[petsc-dev] Performance of Petsc Cholesky vs LU

[Dave Nystrom]

Jed Brown writes:
 > 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

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.