[petsc-users] Using MatCreateSBAIJ and MatZeroRowsColumns in parallel

[Matthew Knepley]

On Fri, Dec 2, 2022 at 7:48 AM Karthikeyan Chockalingam - STFC UKRI via
petsc-users <petsc-users at mcs.anl.gov> 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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-- 
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https://www.cse.buffalo.edu/~knepley/ <http://www.cse.buffalo.edu/~knepley/>
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