[petsc-dev] fieldsplit + composite + ksp

[Pierre Jolivet]

Hello,
I’m solving the following dummy system http://jolivet.perso.enseeiht.fr/composite_ksp.tar.gz <http://jolivet.perso.enseeiht.fr/composite_ksp.tar.gz>
[A, B];[C, D], with a PCFIELDSPLIT. For the PC of D, I’m using a PCCOMPOSITE with two sub PCs. One of which is a PCKSP.
Could you please help me figure out what is wrong in the following piece of code, that may be launched with the following arguments:
$ mpirun -n 1 ./a.out -ksp_type preonly -pc_type fieldsplit -fieldsplit_1_pc_type composite -fieldsplit_1_sub_1_pc_type ksp -fieldsplit_1_sub_1_ksp_ksp_type gmres -fieldsplit_1_sub_1_ksp_pc_type gamg -fieldsplit_1_sub_1_ksp_ksp_converged_reason -fieldsplit_1_sub_1_ksp_pc_gamg_sym_graph 1 -fieldsplit_1_sub_1_ksp_pc_gamg_square_graph 10 -fieldsplit_1_sub_1_ksp_ksp_rtol 1e-8

It solves the dummy system twice, with a varying block D.

It should give you:
      Linear fieldsplit_1_sub_1_ksp_ solve converged due to CONVERGED_RTOL iterations 8
solve #0: 16098.3
      Linear fieldsplit_1_sub_1_ksp_ solve did not converge due to DIVERGED_PC_FAILED iterations 0
                     PC_FAILED due to SUBPC_ERROR
solve #1: inf

If I switch line 70 to #if 0, I get the expected output:
      Linear fieldsplit_1_sub_1_ksp_ solve converged due to CONVERGED_RTOL iterations 8
solve #0: 16098.3
      Linear fieldsplit_1_sub_1_ksp_ solve converged due to CONVERGED_RTOL iterations 8
solve #1: 325.448

I’m realizing that this has probably nothing to do with the outer PCFIELDSPLIT, but this comes from a rather large FSI solver, so reproducing this behavior in “only” 97 SLOC is good enough for your I hope.

Thanks in advance,
Pierre
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