[petsc-users] Norm_2 calculation

[Fazlul Huq]

Input string:
mpiexec -np 1 ./poisson_m -n 10000000 -pc_type cholesky  -ksp_view
-ksp_converged_reason -ksp_monitor_true_residual

Output is attached herewith.

Thank you.
Sincerely,
Huq

On Fri, Jun 5, 2020 at 10:16 AM Matthew Knepley <knepley at gmail.com> wrote:

> On Fri, Jun 5, 2020 at 11:04 AM Fazlul Huq <huq2090 at gmail.com> wrote:
>
>> Hello All,
>> I'm trying to calculate the norm_2 error of a solution.
>> Here, vector s: Analytical solution
>>           vector x: Numerical solution
>>
>> ierr = VecView(s,PETSC_VIEWER_STDOUT_WORLD);CHKERRQ(ierr);
>> ierr = VecView(x,PETSC_VIEWER_STDOUT_WORLD);CHKERRQ(ierr);
>>
>>  ierr = VecAXPY(x,-1.0,s);CHKERRQ(ierr);
>>  ierr = VecNorm(x,NORM_2,&norm);CHKERRQ(ierr);
>>  ierr = VecView(x,PETSC_VIEWER_STDOUT_WORLD);CHKERRQ(ierr);
>>  ierr = KSPGetIterationNumber(ksp,&its);CHKERRQ(ierr);
>>  if (norm > tol) {
>>     ierr = PetscPrintf(PETSC_COMM_WORLD,"Second Norm of error %g\n",
>> double)norm);CHKERRQ(ierr);
>>     ierr = PetscPrintf(PETSC_COMM_WORLD,"Iterations
>> %D\n",its);CHKERRQ(ierr);
>>   }
>>
>> Am I calculating the "Norm_2" error correctly or making any mistake?
>> Unfortunately, for large sized matrix, say 10^6, I am getting very high
>> value of "Norm_2" error.
>>
>
> 1) I am guessing x comes from a KSPSolve(). It is only as accurate as your
> tolerance
>
> 2) This is the l_2 norm, not the L_2 norm, so if you are using a continuum
> method like FEM, this is likely wrong.
>
>   Thanks,
>
>      Matt
>
>
>> Thank you.
>> Sincerely,
>> Huq
>>
>> --
>>
>> Fazlul Huq
>> Graduate Research Assistant
>> Department of Nuclear, Plasma & Radiological Engineering (NPRE)
>> University of Illinois at Urbana-Champaign (UIUC)
>> E-mail: huq2090 at gmail.com
>>
>
>
> --
> 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/
> <http://www.cse.buffalo.edu/~knepley/>
>


-- 

Fazlul Huq
Graduate Research Assistant
Department of Nuclear, Plasma & Radiological Engineering (NPRE)
University of Illinois at Urbana-Champaign (UIUC)
E-mail: huq2090 at gmail.com
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