[petsc-dev] PCFIELDSPLIT with inner -ksp_reuse_preconditioner

[Pierre Jolivet]

Hello,
A user(*) wants to switch back and forth with the option -fieldsplit_1_ksp_reuse_preconditioner, but it’s generating results that I don’t understand.
In the following example I solve three successive linear systems.
If I run
$ ./ex1111 -pc_type fieldsplit -fieldsplit_pc_type lu -log_view -info -options_view -switch false
-fieldsplit_1_ksp_reuse_preconditioner is always false, and I properly get:
1) convergence in a single iteration every time 
2) [0] PCSetUp(): Setting up PC with same nonzero pattern for the last solve
3) MatLUFactorNum         3 in -log_view

Now, if I run
$ ./ex1111 -pc_type fieldsplit -fieldsplit_pc_type lu -log_view -info -options_view -switch true
-fieldsplit_1_ksp_reuse_preconditioner is set to true for the second solve and switched back to false for the last solve.
I (wrongfully, IMHO?) get:
1) convergence in 2 iterations for the last solve (instead of 1?)
2) [0] PCSetUp(): Leaving PC with identical preconditioner since reuse preconditioner is set (which should not be set for the last solve?)
3) MatLUFactorNum         1 (instead of 2?)
4) #PETSc Option Table entries:
-fieldsplit_1_ksp_reuse_preconditioner false (though from -info, cf. point #2 just above, this option value is true?)

Does it make sense or do you agree that something is off?

Thanks,
Pierre

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(*) https://community.freefem.org/t/supply-petsc-numbering-for-shared-memory-block-matrix/486/18