Symmetric matrices.

[Hong Zhang]

> Which solvers are more appropriate for symmetric matrices?

Iterative solvers: ksp_type cg, pc_type icc
Direct solvers:    ksp_type preonly, pc_type cc

Non-symmetric solvers also work, but the above solvers
are more efficient in general.

You can run petsc code with various ksp/pc combinations,
and use the option '-log_summary' to evaluate the performance.
Using '-help' to see all solver options.

Hong