[petsc-users] Eliminating rows and columns which are zeros

[Barry Smith]

  Yes, after the solve the x will contain correct values for ALL the locations including the (zeroed out rows). You use case is exactly what redistribute it for.

  Barry


> On Jan 10, 2023, at 11:25 AM, Karthikeyan Chockalingam - STFC UKRI <karthikeyan.chockalingam at stfc.ac.uk> wrote:
> 
> Thank you Barry. This is great!
>  
> I plan to solve using ‘-pc_type redistribute’ after applying the Dirichlet bc using
> MatZeroRowsColumnsIS(A, isout, 1, x, b); 
>  
> While I retrieve the solution data from x (after the solve) – can I index them using the original ordering (if I may say that)?
>  
> Kind regards,
> Karthik.
>  
> From: Barry Smith <bsmith at petsc.dev <mailto:bsmith at petsc.dev>>
> Date: Tuesday, 10 January 2023 at 16:04
> To: Chockalingam, Karthikeyan (STFC,DL,HC) <karthikeyan.chockalingam at stfc.ac.uk <mailto:karthikeyan.chockalingam at stfc.ac.uk>>
> Cc: petsc-users at mcs.anl.gov <mailto:petsc-users at mcs.anl.gov> <petsc-users at mcs.anl.gov <mailto:petsc-users at mcs.anl.gov>>
> Subject: Re: [petsc-users] Eliminating rows and columns which are zeros
> 
>  
> https://petsc.org/release/docs/manualpages/PC/PCREDISTRIBUTE/#pcredistribute   -pc_type redistribute
>  
>  
> It does everything for you. Note that if the right hand side for any of the "zero" rows is nonzero then the system is inconsistent and the system does not have a solution.
>  
> Barry
>  
> 
> 
> On Jan 10, 2023, at 10:30 AM, Karthikeyan Chockalingam - STFC UKRI via petsc-users <petsc-users at mcs.anl.gov <mailto:petsc-users at mcs.anl.gov>> wrote:
>  
> Hello,
>  
> I am assembling a MATIJ of size N, where a very large number of rows (and corresponding columns), are zeros. I would like to potentially eliminate them before the solve.
>  
> For instance say N=7
>  
> 0 0  0  0 0 0 0
> 0 1 -1  0 0 0 0
> 0 -1 2  0 0 0 -1
> 0 0  0  0 0 0 0
> 0 0  0  0 0 0 0
> 0 0  0  0 0 0 0
> 0 0  -1 0 0 0 1
>  
> I would like to reduce it to a 3x3
>  
> 1 -1 0
> -1 2 -1
> 0 -1 1
>  
> I do know the size N.
>  
> Q1) How do I do it?
> Q2) Is it better to eliminate them as it would save a lot of memory?
> Q3) At the moment, I don’t know which rows (and columns) have the zero entries but with some effort I probably can find them. Should I know which rows (and columns) I am eliminating?
>  
> Thank you.
>  
> Karthik.
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