[petsc-dev] VecNest

[Pierre Jolivet]

Hello,
Could anyone help me understand what is going on in the following example, please?
I have a VecNest.
I either: a) initialize all values to 0.0, then set a specific part of the vector to nonzero or b) initialize a part of the vector to 0.0 and set the other part to nonzero.
I don’t see why a) and b) produce different results.

$ ./ex1111 -pc_type fieldsplit -ksp_monitor_true_residual -ksp_converged_reason -fieldsplit_pc_type jacobi -ksp_pc_side right -ksp_view_final_residual -nest_subvec true 
  0 KSP unpreconditioned resid norm 8.375635517980e-01 true resid norm 8.375635517980e-01 ||r(i)||/||b|| 1.000000000000e+00
  1 KSP unpreconditioned resid norm 4.748816884247e-01 true resid norm 4.748816884247e-01 ||r(i)||/||b|| 5.669798875623e-01
  2 KSP unpreconditioned resid norm 4.713006778679e-01 true resid norm 4.713006778679e-01 ||r(i)||/||b|| 5.627043784990e-01
  3 KSP unpreconditioned resid norm 7.092979927129e-02 true resid norm 7.092979927129e-02 ||r(i)||/||b|| 8.468587144106e-02
  4 KSP unpreconditioned resid norm 1.457836310255e-02 true resid norm 1.457836310255e-02 ||r(i)||/||b|| 1.740567992870e-02
  5 KSP unpreconditioned resid norm 1.625040500524e-14 true resid norm 1.633468028779e-14 ||r(i)||/||b|| 1.950261595401e-14
Linear solve converged due to CONVERGED_RTOL iterations 5
KSP final norm of residual 1.63347e-14
$ ./ex1111 -pc_type fieldsplit -ksp_monitor_true_residual -ksp_converged_reason -fieldsplit_pc_type jacobi -ksp_pc_side right -ksp_view_final_residual -nest_subvec false
  0 KSP unpreconditioned resid norm 0.000000000000e+00 true resid norm 8.375635517980e-01 ||r(i)||/||b||            inf
Linear solve converged due to CONVERGED_ATOL iterations 0
KSP final norm of residual 0.837564

Thanks,
Pierre

-------------- next part --------------
A non-text attachment was scrubbed...
Name: ex1111.c
Type: application/octet-stream
Size: 1498 bytes
Desc: not available
URL: <http://lists.mcs.anl.gov/pipermail/petsc-dev/attachments/20220711/0530c8e9/attachment.obj>