[petsc-dev] Performance of Petsc Cholesky vs LU

[Jed Brown]

On Tue, Nov 29, 2011 at 23:53, Dave Nystrom <dnystrom1 at comcast.net> wrote:

> I have a resistive mhd code that I have recently interfaced to petsc which
> has 7 linear solves that are all symmetric.  I recently tried using
> -pc_type
> cholesky -ksp_type preonly for a run and found that it was taking about 6
> times as long per linear solve as when I was using -pc_type lu -ksp_type
> preonly.
>

Try -pc_factor_mat_ordering_type nd


>  I was wondering if that was reasonable behavior.  I would not have
> thought that using a cholesky direct solve would take longer than an LU
> direct solve in petsc for the serial case and was hoping it would be
> faster.
> Does this behavior seem reasonable?
>

Try this:

-pc_type cholesky -pc_factor_mat_ordering_type nd

Barry, why is natural ordering still the default for Cholesky? It is so
slow that it is worthless.
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