[petsc-users] Why use MATMPIBAIJ?

Hoang Giang Bui hgbk2008 at gmail.com
Mon Jan 18 08:29:30 CST 2016

On Thu, Jan 14, 2016 at 8:08 PM, Barry Smith <bsmith at mcs.anl.gov> wrote:

> > 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".

Using block size 4 with the scaling, the hypre AMG does not converge. So
it's somehow right.

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

Does PCFIELDSPLIT support variable block size? For example using P2/P1
discretization, the number of nodes carrying [u_x u_y u_z] is different
with number of nodes carrying p. PCFieldSplitSetBlockSize would not be
correct in this case.

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