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

[Matthew Knepley]

On Tue, Jan 21, 2025 at 9:46 AM Daniel Stone <daniel.stone at opengosim.com>
wrote:

> Is there a quick way for me to find out which version of
> PETSc KSPConvergedDefaultCreate() was introduced with?
> I'll likely need to conditionally pragma-out these calls for back
> compatibility.
>

We have changes docs: https://urldefense.us/v3/__https://petsc.org/main/changes/__;!!G_uCfscf7eWS!cAwR4aLIcKH3Pn0R90bQMD51mNRpiPcM7Ic_X9IQC6ANOcmJIekcW1TYwAdodLUkR1pyNfHatMuaoZiRhrFA$ 

main *$:~/PETSc4/petsc/petsc-pylith$ find doc/changes/ | xargs grep
KSPConvergedDefaultCreate
doc/changes//35.rst:   ``KSPConvergedDefaultCreate()``,

  Thanks,

     Matt

Thanks,
>
> Daniel
>
> On Tue, Jan 21, 2025 at 1:16 PM Barry Smith <bsmith at petsc.dev> wrote:
>
>>
>>    Yes, that can be passed in also with KSPSetConvergenceTest()
>>
>>
>> On Jan 21, 2025, at 4:25 AM, Daniel Stone <daniel.stone at opengosim.com>
>> wrote:
>>
>> 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
>>> >
>>> >
>>> >
>>>
>>>
>>

-- 
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

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