[petsc-users] consider an error message as a petscinfo??

[Kong, Fande]

Thanks a lot for your explanation, Barry,

This makes sense!

Fande,

On Fri, Feb 24, 2017 at 1:56 PM, Barry Smith <bsmith at mcs.anl.gov> wrote:

>
>     Fande,
>
>     Yes. Say one is doing a timestepping and using a direct solver. With
> time stepping we do not want to necessarily stop (in fact we almost never)
> on a failed solve (due to for example a failed factorization). We want the
> code to continue up the stack until it gets to the time stepper with the
> information that the linear solver failed; then the timestepper can decide
> what to do, decrease the time step and try again (common) or switch to
> another method etc. The same could be true for linear solvers called from
> within optimization routines.
>
>     The reason we do the funny business with putting infinity into the
> vector is so that errors that occur on any process get propagated to all
> processes during the next inner product or norm computation, otherwise we
> would need to have some "side channel" communication which MPI doesn't
> really provide to propagate errors to all processes, this would be a lot of
> performance crippling communication that would have to take place during
> the entire solution process. Plus our approach since it doesn't change the
> flow of control cannot result in lost memory etc. Note that C (and MPI)
> don't have any exception mechanism that could be used to handle these
> "exceptional cases" directly.
>
>    Note that you can use options like -snes_error_if_not_converged or
> -ksp_error_if_not_converged to force the error to be set as soon as any
> problem with convergence or zero factorization is found (good for
> debugging, but not usually for production unless production is solving a
> linear system only).
>
>    Barry
>
> > On Feb 24, 2017, at 2:40 PM, Kong, Fande <fande.kong at inl.gov> wrote:
> >
> > Hi All,
> >
> > In MatSolve(), there is a piece of code:
> >
> >   if (mat->errortype) {
> >     ierr = PetscInfo1(mat,"MatFactorError %D\n",mat->errortype);CHKERRQ(
> ierr);
> >     ierr = VecSetInf(x);CHKERRQ(ierr);
> >   } else {
> >     ierr = (*mat->ops->solve)(mat,b,x);CHKERRQ(ierr);
> >   }
> >
> > If a direct solver such as LU or superlu fails to create a
> factorization, why we do not stop here and throw out an error message here
> or earlier?  Now we just let solver keep doing garbage computation, and
> finally we have a solution like this:
> >
> > Vec Object: 1 MPI processes
> >   type: mpi
> > Process [0]
> > inf.
> > inf.
> > inf.
> > inf.
> > inf.
> > inf.
> > inf.
> > inf.
> > inf.
> > inf.
> > inf.
> > inf.
> > inf.
> > inf.
> > inf.
> > inf.
> >
> > Any particular reason to handle the thing in this way?
> >
> > Fande,
> >
> >
>
>
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