On 20 Mar 2020, at 10:53 PM, Lawrence Mitchell <wence@gmx.li> wrote:

On Fri, 20 Mar 2020 at 21:07, Pierre Jolivet <pierre.jolivet@enseeiht.fr> wrote:

On 19 Mar 2020, at 7:56 PM, Matthew Knepley <knepley@gmail.com> wrote:

On Fri, Mar 13, 2020 at 11:20 AM Pierre Jolivet <pierre.jolivet@enseeiht.fr> wrote:

On 13 Mar 2020, at 2:36 PM, Matthew Knepley <knepley@gmail.com> wrote:

On Fri, Mar 13, 2020 at 3:19 AM Pierre Jolivet <pierre.jolivet@enseeiht.fr> wrote:

On 12 Mar 2020, at 11:40 PM, Matthew Knepley <knepley@gmail.com> wrote:

On Thu, Mar 12, 2020 at 5:59 PM Jed Brown <jed@jedbrown.org> wrote:

Pierre Jolivet <pierre.jolivet@enseeiht.fr> writes:

> Hello,
> Has there been any follow-up on this https://lists.mcs.anl.gov/pipermail/petsc-users/2015-January/024020.html <https://lists.mcs.anl.gov/pipermail/petsc-users/2015-January/024020.html>?
> Given a 3x3 MatNest A = [A_00,0,0 ; 0,A_11,0 ; 0,0,A_22], I’d like to setup a two-way fieldsplit coupling [A_00,0 ; 0,A_11] and [A_22] but I can’t figure out the proper options.

Are you looking for a Schur split or additive/multiplicative?

Don’t know yet which will perform best, do you have a specific solution in mind for one scenario or the other?

I was mostly wondering if it was possible in a general context, not taking -pc_fieldsplit_type into account.

-pc_fieldsplit_field_0 0,1 -pc_fieldsplit_field_1 2 -pc_fieldsplit_type schur

These flags, used with my .cpp, yield:

[0]PETSC ERROR: Arguments are incompatible

[0]PETSC ERROR: To use Schur complement preconditioner you must have exactly 2 fields

If I use -pc_fieldsplit_%d_fields <a,b,..> as advocated in the manual (instead of -pc_fieldsplit_field_%d as you suggested), I get the same error.

Okay it is  -pc_fieldsplit_%d_fields. When you use this, how many fields does it think you have?

Three.

In case it’s not clear, the MWE is at the bottom of my first email (nest.cpp).

$ mpicxx nest.cpp -I$PETSC_DIR/$PETSC_ARCH/include -I$PETSC_DIR/include -L$PETSC_DIR/$PETSC_ARCH/lib -lpetsc

$ ./a.out -pc_type fieldsplit -pc_fieldsplit_0_fields 0,1 -pc_fieldsplit_1_fields 2 -pc_fieldsplit_type schur

[…]

[0]PETSC ERROR: Arguments are incompatible

[0]PETSC ERROR: To use Schur complement preconditioner you must have exactly 2 fields

[…]

    Split info:

    Split number 0 Defined by IS

    Split number 1 Defined by IS

    Split number 2 Defined by IS

[…]

I see now. If you call PCFieldsplitSetIS(), this overrides anything else. We do not even try to discover the split. If you want

a split determined by command line arguments, you have to defer the split. It looks like it can get splits from the MatNest,

so just do not call SetIS().

So, I’ve commented the three PCFieldSplitSetIS.

$ ./a.out -pc_type fieldsplit -pc_fieldsplit_0_fields 0,1 -pc_fieldsplit_1_fields 2 -pc_fieldsplit_block_size 3

is starting to give me something meaningful in the KSPView, i.e.:

[…]

  Split number 0 Fields  0, 1

[…]

  Split number 1 Fields  2

But it’s surrounded by a bunch of errors such as:

[0]PETSC ERROR: Arguments are incompatible

[0]PETSC ERROR: Could not find index set

Do I need some other command-line options?

If you have a flat 3x3 matnest, the index set matching will not find a subset that is anything other than a 1-field split. I have a patch lying around that i think fixes this. I can try and dig it out on Monday. 

Lawrence