[petsc-users] PCFieldSplit gives different results for direct and iterative solver

[Y. Shidi]

Hello,

I am trying to solve the incompressible n-s equations by
PCFieldSplit.

The large matrix and vectors are formed by MatCreateNest()
and VecCreateNest().
The system is solved directly by the following command:
     -ksp_type fgmres \
     -pc_type fieldsplit \
     -pc_fieldsplit_type schur \
     -pc_fieldsplit_schur_fact_type full \
     -ksp_converged_reason \
     -ksp_monitor_true_residual \
     -fieldsplit_0_ksp_type preonly \
     -fieldsplit_0_pc_type cholesky \
     -fieldsplit_0_pc_factor_mat_solver_package mumps \
     -mat_mumps_icntl_28 2 \
     -mat_mumps_icntl_29 2 \
     -fieldsplit_1_ksp_type preonly \
     -fieldsplit_1_pc_type jacobi \
Output:
   0 KSP unpreconditioned resid norm 1.214252932161e+04 true resid norm 
1.214252932161e+04 ||r(i)||/||b|| 1.000000000000e+00
   1 KSP unpreconditioned resid norm 1.642782495109e-02 true resid norm 
1.642782495109e-02 ||r(i)||/||b|| 1.352916226594e-06
Linear solve converged due to CONVERGED_RTOL iterations 1

The system is solved iteratively by the following command:
     -ksp_type fgmres \
     -pc_type fieldsplit \
     -pc_fieldsplit_type schur \
     -pc_fieldsplit_schur_factorization_type diag \
     -ksp_converged_reason \
     -ksp_monitor_true_residual \
     -fieldsplit_0_ksp_type preonly \
     -fieldsplit_0_pc_type gamg \
     -fieldsplit_1_ksp_type minres \
     -fieldsplit_1_pc_type none \
Output:
   0 KSP unpreconditioned resid norm 1.214252932161e+04 true resid norm 
1.214252932161e+04 ||r(i)||/||b|| 1.000000000000e+00
   1 KSP unpreconditioned resid norm 2.184037364915e+02 true resid norm 
2.184037364915e+02 ||r(i)||/||b|| 1.798667565109e-02
   2 KSP unpreconditioned resid norm 2.120097409539e+02 true resid norm 
2.120097409635e+02 ||r(i)||/||b|| 1.746009709742e-02
   3 KSP unpreconditioned resid norm 4.364091658268e+01 true resid norm 
4.364091658575e+01 ||r(i)||/||b|| 3.594054865332e-03
   4 KSP unpreconditioned resid norm 2.632671796885e+00 true resid norm 
2.632671797020e+00 ||r(i)||/||b|| 2.168141189773e-04
   5 KSP unpreconditioned resid norm 2.209213998004e+00 true resid norm 
2.209213980361e+00 ||r(i)||/||b|| 1.819401808180e-04
   6 KSP unpreconditioned resid norm 4.683775185840e-01 true resid norm 
4.683775085753e-01 ||r(i)||/||b|| 3.857330677735e-05
   7 KSP unpreconditioned resid norm 3.042503284736e-02 true resid norm 
3.042503349258e-02 ||r(i)||/||b|| 2.505658638883e-06


Both methods give answers, but they are different so I am wondering
if it is possible that you can help me figure out which part I am
doing wrong.

Thank you for your time.

Kind Regards,
Shidi