[petsc-dev] Multigrid is confusing

[Jed Brown]

On Wed, May 23, 2012 at 2:52 PM, Jed Brown <jedbrown at mcs.anl.gov> wrote:

> On Wed, May 23, 2012 at 2:26 PM, Barry Smith <bsmith at mcs.anl.gov> wrote:
>
>>
>>  Note that you could use -pc_type eisenstat perhaps in this case instead.
>> Might save lots of flops?  I've often wondered about doing Mark's favorite
>> chebyshev smoother with Eisenstat, seems like it should be a good match.
>>
>
> [0]PETSC ERROR: --------------------- Error Message
> ------------------------------------
> [0]PETSC ERROR: No support for this operation for this object type!
> [0]PETSC ERROR: Cannot have different mat and pmat!
>

Also, I'm having trouble getting Eisenstat to be more than very marginally
faster than SOR.


I think we should later be getting the eigenvalue estimate by applying the
preconditioned operator to a few random vectors, then orthogonalizing. The
basic algorithm is to generate a random matrix X (say 5 or 10 columns),
compute

Y = (P^{-1} A)^q X

where q is 1 or 2 or 3, then compute

Q R = Y

and compute the largest singular value of the small matrix R. The
orthogonalization can be done in one reduction and all the MatMults can be
done together. Whenever we manage to implement a MatMMult and PCMApply or
whatever (names inspired by VecMDot), this will provide a very low
communication way to get the eigenvalue estimates.


I want to turn off norms in Chebyshev by default (they are very wasteful),
but how should I make -mg_levels_ksp_monitor turn them back on? I'm already
tired of typing -mg_levels_ksp_norm_type unpreconditioned.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.mcs.anl.gov/pipermail/petsc-dev/attachments/20120524/59cbe534/attachment.html>