[petsc-users] GAMG PC dependence on right-hand side

[Mark F. Adams]

As Jed mentioned you should not see a significant difference with '-random_seed 1'.  This fix may just be papering over a problem that will bite you eventually.  If you run with '-XXX_gamg_verbose 2' the eigenvalues used will be printed out.  Try running with and without '-random_seed 1' and send the output.  We are looking to see if the computed eigenvalues on these second (bad) solves is different in the two runs. 

And I'm a bit puzzled because I don't know how '-random_seed 1' could be used in GAMG to compute eigen estimates...


On Jul 11, 2013, at 4:47 PM, John Mousel <john.mousel at gmail.com> wrote:

> I have two KSP contexts, helm%ksp and proj%ksp. I switch between the two by calling KSPSetOperators. There was no performance difference, just a different answer. 
> 
> 
> On Thu, Jul 11, 2013 at 3:41 PM, Jed Brown <jedbrown at mcs.anl.gov> wrote:
> John Mousel <john.mousel at gmail.com> writes:
> 
> > Jed,
> >
> > I alternate between solving Helmholtz and Poisson equations in an outer
> > loop. The matrix is fixed for both operators during the process, and only
> > the right-hand side is updated for each system.
> 
> Do you have one KSP or two?  If one, I assume you're trying to preserve
> memory, but there is likely more memory in the preconditioner than the
> Krylov space and if you want to reuse the preconditioner, you have to
> set it up anew each time.
> 
> > I just tried -random_seed 1, and it seems to have made all the difference
> > in the world!
> 
> That's disconcerting because we would like the algorithm to be
> insensitive to such things.  How big was the performance difference?
> Can you give us some information to reproduce?  (Maybe a smallish
> example matrix that demonstrates this problem.)
> 

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