[petsc-dev] Can I modify x on SampleShellPCApply callback ?

[Matthew Knepley]

On Sun, Jul 16, 2017 at 9:19 AM, Franck Houssen <franck.houssen at inria.fr>
wrote:

> The pc (shell) I try to write is made of 2 levels.
> I use y to "store" the contribution of the 1st level as I possibly need to
> reuse y for the 2nd level.
> I wanted to use x to "store" the contribution of the 2nd level (for that,
> need possibly to reuse y), to finally return y += x.
> This would have made less code (create, destroy, data in context, ...) to
> deal with.
>

You can build the subspace correct part using


http://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/PC/PCGALERKIN.html

which might make this easier.

  Thanks,

    Matt


> Not such a big deal anyway
>
> Franck
>
>
> ------------------------------
>
> *De: *"Matthew Knepley" <knepley at gmail.com>
> *À: *"Franck Houssen" <franck.houssen at inria.fr>
> *Cc: *"petsc-dev" <petsc-dev at mcs.anl.gov>
> *Envoyé: *Dimanche 16 Juillet 2017 16:09:34
> *Objet: *Re: [petsc-dev] Can I modify x on SampleShellPCApply callback ?
>
> On Sun, Jul 16, 2017 at 9:07 AM, Franck Houssen <franck.houssen at inria.fr>
> wrote:
>
>> OK, so I need to create a new x. Thanks for the quick answer.
>>
>
> Why would you create a new x. There is some misunderstanding about this
> routine. You
> should never need to create a "new x".
>
>   Matt
>
>
>> Franck
>>
>>
>> ------------------------------
>>
>> *De: *"Matthew Knepley" <knepley at gmail.com>
>> *À: *"Franck Houssen" <franck.houssen at inria.fr>
>> *Cc: *"petsc-dev" <petsc-dev at mcs.anl.gov>
>> *Envoyé: *Dimanche 16 Juillet 2017 15:59:09
>> *Objet: *Re: [petsc-dev] Can I modify x on SampleShellPCApply callback ?
>>
>> On Sun, Jul 16, 2017 at 8:51 AM, Franck Houssen <franck.houssen at inria.fr>
>> wrote:
>>
>>> Can I modify x on SampleShellPCApply callback ?
>>>
>>> I am running debian (gcc-6.4), using petsc-dev synchronized on:
>>> >> git log
>>> commit f9d5775f43f69cbce5a7014a6ce3b24cc0e1214a (HEAD -> master,
>>> origin/master, origin/HEAD)
>>>
>>> when I run petsc/src/ksp/ksp/examples/tutorials/ex15.c, I get:
>>> >> mpirun -n 2 ./ex15.exe -user_defined_pc
>>> Norm of error 5.90715e-08 iterations 15
>>>
>>> OK, so far, so good.
>>>
>>> Now, I just modify x in SampleShellPCApply (reset to 0. at the end =
>>> doing nothing):
>>> >> git diff
>>> diff --git a/src/ksp/ksp/examples/tutorials/ex15.c
>>> b/src/ksp/ksp/examples/tutorials/ex15.c
>>> index badccd93b5..482e5ae6b5 100644
>>> --- a/src/ksp/ksp/examples/tutorials/ex15.c
>>> +++ b/src/ksp/ksp/examples/tutorials/ex15.c
>>> @@ -296,6 +296,7 @@ PetscErrorCode SampleShellPCApply(PC pc,Vec x,Vec y)
>>>
>>>    ierr = PCShellGetContext(pc,(void**)&shell);CHKERRQ(ierr);
>>>    ierr = VecPointwiseMult(y,x,shell->diag);CHKERRQ(ierr);
>>> +  ierr = VecSet(x, 0.);CHKERRQ(ierr);
>>>
>>>    return 0;
>>>  }
>>> >> mpirun -n 2 ./ex15.exe -user_defined_pc
>>> [0]PETSC ERROR: --------------------- Error Message
>>> --------------------------------------------------------------
>>> [0]PETSC ERROR: Object is in wrong state
>>> [0]PETSC ERROR:  Vec is locked read only, argument # 1
>>> [1]PETSC ERROR: #1 VecSet() line 547 in /home/fghoussen/Documents/
>>> INRIA/petsc/src/vec/vec/interface/rvector.c
>>> [1]PETSC ERROR: #2 SampleShellPCApply() line 299 in
>>> /home/fghoussen/Documents/INRIA/petsc/local/../src/ksp/
>>> ksp/examples/tutorials/ex15.c
>>>
>>> Why is that ? Is there a way to "unlock" x ?
>>>
>>> It seems, it's possible to modify y but not x: why ?
>>>
>>
>> x is the input and should be read-only. Changes cannot have any
>> mathematical significance because you cannot know
>> where the x vector would be used subsequently anyway.
>>
>>    Matt
>>
>>   Matt
>>
>>
>>> >> git diff
>>> diff --git a/src/ksp/ksp/examples/tutorials/ex15.c
>>> b/src/ksp/ksp/examples/tutorials/ex15.c
>>> index badccd93b5..98ae001319 100644
>>> --- a/src/ksp/ksp/examples/tutorials/ex15.c
>>> +++ b/src/ksp/ksp/examples/tutorials/ex15.c
>>> @@ -294,6 +294,7 @@ PetscErrorCode SampleShellPCApply(PC pc,Vec x,Vec y)
>>>    SampleShellPC  *shell;
>>>    PetscErrorCode ierr;
>>>
>>> +  ierr = VecSet(y, 0.);CHKERRQ(ierr);
>>>    ierr = PCShellGetContext(pc,(void**)&shell);CHKERRQ(ierr);
>>>    ierr = VecPointwiseMult(y,x,shell->diag);CHKERRQ(ierr);
>>> >> mpirun -n 2 ./ex15.exe -user_defined_pc
>>> Norm of error 5.90715e-08 iterations 15
>>>
>>> Does PETSc uses x even after the callback has been called ?
>>> My understanding is that at the end of the callback only y matters: y
>>> replaces x that has become no-use (y is the new residu): am I wrong ?
>>>
>>> If possible, I would like to reuse x (avoiding to re-create a vector) ?
>>> Is there a way to "unlock" x ?
>>>
>>> Franck
>>>
>>>
>>>
>>
>>
>> --
>> 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
>>
>> 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
>
> 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

http://www.caam.rice.edu/~mk51/
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.mcs.anl.gov/pipermail/petsc-dev/attachments/20170716/1463becd/attachment.html>