[petsc-users] Obtain the preconditioned matrix

[Matthew Knepley]

On Sat, Mar 24, 2018 at 8:15 AM, 我 <dayedut123 at 163.com> wrote:

> Hi Matt,
> Sorry to distribute you again. I still have trouble with
> KSPComputeExplicitOperator. I reduced the matrix size to around 1000*1000.
> The time-cost of this function is acceptable, but when I output the
> preconditioned matrix, all the values are equal to zero! Here is part of my
> codes, do I operate error with this function?
>
> PC pc;
> KSP ksp;
> Mat A,PA;
> Vec b,x;
> ierr=KSPCreate(PETSC_COMM_WORLD,&ksp);CHKERRQ(ierr);
> ierr=KSPSetOperators(ksp,A,A);CHKERRQ(ierr);
> ierr=KSPGetPC(ksp,&pc);CHKERRQ(ierr);
> ierr=PCSetType(pc,PCHYPRE);CHKERRQ(ierr);
> ierr=KSPSetType(ksp,KSPGMRES);CHKERRQ(ierr);
> ierr=KSPSetFromOptions(ksp);CHKERRQ(ierr);
> ierr=KSPSolve(ksp,b,x);CHKERRQ(ierr);
> ierr=KSPComputeExplicitOperator(ksp,&PA);
>

There is no need to assemble the output matrix.


> ierr=MatAssemblyBegin(PA,MAT_FINAL_ASSEMBLY);CHKERRQ(ierr);
> ierr=MatAssemblyEnd(PA,MAT_FINAL_ASSEMBLY);CHKERRQ(ierr);
>

What kind of viewer are you using? The code above should work. I would step
through
in the debugger to see why one column is all zeros, or try it in an example
like
SNES ex5 to make sure its not a problem in your code.

  Thanks,

    Matt


> ierr=MatView(PA,viewer);CHKERRQ(ierr);
>
> Thank you very much!
> Daye
>
>
>
>
>
>
> At 2018-03-23 07:41:26, "Matthew Knepley" <knepley at gmail.com> wrote:
>
> On Thu, Mar 22, 2018 at 8:29 PM, 我 <dayedut123 at 163.com> wrote:
>
>> Hi all,
>> I want to analyze the preconditioned matrix. But the
>> KSPComputeExplicitOperator costs too much time to obtain the matrix. My
>> origin matrix is about 3000*3000 sparse one. I noticed this function is
>> applicable for the relative small system. What's the matrix-size limitation
>> for this function? And for my case, any suggestions to solve it with less
>> time?
>>
>
> This is just a really expensive operation. There is no way around it.
>
>    Matt
>
>
>> Thank you very much!
>> daye
>>
>>
>>
>>
>
>
>
> --
> 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.caam.rice.edu/~mk51/>
>
>
>
>
>



-- 
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.caam.rice.edu/~mk51/>
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