[petsc-users] about pclu

[Jed Brown]

On Thu, Apr 7, 2011 at 15:49, Ryan Yan <vyan2000 at gmail.com> wrote:

> I agree that solves are faster than factorization and GMRES
> use more MatVecMult, even with exact preconditioner. So I guess
> PREONLY is meaning direct solve without forming any sub-space and just
> one forward and backward substitutions?
>

Yes. Because of the way GMRES works with zero initial guess, there will be
two MatSolve and one MatMult even when using a direct solver. PREONLY does
one MatSolve and zero MatMult.
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