[petsc-users] FieldSplit, multigrid and blocksize

[Karin&NiKo]

Dear PETSc gurus,

I am using FieldSplit to solve a poro-mechanics problem. Thus, I am dealing
with 3 displacement DOF and 1 pressure DOF.
In order to precondition the 00 block (aka the displacement block), I am
using a multigrid method (ml or gamg). Nevertheless, I have the feeling
that the multigrids performance is much lower than in the case where they
are used on pure displacement problems (say elasticity). Indeed, I do not
know how to set the block size of the 00 block when using FieldSplit!
Could you please give me some hint on that? (the phrase "The fieldsplit
preconditioner cannot currently be used with the BAIJ or SBAIJ data formats
if the blocksize is larger than 1." is not clear enough for me...).

Thanks in advance,
Nicolas
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