[petsc-users] Fieldsplit schur complement with preonly solves

[Luc Berger-Vergiat]

Hi all,
I am testing some preconditioners for a FEM problem involving different types of fields (displacements, temperature, stresses and plastic strain).
To make sure that things are working correctly I am first solving this problem with: 
-ksp_type preonly -pc_type lu, which works fine obviously.

Then I move on to do:
-ksp_type gmres -pc_type lu, and I get very good convergence (one gmres linear iteration per time step) which I expected.

So solving the problem exactly in a preconditioner to gmres leads to optimal results.
This can be done using a Schur complement, but when I pass the following options:
-ksp_type gmres -pc_type fieldsplit -pc_fieldsplit_type schur -pc_fieldsplit_schur_factorization_type full -pc_fieldsplit_0_fields 2,3 -pc_fieldsplit_1_fields 0,1 -fieldsplit_0_ksp_type preonly -fieldsplit_0_pc_type lu -fieldsplit_1_ksp_type preonly -fieldsplit_1_pc_type lu
My results are terrible, gmres does not converge and my FEM code reduces the size of the time step in order to converge.
This does not make much sense to me...

Curiously if I use the following options:
-ksp_type gmres -pc_type fieldsplit -pc_fieldsplit_type schur -pc_fieldsplit_schur_factorization_type full -pc_fieldsplit_0_fields 2,3 -pc_fieldsplit_1_fields 0,1 -fieldsplit_0_ksp_type gmres -fieldsplit_0_pc_type lu -fieldsplit_1_ksp_type gmres -fieldsplit_1_pc_type lu
then the global gmres converges in two iterations.

So using a pair of ksp gmres/pc lu on the A00 block and the Schur complements works, but using lu directly doesn't.

Because I think that all this is quite strange, I decided to dump some matrices out. Namely, I dumped the complete FEM jacobian, I also do a MatView on jac->B, jac->C and the result of KSPGetOperators on kspA. These returns three out of the four blocks needed to do the Schur complement. They are correct and I assume that the last block is also correct.
When I import jac->B, jac->C and the matrix corresponding to kspA in MATLAB to compute the inverse of the Schur complement and pass it to gmres as preconditioner the problem is solved in 1 iteration.

So MATLAB says:
-ksp_type gmres -pc_type fieldsplit -pc_fieldsplit_type schur -pc_fieldsplit_schur_factorization_type full -pc_fieldsplit_0_fields 2,3 -pc_fieldsplit_1_fields 0,1 -fieldsplit_0_ksp_type preonly -fieldsplit_0_pc_type lu -fieldsplit_1_ksp_type preonly -fieldsplit_1_pc_type lu
should yield only one iteration (maybe two depending on implementation).

Any ideas why the Petsc doesn't solve this correctly?

Best,
Luc

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