeqDense(PETSC_COMM_SELF, M, N, PETSC_NULL, &x);
>>>
>>>
>>
>> Replace with
>>
>>
>> MatDuplicate(B,MAT_DO_NOT_COPY_VALUES,&x);
>>
>>
>>
>>> for(j=0; j<N; j++)
>>>
>>
>> change the loop bounds here:
>>
>> for(j=0; j<n; j++)
>>
>>
>>> {
>>> for(i=0; i<M; i++) {
>>>
>>>
>>
>> Good, now compute value as the value that goes in (i,cstart+j).
>>
>>
>> MatSetValues(B, 1, &i, 1, &j, &value, INSERT_VALUES);
>>
>>> }
>>> }
>>> MatAssemblyBegin(...);
>>> MatAssemblyEnd(...)
>>>
>>>
>>
>> This part is correct.
>>
>>
>>
>>> MPI_Scan(&n, &cstart, 1, MPIU_INT, MPI_SUM, PETSC_COMM_WORLD);
>>> cstart -= n;
>>>
>>
>> We did this already.
>>
>>
>>
>>>
>>> MatConvert(...);
>>> MatCholeskyFactor(...);
>>> MatMatSolve(...);
>>>
>>>
>>
>> Yes.
>>
>>
>>
>> You can gather the matrix x onto all processes if you need the whole
>> result everywhere, but for performance reasons if you scale further, you
>> should avoid it if possible.
>>
>
>
>
>
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