[petsc-users] Reuse KSP

[Smith, Barry F.]

   If the matrix has a null space then, in general, direct LU factorizations should fail. If they don't fail they can produce an incorrect answer 

   If your matrix has a null space then you should not remove the  [Set nullspace of A with MatSetNullSpace] 

   Barry

> On Apr 17, 2018, at 7:48 AM, Young, Matthew, Adam <may at bu.edu> wrote:
> 
> 
> 
> 
> From: Matthew Knepley <knepley at gmail.com>
> Sent: Tuesday, April 17, 2018 8:38 AM
> To: Young, Matthew, Adam
> Cc: petsc-users at mcs.anl.gov
> Subject: Re: [petsc-users] Reuse KSP
>  
> On Tue, Apr 17, 2018 at 8:33 AM, Young, Matthew, Adam <may at bu.edu> wrote:
> My code (C/C++) uses PETSc to solve a linear system at each iteration in a time-stepping scheme. The PETSc routines are separated out because the scheme that uses them is optional in the larger code. 
> 
> At each time step, the PETSc scheme calls a routine that includes:
> 
> {
>   Mat A;
>   Vec x,b;
>   KSP ksp;
> 
>   [Set up A,x,b]
> 
>   KSPCreate(PETSC_COMM_WORLD,&ksp);
> 
>   [Build A and b from simulation data]
> 
>   [Set nullspace of A with MatSetNullSpace]
> 
>   KSPSetOperators(ksp,A,A);
>   KSPSetFromOptions(ksp);
>   KSPSolve(ksp,b,x);
> 
>   [Write x into an instance of our array class, perform checks, etc.]
> 
>   KSPDestroy(&ksp);
> 
>   return(0);
> }
> 
> My question is: If I want to use superLU_DIST with the option -mat_superlu_dist_fact SamePattern (or SamePattern_SameRowPerm), should I move KSPCreate/Destroy outside of the time-step loop in the larger code?
> 
> 1) Yes
> 
> 2) How are you using LU when A has a nullspace?
> My bad - that code is in there from when I was trying iterative solvers. Should I remove it if I use LU or will PETSc just ignore it?
> 
> 3) You can also move out SetFromOptions().
> Thanks for the advice. So, that means I'll build A and b from simulation data, SetOperators(), Solve(), and carry on?
> 
>   Matt
>  
> Thanks.
> ----------------------------
> Matthew Young
> PhD Candidate
> Astronomy Department
> Boston University
> 
> 
> 
> 
> 
> -- 
> 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
> 
> https://www.cse.buffalo.edu/~knepley/