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

dayedut123 at 163.com
Wed Mar 21 08:21:01 CDT 2018

```Thank you very much for your consistent help!

At 2018-03-21 20:12:58, "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).

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/

--

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/
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