[petsc-dev] Problematic Merge of FieldSplit

Dmitry Karpeev karpeev at mcs.anl.gov
Fri Jul 6 06:04:30 CDT 2012

I have the following problem with the prefix choice for the
MatSchurComplement KSP introduced in this changeset (
I'm talking about the  "inner" KSP for A00, effecting inv(A00) in the
definition S = A11 - A10 inv(A00) A01.
We also have the "outer" inv(A00) KSP, which gets prefix "0".  I
recently set the "inner" inv(A00) KSP
prefix to "0", simply by inheriting it from the "outer" solver.  Now, it is
completely reasonable
to expect the inner and outer A00 KSPs to have different prefixes so that
they can be configured differently.
In fact, there was a recent petsc-users request related to this (
However, currently the inner A00 KSP inherits the prefix from the A11 KSP
corresponding to the "1" field. With this prefix choice
I end up configuring inv(A00) and inv(S) identically, which isn't what I
want at all.
I'm not sure what the right approach is, but the current one doesn't work
for me.

Note also that if A00 is treated with a recursive split, there may be
numerous options for the A00 KSP.
Do we want to repeat them for the inner and outer KSPs, if we want to
configure them identically?
It's automatic, if the two A00 KSPs share a prefix.  Again, this takes away
some flexibility, so maybe it's not the best solution,
but I think retaining a simple option for using identical configurations
 is also highly desirable.

Any ideas on how to handle this?

On Tue, Jun 26, 2012 at 6:13 AM, Matthew Knepley <knepley at gmail.com> wrote:

> It turns out that 'hg rollback' during an 'hg rebase' does not do what I
> thought it did. I think
> everything is cleaned up with this push, but if you made FS changes in the
> past month, please
> check that it is doing what you want with prefixes, etc.
> Now, nested fieldsplits from the command line work, ala
> -ksp_type fgmres
> -pc_type fieldsplit -pc_fieldsplit_type additive
>   -pc_fieldsplit_0_fields 0,1
>     -fieldsplit_0_pc_type fieldsplit
>     -fieldsplit_0_pc_fieldsplit_type schur
> -fieldsplit_0_pc_fieldsplitschur_factorization_type full
>       -fieldsplit_0_fieldsplit_velocity_ksp_type preonly
>       -fieldsplit_0_fieldsplit_velocity_pc_type lu
>       -fieldsplit_0_fieldsplit_pressure_ksp_rtol 1e-10
>       -fieldsplit_0_fieldsplit_pressure_pc_type jacobi
>   -pc_fieldsplit_1_fields 2
>     -fieldsplit_temperature_ksp_type preonly
>     -fieldsplit_temperature_pc_type lu
> A split with only one field gets the field name, and otherwise a split
> number.
>     Matt
> --
> 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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