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

[Matthew Knepley]

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/
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.mcs.anl.gov/pipermail/petsc-dev/attachments/20170716/b2b7b699/attachment.html>