[petsc-users] Options to set when solving Ax=b if matrix A changes its non-zero position at each solve

[TAY wee-beng]

On 10/6/2012 2:42 PM, Matthew Knepley wrote:
> On Sun, Jun 10, 2012 at 5:42 PM, TAY wee-beng <zonexo at gmail.com 
> <mailto:zonexo at gmail.com>> wrote:
>
>     Hi,
>
>     Currently, my matrix A changes its non zero position at each time
>     step.
>
>     For e.g.: At a particular node, at time step = 1, I  will insert
>     values into its east, west neighbours. At time step = 2, I  will
>     insert values into its east, north neighbours. Hence, the non-zero
>     positions at each row will be different at each time step.
>     However, the max values per row will always be 5 or less.
>
>     Currently, after MatSetValues and MatAssemblyBegin/End, I call:
>
>     /call MatSetOption(A_mat,MAT_NEW_NONZERO_LOCATIONS,PETSC_TRUE,ierr)
>
>     call KSPSetOperators(ksp,A_mat,A_mat,DIFFERENT_NONZERO_PATTERN,ierr)
>
>     call KSPGetPC(ksp,pc,ierr)
>
>     ksptype=KSPBCGS
>
>     call KSPSetType(ksp,ksptype,ierr)
>
>     call KSPSetFromOptions(ksp,ierr)
>
>     call KSPSolve(ksp,b_rhs,xx,ierr)/
>
>     Is this sufficient and correct? If so, apart from KSPSolve,
>     MatSetValues and MatAssemblyBegin/End, can I only call the other
>     subroutines once at the 1st time step?
>
>     Or do I have to explicitly destroy the matrix and create a new
>     one? I think I ask this question a while ago but I can't find the
>     email now.
>
>
> After assembly the nonzero pattern is fixed. The right thing to do is 
> put in explicit zeros for the rest of the stencil.
>
>   Matt
Hi,

Just to clarify, do you mean that I create a bigger stencil and put in 
explicit zeros for those which I don't need at each time step?

I realise that for my problem, instead of the original 5 non-zero 
locations (east, west, north, south, center) for the stencil, I now have 
4 more (east-east, west-west etc). So now I call MATCREATEAIJ with dnz + 
o_nz = 9 (instead of previously 5). Is that correct?
>
>
>     -- 
>     Yours sincerely,
>
>     TAY wee-beng
>
>
>
>
> -- 
> What most experimenters take for granted before they begin their 
> experiments is infinitely more interesting than any results to which 
> their experiments lead.
> -- Norbert Wiener
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