[petsc-users] Why use MATMPIBAIJ?

[Matthew Knepley]

On Mon, Jan 18, 2016 at 8:29 AM, Hoang Giang Bui <hgbk2008 at gmail.com> wrote:

>
>
> 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.
>

You misunderstand the blocking. You would put ALL velocities (P2) in one
block and ALL pressure (P1) in another.
The PCFieldSplitSetBlockSize() call is for co-located discretizations,
which P2/P2 is not.

   Matt


> Giang
>
>


-- 
What most experimenters take for granted before they begin their
experiments is infinitely more interesting than any results to which their
experiments lead.
-- Norbert Wiener
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