[petsc-users] Why use MATMPIBAIJ?

[Barry Smith]

> On Jan 14, 2016, at 12:57 PM, Jed Brown <jed at jedbrown.org> wrote:
> 
> Hoang Giang Bui <hgbk2008 at gmail.com> writes:
>> One more question I like to ask, which is more on the performance of the
>> solver. That if I have a coupled problem, says the point block is [u_x u_y
>> u_z p] in which entries of p block in stiffness matrix is in a much smaller
>> scale than u (p~1e-6, u~1e+8), then AMG with hypre in PETSc still scale?
> 
> You should scale the model (as Barry says).  But the names of your
> variables suggest that the system is a saddle point problem, in which
> case there's a good chance AMG won't work at all.  For example,
> BoomerAMG produces a singular preconditioner in similar contexts, such
> that the preconditioned residual drops smoothly while the true residual
> stagnates (the equations are not solved at all).  So be vary careful if
> you think it's "working".

   The PCFIEDSPLIT preconditioner is designed for helping to solve saddle point problems.

   Barry