[petsc-dev] [petsc-users] Multiple solves with PCMG fail

[Jed Brown]

Mark Adams <mfadams at lbl.gov> writes:

>> My real concern with randomness is that results are not reproducible.
>>
>
> My feeling is that this will be small but it will finite in exact
> arithmetic, which is not ideal.
>
> If the solve is sensitive to this then I think the you are in trouble
> anyway (if a plane craters you don't really care how deep the hole is).

I'm more concerned with the process users go through.  If they are
solving a borderline system and it sometimes converges and sometimes
fails, they may well conclude "PETSc is flaky software".  Especially if
attempts to reduce aren't reproducible.  I don't look forward to getting
a matrix in the mail with a report "I tried ex10 with these options and
it failed twice in 20 runs".  We have an idea of where to start, but
it's much worse if the user doesn't write us.

> If it did indeed lead to an irreplaceable error then you can look at the
> verbose output and see that the failures had lower estimates.  At least it
> is debuggable and since it has never actually happened, has it, then I
> think we could live with the possibility.

There is also the issue that some people simulate unstable systems with
rare events, such as certain failure modes in fracture or transitions in
climate.  In order to investigate the rare events, sometimes they need
to be reproducible.  We shouldn't let this prevent us from choosing a
more robust default, but we should strive to document every point of
non-reproducibility and how to make results reproducible.  This is extra
cognitive load and more to maintain, so it would be nice to keep the
number of such points small.

> You can compute it before you create the RHS, no?  

Depends on the problem.

> Perhaps we add a SetVectos(Vec[],numvec) that you call before setup()
> to if you really care about memory?

No, too much to think about.
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