[petsc-users] KSPBuildResidual and KSPType compatibility

[Barry Smith]

When the preconditioned and non-preconditioner residual norms as so hugely different this usually indicates something is badly scaled or something "bad" is happening within the preconditioner. 

0 KSP preconditioned resid norm 2.434689662304e-01 true resid norm 1.413757649058e+09 ||r(i)||/||b|| 1.442188243865e-04
....
19 KSP preconditioned resid norm 4.505576228085e-15 true resid norm 1.810377537457e-02 ||r(i)||/||b|| 1.846784138157e-15

notice that at the end the true residual is not particularly small. Is your b really big and does the matrix have really large values in it?

What are the two runs? Left and right preconditioning with ibcgs or something else?


> On Dec 7, 2021, at 12:44 PM, Fischer, Greg A. <fischega at westinghouse.com> wrote:
> 
> Attached are outputs with these options. 
>  
> What should I make of these? 
>  
> From: Barry Smith <bsmith at petsc.dev <mailto:bsmith at petsc.dev>> 
> Sent: Monday, December 6, 2021 2:16 PM
> To: Fischer, Greg A. via petsc-users <petsc-users at mcs.anl.gov <mailto:petsc-users at mcs.anl.gov>>
> Cc: Fischer, Greg A. <fischega at westinghouse.com <mailto:fischega at westinghouse.com>>
> Subject: Re: [petsc-users] KSPBuildResidual and KSPType compatibility
>  
> [External Email]
>  
> 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 <mailto:jed at jedbrown.org>> wrote:
>  
> "Fischer, Greg A. via petsc-users" <petsc-users at mcs.anl.gov <mailto: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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> <ksp-monitor_output.txt><ksp-monitor_output-pc_side_right.txt>

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