[petsc-users] tfqmr true residual norm monitoring

[Алексей Рязанов]

Hi, Im solving a linear system with different methods for comparison goals.
And I want to make Krylov iterative methods work until they reach the
predefined value of the *TRUE* residual norm.
So Im setting up tolerances (in fact only *atol*) with KSPSetTolerances(KSP,
1.e-50, 1.e-10, PETSC_DEFAULT, PETSC_DEFAULT) and watching for -ksp_monitor
(or -ksp_monitor_true_residual).
By default most of methods monitor their preconditioned residual norm and
stop working then the preconditioned residual norm reaches the predefined
value.
That of course doesn't mean true residual norm also reached the value.

So I need to change this behavior and use KSPSetNormType(KSP,
KSP_NORM_UNPRECONDITIONED) to use unpreconditioned residual or
KSPSetPreconditionerSide(KSP, PC_RIGHT) to make preconditioned residual be
the same as unpreconditioned and achieve my goal anyway.

So what I have:

1)
KSPSetNormType(KSP, KSP_NORM_UNPRECONDITIONED) is only supported by CG,
RICHARDSON AND CHEBYCHEV (a said in user manual).
So as it is expected, CG, RICHARDSON AND CHEBYCHEV print error message: "No
right preconditioning for KSPCG, KSPRICHARDSON, KSPCHEBYCHEV!" and work
great with KSPSetNormType(KSP, KSP_NORM_UNPRECONDITIONED)
Great!

2)
Practically the same behavior does KSPLSQR (except the great work, actually
it doesn't work at all with my linear system, but I don't care about lsqr)
So it's ok.

3)
KSPBCGS and KSPBICG don't support right preconditioning, so their true
residual norm values cant be set as the finish point for iterating.
Am I right?

4)
KSPGMRES print error message: "Use right preconditioning -ksp_right_pc if
want unpreconditioned norm", when I use it with KSPSetNormType(KSP,
KSP_NORM_UNPRECONDITIONED).
And works great with  KSPSetPreconditionerSide(KSP, PC_RIGHT).
As expected.

5)
KSPCGS prints no error, when I use it with KSPSetNormType(KSP,
KSP_NORM_UNPRECONDITIONED) and works with preconditioned norm.
And works great with  KSPSetPreconditionerSide(KSP, PC_RIGHT).
So it's ok.

6)
KSPTFQMR and KSPTCQMR print no errors with any settings and works with
preconditioned norm.
Is there any methods to make KSPTFQMR use unpreconditioned norm or right
preconditioning?

Please help me to understand situations 3) and 6)

Thank in advance!

-- 
Best regards,
Alexey Ryazanov
______________________________________
Nuclear Safety Institute of Russian Academy of Sciences
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