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

[Mark Adams]

On Wed, Oct 22, 2014 at 2:25 AM, Jed Brown <jed at jedbrown.org> wrote:

> Barry Smith <bsmith at mcs.anl.gov> writes:
>
> >   Jed
> >
> >     There are some additional issues when the GMRES runs for zero
> iterations in computing eigenvalues for Cheby.
>
> Hmm, what is right?  Suggest that the user set
> -ksp_chebyshev_estimate_eigenvalues_random?  Automatically fall back to
> that?
>
>
Are you implying that PETSc is using the RHS to compute the spectra for
cheby?


> Just picking arbitrary values doesn't seem right because the solver
> won't work right on the next solve.
>
> > static PetscErrorCode KSPChebyshevComputeExtremeEigenvalues_Private(KSP
> kspest,PetscReal *emin,PetscReal *emax)
> > {
> >   PetscErrorCode ierr;
> >   PetscInt       n,neig;
> >   PetscReal      *re,*im,min,max;
> >
> >   PetscFunctionBegin;
> >   ierr = KSPGetIterationNumber(kspest,&n);CHKERRQ(ierr);
> >   ierr = PetscMalloc2(n,&re,n,&im);CHKERRQ(ierr);
> >   ierr = KSPComputeEigenvalues(kspest,n,re,im,&neig);CHKERRQ(ierr);
> >   min  = PETSC_MAX_REAL;
> >   max  = PETSC_MIN_REAL;
> >   for (n=0; n<neig; n++) {
> >     min = PetscMin(min,re[n]);
> >     max = PetscMax(max,re[n]);
> >   }
> >   ierr  = PetscFree2(re,im);CHKERRQ(ierr);
> >   *emax = max;
> >   *emin = min;
> >
> > * thread #1: tid = 0x3c16cf, 0x00000001041b6777
> libpetsc.3.5.dylib`KSPSolve_Chebyshev(ksp=0x00007fb85197ac60) + 1495 at
> cheby.c:378, queue = 'com.apple.main-thread', stop reason = step over
> >     frame #0: 0x00000001041b6777
> libpetsc.3.5.dylib`KSPSolve_Chebyshev(ksp=0x00007fb85197ac60) + 1495 at
> cheby.c:378
> >    375            cheb->emin = cheb->tform[0]*min + cheb->tform[1]*max;
> >    376            cheb->emax = cheb->tform[2]*min + cheb->tform[3]*max;
> >    377
> > -> 378            cheb->estimate_current = PETSC_TRUE;
> >    379          }
> >    380
> >    381          ksp->its = 0;
> > (lldb) p cheb->emin
> > (PetscReal) $11 = -1.7976931348623158E+307
> > (lldb) p cheb->emax
> > (PetscReal) $12 = -Inf
> >
> > This puts giberish into the eigenvalue estimates.
> >
> > I am not sure how you want to handle this?
> >
> > The zero right hand side zero, initial guess case is a great corner case
> for testing complicated solvers :-) I’d forgotten about it for many years
>
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