[petsc-users] Increasing norm with finer mesh

[Ellen M. Price]

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/>