On Fri, Dec 2, 2022 at 7:48 AM Karthikeyan Chockalingam - STFC UKRI via petsc-users <[email protected]> wrote:

Hello,

I have system matrix which is symmetric and thought I could make use of MatCreateSBAIJ. I don’t understand how to set the blocksize bs. I believe it has taken into account, when having multiple components/dofs per node.

(i) Currently, I have only one scalar field to solve so I set bs = 1 as below. Is it correct?

ierr = MatCreateSBAIJ(PETSC_COMM_WORLD, 1, PETSC_DECIDE, PETSC_DECIDE, N, N, d_nz, PETSC_NULL, o_nz, PETSC_NULL, &A); CHKERRQ (ierr);

ierr = MatCreateVecs(A, &right, &left);CHKERRQ(ierr);

(ii) If you multiple components/dofs per node, say 2, is the then block size = 2?

(iii) To apply homogenous Dirichlet boundary condition, I make use of MatZeroRowsColumns. It works in serial but while applying in parallel the following error is thrown,

[0]PETSC ERROR: No method zerorowscolumns for Mat of type mpisbaij

How do I fix it?

(iv) Is there performance again, when using MatCreateSBAIJ for large symmetric system matrix? I read there is more communication involved.

For block size 1, there is no improvement. Even for 2 I would say it is small. The SBAIJ matrix will do the same number of flops and use about the same bandwidth, but it will save on storage. Unless storage is a big deal for you, I would use AIJ, get everything working, and profile to see how important various operations are. Thanks, Matt

Best,

Karthik.

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