[petsc-users] about MatTranspose

[Likun Tan]

I see,
thank you.



On Fri, September 2, 2011 7:33 pm, Jed Brown wrote:
> 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.
>>>
>>
>>
>>
>>
>