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

[Franck Houssen]

OK, so I need to create a new x. Thanks for the quick answer. 

Franck 

----- Mail original -----

> 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/
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