On Jul 16, 2011, at 4:55 AM, ??????? ??????? wrote:
> 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?
Actually in petsc-dev
http://www.mcs.anl.gov/petsc/petsc-as/developers/index.html KSPBCGS does
support both left and right preconditioning. KSPBICG is coded only for left
preconditioning
>
>
> 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?
These two algorithms actually do work with both left and right
preconditioner but they are different than all the other methods in that:
The "residual norm" computed in this algorithm is actually just an upper
bound on the actual residual norm.
That is for left preconditioning it is a bound on the preconditioned
residual and for right preconditioning
it is a bound on the true residual.
I have added this information to their manual pages in petsc-dev for
clarity.
Note that our GMRES is implemented only with left preconditioning and
preconditioned residual norm but you can use the FGMRES which is implemented
only for right preconditioning and the true residual norm for testing.
If you not particularly interested in speed and just want the iterations to
stop when the true residual norm reaches some tolerance you can easily provide
your own KSPConvergedTest() by calling KSPSetConvergenceTest() and writing a
routine that calls KSPBuildResidual() (which always builds the true residual)
and compute the norm of the result.
Barry
>
>
> Please help me to understand situations 3) and 6)
>
> Thank in advance!
>
> --
> Best regards,
> Alexey Ryazanov
> ______________________________________
> Nuclear Safety Institute of Russian Academy of Sciences
>
>
