[petsc-users] direct solver question
Tabrez Ali
stali at geology.wisc.edu
Tue Aug 7 13:53:57 CDT 2012
Matt
So if I want to use a direct solver I should really use (2) instead of (3).
Tabrez
On 08/07/2012 01:05 PM, Matthew Knepley wrote:
> On Tue, Aug 7, 2012 at 12:48 PM, Tabrez Ali <stali at geology.wisc.edu
> <mailto:stali at geology.wisc.edu>> wrote:
>
> Barry
>
> So which one of the two gives the correct solution? And yes the
> condition number is high.
>
> Actually I am trying to solve a trivial quasi-static problem with
> the following flags
>
> (1) -ksp_type gmres -pc_type asm
> (2) -ksp_type gmres -pc_type lu -pc_factor_shift_type nonzero
> (3) -ksp_type preonly -pc_type lu -pc_factor_shift_type nonzero
>
>
> This does no iterative refinement, so you are stuck with the initial
> accuracy of the LU
> factorization, which can be bad for an ill-conditioned matrix.
>
> Matt
>
> The results at t=0 for all three cases are equivalent.
>
> For the time dependent part (linear system is solved again),
> results from (1) and (2) are the same. But (3) gives a very
> different answer.
>
> I also solve the same quasi-static problem using a different code
> (does not use PETSc) and the results obtained are similar to (1)
> and (2).
>
> So basically I am trying to understand why '-ksp_type preonly'
> gives me a (potentially) wrong solution.
>
> Thanks
> Tabrez
>
>
>
> On 08/07/2012 12:01 PM, Barry Smith wrote:
>
> On Aug 7, 2012, at 12:06 PM, Tabrez Ali<stali at geology.wisc.edu
> <mailto:stali at geology.wisc.edu>> wrote:
>
> Hello list
>
> By any chance is using
>
> -ksp_type gmres -pc_type lu
>
> similar to
>
> -ksp_type preonly -pc_type lu?
>
> Should the results be equivalent?
>
> T
>
> The should be similar but will not be identical
> necessarily, in particular when the matrix is ill conditioned
> they can be very different and in fact GMRES may iterate a few
> times.
>
> Barry
>
>
>
> --
> No one trusts a model except the one who wrote it; Everyone trusts
> an observation except the one who made it- Harlow Shapley
>
>
>
>
> --
> 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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