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

[Richard Tran Mills]

On Wed, Mar 21, 2018 at 6:12 AM, Matthew Knepley <knepley at gmail.com> wrote:

> 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).
>

Let me also point out: If you want to know all of the details of what
solvers and preconditioners are being used, you can run with "-ksp_view"
and you will be given all of the details of what PETSc is using. If you are
using SNES to solve nonlinear problems, then you can also do "-snes_view".

--Richard


>
>
>> 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/>
>
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