[petsc-users] KSPBuildResidual and KSPType compatibility

[Barry Smith]

What do you get for -ksp_type ibcgs -ksp_monitor -ksp_monitor_true_residual

with and without  -ksp_pc_side right ?


> On Dec 6, 2021, at 12:00 PM, Jed Brown <jed at jedbrown.org> wrote:
> 
> "Fischer, Greg A. via petsc-users" <petsc-users at mcs.anl.gov> writes:
> 
>> Hello petsc-users,
>> 
>> I would like to check convergence against the infinity norm, so I defined my own convergence test routine with KSPSetConvergenceTest. (I understand that it may be computationally expensive.)
>> 
>> I would like to do this with the "ibcgs" method. When I use KSPBuildResidual and calculate the NORM_2 against the output vector, I get a value that differs from the 2-norm that gets passed into the convergence test function. However, when I switch to the "gcr" method, the value I calculate matches the function input value.
> 
> IBCGS uses the preconditioned norm by default while GCR uses the unpreconditioned norm. You can use -ksp_norm_type unpreconditioned or KSPSetNormType() to make IBCGS use unpreconditioned.
> 
>> Is the KSPBuildResidual function only compatible with a subset of the KSPType methods? If I want to evaluate convergence against the infinity norm, do I need to set KSPSetInitialGuessNonzero and continually re-start the solver with a lower tolerance values until I get a satisfactory value of the infinity norm?
>> 
>> Thanks,
>> Greg
>> 
>> 
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