[petsc-users] dense matrix problem

[Matthew Knepley]

On Thu, Jan 31, 2013 at 12:31 PM, Alex Salazar <alejandrocof at gmail.com>wrote:

> Thanks Jed, sorry I new using Petsc.
>
> I am runing my program  with default options :
> mpirun -np 1 ./a.out
> runing the program with a few points (10) the interpolation is aceptable,
>
>
> using the monitor options ie.
> mpirun -np 1 ./a.out -ksp_converged_reason -ksp_monitor_true_residual
> the output is the next
>  0 KSP preconditioned resid norm 7.415666989515e+05 true resid norm
> 1.510533668194e+04 ||r(i)||/||b|| 1.000000000000e+00
>   1 KSP preconditioned resid norm 4.935801698308e+02 true resid norm
> 1.721762948997e+01 ||r(i)||/||b|| 1.139837519183e-03
>   2 KSP preconditioned resid norm 1.038493428107e-01 true resid norm
> 9.406615966569e+00 ||r(i)||/||b|| 6.227346112594e-04
> Linear solve converged due to CONVERGED_RTOL iterations 2
>
> But if encrease the number of points to interpolate, the result is
> incorrect and the time of convergece encrease, the output in this case is:
>
> .....
> 138 KSP preconditioned resid norm 4.897817095518e-06 true resid norm
> 5.455886375264e-08 ||r(i)||/||b|| 4.852601039969e-10
> 139 KSP preconditioned resid norm 3.535973415166e-06 true resid norm
> 3.717900878947e-08 ||r(i)||/||b|| 3.306793512687e-10
> 140 KSP preconditioned resid norm 2.450794339359e-06 true resid norm
> 3.009597005096e-08 ||r(i)||/||b|| 2.676810430480e-10
> Linear solve converged due to CONVERGED_RTOL iterations 140
> [0] time (sec) 89.940000
>

You have an ill-conditioned system, so you will have to lower the
tolerance. Also note that your
preconditioner is a piece of crap.

    Matt


>
> the  code where the solution is implemented is the next:
> KSP ksp;
> KSPCreate(PETSC_COMM_WORLD,&ksp);
> KSPSetOperators(ksp,A,A,DIFFERENT_NONZERO_PATTERN);
> KSPSetTolerances(ksp,1.e-5/(N+nt+1),1.e-50,PETSC_DEFAULT,PETSC_DEFAULT);
> KSPSetFromOptions(ksp);
> KSPSolve(ksp,b,x);
>
> Let me know if you need another information.
>
> Regard Alex
>
>
> 2013/1/31 Jed Brown <jedbrown at mcs.anl.gov>
>
>> "result is incorrect" is not helpful. Please include diagnostics. Are you
>> using an iterative or direct solver? Start here if you don't know what to
>> check:
>>
>>
>> http://scicomp.stackexchange.com/questions/513/why-is-my-iterative-linear-solver-not-converging
>>
>>
>>
>> On Thu, Jan 31, 2013 at 9:58 AM, Alex Salazar <alejandrocof at gmail.com>wrote:
>>
>>> Hello, everyone
>>>
>>> I want to get the solution of algrebraic system (with a  dense
>>> zero-diagonal simetric matrix) of a interpolation problem (using radial
>>> basis functions) with Petsc.
>>>
>>>  For a few points (around 10 elements) using KSP I get the correct
>>> solution,  however when  I increase the number of points,  the result is
>>> incorrect.
>>>
>>> Any Idea ?
>>>
>>> Regards Alejandro.
>>>
>>>
>>
>


-- 
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
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