[petsc-users] (Fast?) assembly of upper triangle and MatZeroRows

[Matthew Knepley]

On Mon, Aug 8, 2011 at 10:31 PM, Jed Brown <jedbrown at mcs.anl.gov> wrote:

> On Mon, Aug 8, 2011 at 17:16, Barry Smith <bsmith at mcs.anl.gov> wrote:
>
>> Add a KSP/SNES./TSSetDirichlet(ksp,snes,ts,IS listofnodes,Vec
>> valuesfornodes)?
>>
>
> This might be worthwhile. It needs a new Mat primitive which zeros the rows
> and columns while also "remembering" the column values that it zeroed (as
> another assembled matrix). I think it's best to apply the transformation
> immediately after residuals are calculated so that norms are correct (not in
> KSP at all). Matrix values are not available at this time so the Dirichlet
> equations may not be scaled correctly for GMG. We have to be careful that
> the residual evaluation passed to MatMFFD has the Dirichlet projection
> inside it.
>

This is exactly the reason that I abandoned this approach when I got to
nonlinear equations. It is complex and fragile, and requires that too
many things be coordinated out of sight of the user.

   Matt

-- 
What most experimenters take for granted before they begin their experiments
is infinitely more interesting than any results to which their experiments
lead.
-- Norbert Wiener
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