[petsc-users] Increasing norm with finer mesh

[Weizhuo Wang]

I just tried both, neither of them make a difference. I got exactly the
same curve with either combination.

Thanks!

Wang weizhuo

On Tue, Oct 16, 2018 at 8:06 PM Matthew Knepley <knepley at gmail.com> wrote:

> On Tue, Oct 16, 2018 at 7:26 PM Weizhuo Wang <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
>
>
>>
>> [image: Disagrees.png]
>>
>> Thanks!
>>
>> Wang Weizhuo
>>
>> On Tue, Oct 9, 2018 at 2:50 AM Mark Adams <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>
>>> wrote:
>>>
>>>> On Mon, Oct 8, 2018 at 9:28 PM Weizhuo Wang <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> wrote:
>>>>>
>>>>>>
>>>>>>
>>>>>> On Mon, Oct 8, 2018 at 6:58 PM Weizhuo Wang <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/~knepley/>
>


-- 
Wang Weizhuo
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