[petsc-users] KSPBuildResidual and KSPType compatibility

Tue Dec 7 15:25:15 CST 2021

I've attached typical examples of "A" and "b". Do these qualify as large values?

The two runs are with and without "-ksp_pc_side right".

From: Barry Smith <bsmith at petsc.dev>
Sent: Tuesday, December 7, 2021 3:44 PM
To: Fischer, Greg A. <fischega at westinghouse.com>
Cc: Fischer, Greg A. via petsc-users <petsc-users at mcs.anl.gov>
Subject: Re: [petsc-users] KSPBuildResidual and KSPType compatibility

[External Email]

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<mailto: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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