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

[Jed Brown]

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?

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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