[petsc-users] Nesting splits based on IS

[Karin&NiKo]

Dear PETSc users,

I use the fieldsplit PC in an application where the splits are
programmatically defined by IS using PCFieldSplitSetIS. Then the user can
specify its own PC at runtime using PETSc options.
My question : is it possible to define nested splits in this case as it can
be done with strided splits (see snes/examples/tutorials/ex19.c with test
suffix fieldsplit_4).

In order to be perfectly clear : let's say I have a 3 fields problem :
velocity (v split), pressure (p split) and temperature (t split).
What I would like to do is something like the following but it fails :

-ksp_type fgmres
-pc_fieldsplit_type multiplicative
-pc_type fieldsplit    -pc_fieldsplit_0_fields fieldsplit_v_,
fieldsplit_p_    -pc_fieldsplit_1_fields fieldsplit_t_

-prefix_push  fieldsplit_0_
-ksp_type fgmres
-pc_fieldsplit_schur_factorization_type upper
-pc_type fieldsplit
-pc_fieldsplit_type schur
-pc_fieldsplit_schur_precondition a11
-prefix_pop

-prefix_push  fieldsplit_1_
-ksp_type fgmres
-pc_type jacobi
-prefix_pop

-prefix_push  fieldsplit_v_
-ksp_type fgmres
-pc_type gamg
-prefix_pop

-prefix_push  fieldsplit_p_
-ksp_type fgmres
-pc_type jacobi
-prefix_pop

I thank you for your help,
Nicolas
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