[petsc-users] Bug report: using a custom ksp convergence function

[Daniel Stone]

That works wonderfully, thanks! Should be paired with

  call KSPConvergedDefaultDestroy(ctx,ierr)

I think.

Best Regards,

Daniel



On Mon, Jan 20, 2025 at 3:20 PM Barry Smith <bsmith at petsc.dev> wrote:

>
>    David,
>
>       KSPConvergedDefault() now takes a non-NULL context that must be
> provided. I have attached a modified version of ex5f.F90 that demonstrates
> how it is constructured before being passed to KSPSetConvergenceTest().
>
>    Barry
>
>
> > On Jan 20, 2025, at 8:54 AM, Daniel Stone <daniel.stone at opengosim.com>
> wrote:
> >
> > Hello PETSc Community,
> >
> > I think I've found a bug -
> >
> > Go to $PETSC_DIR/src/ksp/ksp/tutorials/
> >
> > open ex5f.F90, and add the following line in MyKSPConverged(), say
> around line 336:
> >
> >  call KSPConvergedDefault(ksp,n,rnorm,flag,dummy,ierr)
> >
> > It should make sense why someone would want to do this - within a
> definition of custom convergence behaviour, get the default convergence
> flags, and, based on certain conditions, overwrite it.
> >
> > Now, building and running the exercise, making sure to include the flag
> to use the custom convergence:
> >
> >  boston at boston-SYS-540A-TR:~/DATA_DRIVE/PETSC_DIRS/petsc_3.22/src/ksp/ksp/tutorials$
> ./ex2f -my_ksp_convergence
> > [0]PETSC ERROR:
> ------------------------------------------------------------------------
> > [0]PETSC ERROR: Caught signal number 11 SEGV: Segmentation Violation,
> probably memory access out of range[...]
> >
> > From my debugging, the issue is at line 1520 of iterativ.c:
> >
> >   KSPConvergedDefaultCtx *cctx = (KSPConvergedDefaultCtx *)ctx;
> >
> > Because a dummy is used for the context object, this cast fails (my
> debugger says "cannot access memory..." for cctx). Then the crash happens
> when trying to access members of cctx:
> >
> >   if (cctx->convmaxits && n >= ksp->max_it) {
> >
> >
> > I'm not sure how to fix it. Crucially, we must note that that this did
> work in older versions of PETSc - I can repeat this test in 3.19.1, for
> example, and it works fine - a debugger shows the cast succeeding and
> producing some
> > defaulted version of the ctx object, the subsequent crash does not
> happen, etc.
> >
> > Does anybody have any advice for working around this for the time being?
> A piece of software I work with uses a custom ksp convergence test in the
> manner described above, and is only functional with older versions of petsc
> because of this.
> > Like in ex2f, I have no need of the ctx object, and just as in ex2f, I
> use 0 instead.
> >
> > Things I'll try next: defining a proper ctx object (I use 0, like in
> ex2f), or looking for some PETSC_NULL_CTX definition somewhere.
> >
> > Many Thanks,
> >
> > Daniel
> >
> >
> >
>
>
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