Jed Brown writes:

On Tue, Nov 29, 2011 at 23:53, Dave Nystrom <[email protected]> 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

Thanks. I'll give this a try and report back on the results.

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