[petsc-users] How to turn off preconditioner in PETSC?

[Matthew Knepley]

On Wed, Mar 21, 2018 at 9:07 AM, 我 <dayedut123 at 163.com> wrote:

> Thanks for your reply! You mean the preconditioner must be the necessary
> choice for the linear iterative method in PETSc?
>

No. As you saw, you can use no preconditioner. The bad convergence has
nothing to do with PETSc. It is a mathematical fact. All
iterative methods behave this way.


> And the default preconditioner in PETSC is which one?
>

ILU(0).


> I want to compare them in order to illustrate that PCHYPRE is best one for
> my problem.
>

Then the right thing to do is read some papers and reproduce what other
people have done, and show that Hypre is better than that.


> If I want to get the matrix after preconditioned (e.g. PAx=Pb, and I want
> to get PA), is there a function in PETSc?
>

http://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/KSP/KSPComputeExplicitOperator.html

It is extremely expensive and should only be used for very small problems.
The whole idea of iterative methods is that you do
NOT compute this operator explicitly.

   Matt


> Thanks again!
> Daye
>
>
>
>
> At 2018-03-21 18:45:18, "Matthew Knepley" <knepley at gmail.com> wrote:
>
> On Wed, Mar 21, 2018 at 3:35 AM, 我 <dayedut123 at 163.com> wrote:
>
>> Hi,
>> I want to compare the time cost between preconditioner and
>> unpreconditioner in PETSc. But I didn't know how to turn off the
>> preconditioner in Petsc. If I choose the PCNONE, but the solution even can
>> not converge.
>>
>
> That is how you turn off a preconditioner, -pc_type none. Without a
> preconditioner, almost nothing converges. You can't have it both ways.
>
>   Thanks,
>
>      Matt
>
>
>> If I do not declare PC at the beginning of my program, will PETSc choose
>> a default preconditioner? I just want to turn off it. Any suggestions?
>> 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/>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.mcs.anl.gov/pipermail/petsc-users/attachments/20180321/c6e02c61/attachment.html>