[petsc-users] Increasing norm with finer mesh

[Weizhuo Wang]

Amazing, right preconditioning fixes the problem. Thanks a lot!


On Tue, Oct 16, 2018 at 8:31 PM Dave May <dave.mayhem23 at gmail.com> wrote:

>
>
> On Wed, 17 Oct 2018 at 03:15, Weizhuo Wang <weizhuo2 at illinois.edu> wrote:
>
>> I just tried both, neither of them make a difference. I got exactly the
>> same curve with either combination.
>>
>
> Try using right preconditioning.
>
>
> https://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/KSP/KSPSetPCSide.html
>
>
> Use the options:
>
> -ksp_type gmres -ksp_pc_side right -ksp_rtol 1e-12
>
> Or;
>
> -ksp_type fgmres  -ksp_rtol 1e-12
>
> Fgmres does right preconditioning my default
>
>
>
>> 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
>>
>

-- 
Wang Weizhuo
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