[petsc-users] Patching in generalized eigen value problems

[Daralagodu Dattatreya Jois, Sathwik Bharadw]

Dear petsc users,


I am solving for generalized eigen value problems using petsc and slepc.

Our equation will be of the form,


A X=λ B X.


I am constructing the A and B matrix of type MATMPIAIJ. Let us consider that

both of my matrices are of dimension 10*10. When we are solving for a closed

geometry, we require to add all the entries of the last (9th) row and column to

the first (0th) row and column respectively for both matrices. In a high density mesh,

I will have a large number of such row to row and column to column additions.

For example, I may have to add last 200 rows and columns to first 200 rows and columns

respectively. We will then zero the copied row and column expect the diagonal

element (9th row/column in the former case).


I understand that MatGetRow, MatGetColumnVector, MatGetValues or any other

MatGet- or VecGet- functions are not collective. Can you suggest any

efficient algorithm or function to achieve this way of patching?


One way I can think of is to obtain the column vector using MatGetColumnVector and

row vector by MatZeroRows and then scatter these vectors to all processes. Once we have

entire row/column vector entries in each process, we can add the values to the matrix by

their global index. Of course, care should be taken to add the values of diagonal element

only once. But this will be a quite slow process.

Any ideas are appreciated.



Thanks,

Sathwik Bharadwaj
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