[petsc-users] Recomputing ksp

[Barry Smith]

> On Jul 9, 2015, at 4:45 PM, Michele Rosso <mrosso at uci.edu> wrote:
> 
> Barry,
> 
> this helped a lot.
> I do multiple solves with the operator changing at each solve. Without forcing recomputing the preconditioner and setting explicitly KSPSetReusePreconditioner,
> I mostly get "PCSetUp(): Leaving PC with identical preconditioner since operator is unchanged" and a few times  "PCSetUp(): Setting up PC with same nonzero pattern".
> Shouldn't I get "PCSetUp(): Setting up PC with same nonzero pattern" all the times since the operator keeps changing?

   The PCSetUp() routine (slightly confusingly) is actually called for every linear iteration (though, of course, it does nothing within a single linear solve). This is just the way the logic of the KSP implementation currently is.

  Barry

> I am solving with CG + MG with coarse operators computed via Galerkin process. 
> 
> Michele 
> 
> 
> 
> 
> On Thu, 2015-07-09 at 15:06 -0500, Barry Smith wrote:
>>   Run with -info ; it prints lots of stuff but you can grep for PCSetUp  and you'll see a message such as 
>> 
>> Setting up PC for first time
>> 
>> Leaving PC with identical preconditioner since operator is unchanged
>> 
>> Setting up PC with different nonzero pattern\n");CHKERRQ(ierr);
>> 
>> Setting up PC with same nonzero pattern
>> 
>> or 
>> 
>> Leaving PC with identical preconditioner since reuse preconditioner is set
>> 
>> 
>> 
>> 
>> > On Jul 9, 2015, at 2:45 PM, Michele Rosso <mrosso at uci.edu> wrote:
>> > 
>> > Barry,
>> > 
>> > thanks you for your help.
>> > Is there a database options that allows me to check weather PC has been recomputed or kept the same?
>> > 
>> > Thanks,
>> > Michele 
>> > 
>> > 
>> > On Thu, 2015-07-09 at 14:43 -0500, Barry Smith wrote:
>> >>    By default in the last two PETSc releases KSP automatically updates the preconditioner whenever the matrix you set with KSPSetOperators() has been changed. That is you have to do nothing and PETSc will recompute the preconditioner as needed. If you wish to keep the same preconditioner even though the matrix has changed then you can use KSPSetReusePreconditioner(ksp,PETSC_TRUE) and it will keep using the same preconditioner until you call KSPSetReusePreconditioner(ksp,PETSC_FALSE) which will switch back to the default mode.
>> >> 
>> >>   You only need to destroy the KSP or call KSPReset() when you change the size of the vectors or matrices.
>> >> 
>> >>   Barry
>> >> 
>> >> 
>> >> > On Jul 9, 2015, at 12:20 PM, Michele Rosso <mrosso at uci.edu> wrote:
>> >> > 
>> >> > Hi,
>> >> > 
>> >> > I need to recompute the preconditioner every once in a while. So far I do this "manually", i.e. I destroy ksp and re-create and reset it whenever needed. 
>> >> > I am wondering if there is a cleaner way of doing this via a PETSc function. I found KSPreset but there are no examples about it so I am not sure it is what I am looking for.
>> >> > Could you help please?
>> >> > 
>> >> > 
>> >> > Thanks,
>> >> > Michele
>> >> 
>> >> 
>> >> 
>> > 
>> 
>> 
>> 
>