[petsc-users] Increasing norm with finer mesh
Fande Kong
fdkong.jd at gmail.com
Tue Oct 16 20:54:52 CDT 2018
Use -ksp_view to confirm the options are actually set.
Fande
Sent from my iPhone
> On Oct 16, 2018, at 7:40 PM, Ellen M. Price <ellen.price at cfa.harvard.edu> wrote:
>
> Maybe a stupid suggestion, but sometimes I forget to call the
> *SetFromOptions function on my object, and then get confused when
> changing the options has no effect. Just a thought from a fellow grad
> student.
>
> Ellen
>
>
>> On 10/16/2018 09:36 PM, Matthew Knepley wrote:
>> On Tue, Oct 16, 2018 at 9:14 PM Weizhuo Wang <weizhuo2 at illinois.edu
>> <mailto:weizhuo2 at illinois.edu>> wrote:
>>
>> I just tried both, neither of them make a difference. I got exactly
>> the same curve with either combination.
>>
>>
>> I have a hard time believing you. If you make the residual tolerance
>> much finer, your error will definitely change.
>> I run tests every day that do exactly this. You can run them too, since
>> they are just examples.
>>
>> Thanks,
>>
>> Matt
>>
>>
>> Thanks!
>>
>> Wang weizhuo
>>
>> On Tue, Oct 16, 2018 at 8:06 PM Matthew Knepley <knepley at gmail.com
>> <mailto:knepley at gmail.com>> wrote:
>>
>> On Tue, Oct 16, 2018 at 7:26 PM Weizhuo Wang
>> <weizhuo2 at illinois.edu <mailto:weizhuo2 at illinois.edu>> wrote:
>>
>> Hello again!
>>
>> After some tweaking the code is giving right answers now.
>> However it start to disagree with MATLAB results
>> ('traditional' way using matrix inverse) when the grid is
>> larger than 100*100. My PhD advisor and I suspects that the
>> default dimension of the Krylov subspace is 100 in the test
>> case we are running. If so, is there a way to increase the
>> size of the subspace?
>>
>>
>> 1) The default subspace size is 30, not 100. You can increase
>> the subspace size using
>>
>> -ksp_gmres_restart n
>>
>> 2) The problem is likely your tolerance. The default solver
>> tolerance is 1e-5. You can change it using
>>
>> -ksp_rtol 1e-9
>>
>> Thanks,
>>
>> Matt
>>
>>
>>
>> Disagrees.png
>>
>> Thanks!
>>
>> Wang Weizhuo
>>
>> On Tue, Oct 9, 2018 at 2:50 AM Mark Adams <mfadams at lbl.gov
>> <mailto:mfadams at lbl.gov>> wrote:
>>
>> To reiterate what Matt is saying, you seem to have the
>> exact solution on a 10x10 grid. That makes no sense
>> unless the solution can be represented exactly by your
>> FE space (eg, u(x,y) = x + y).
>>
>> On Mon, Oct 8, 2018 at 9:33 PM Matthew Knepley
>> <knepley at gmail.com <mailto:knepley at gmail.com>> wrote:
>>
>> On Mon, Oct 8, 2018 at 9:28 PM Weizhuo Wang
>> <weizhuo2 at illinois.edu
>> <mailto:weizhuo2 at illinois.edu>> wrote:
>>
>> The code is attached in case anyone wants to
>> take a look, I will try the high frequency
>> scenario later.
>>
>>
>> That is not the error. It is superconvergence at the
>> vertices. The real solution is trigonometric, so your
>> linear interpolants or whatever you use is not going
>> to get the right value in between mesh points. You
>> need to do a real integral over the whole interval
>> to get the L_2 error.
>>
>> Thanks,
>>
>> Matt
>>
>>
>> On Mon, Oct 8, 2018 at 7:58 PM Mark Adams
>> <mfadams at lbl.gov <mailto:mfadams at lbl.gov>> wrote:
>>
>>
>>
>> On Mon, Oct 8, 2018 at 6:58 PM Weizhuo Wang
>> <weizhuo2 at illinois.edu
>> <mailto:weizhuo2 at illinois.edu>> wrote:
>>
>> The first plot is the norm with the flag
>> -pc_type lu with respect to number of
>> grids in one axis (n), and the second
>> plot is the norm without the flag
>> -pc_type lu.
>>
>>
>> So you are using the default PC w/o LU. The
>> default is ILU. This will reduce high
>> frequency effectively but is not effective
>> on the low frequency error. Don't expect
>> your algebraic error reduction to be at the
>> same scale as the residual reduction (what
>> KSP measures).
>>
>>
>>
>>
>> --
>> Wang Weizhuo
>>
>>
>>
>> --
>> What most experimenters take for granted before they
>> begin their experiments is infinitely more
>> interesting than any results to which their
>> experiments lead.
>> -- Norbert Wiener
>>
>> https://www.cse.buffalo.edu/~knepley/
>> <https://urldefense.proofpoint.com/v2/url?u=http-3A__www.cse.buffalo.edu_-7Eknepley_&d=DwMFaQ&c=OCIEmEwdEq_aNlsP4fF3gFqSN-E3mlr2t9JcDdfOZag&r=hsLktHsuxNfF6zyuWGCN8x-6ghPYxhx4cV62Hya47oo&m=EFM29ATgv4U8PjXEtfgMkuxKr5DGscMlH-j769W5W_4&s=grgSL2LaDCthvYvvFITmeOOWPCwgmNfYRPs94N8kmOs&e=>
>>
>>
>>
>> --
>> Wang Weizhuo
>>
>>
>>
>> --
>> What most experimenters take for granted before they begin their
>> experiments is infinitely more interesting than any results to
>> which their experiments lead.
>> -- Norbert Wiener
>>
>> https://www.cse.buffalo.edu/~knepley/
>> <http://www.cse.buffalo.edu/%7Eknepley/>
>>
>>
>>
>> --
>> Wang Weizhuo
>>
>>
>>
>> --
>> What most experimenters take for granted before they begin their
>> experiments is infinitely more interesting than any results to which
>> their experiments lead.
>> -- Norbert Wiener
>>
>> https://www.cse.buffalo.edu/~knepley/
>> <http://www.cse.buffalo.edu/%7Eknepley/>
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