[petsc-users] KSP linear solver

[Que Cat]

Dear Matt,

On Sat, Jan 18, 2014 at 1:54 PM, Matthew Knepley <knepley at gmail.com> wrote:

> On Sat, Jan 18, 2014 at 1:46 PM, Que Cat <quecat001 at gmail.com> wrote:
> Dear Petsc-Users,
>>
>>
>> I debug my code. I solve the small linear system using KSP.
>> The matrix is:
>>
>> row 0: (0, -123354)  (1, 6.92922e-310)  (2, 123354)  (3, 1.38582e-309)
>> row 1: (0, 1.3858e-309)  (1, 91156.6)  (2, 6.92901e-310)  (3, 91156.6)
>> row 2: (0, 8768.96)  (1, 2.47033e-323)  (2, -171955)  (3, 1.02212e-316)
>>  (4, 163186)  (5, 0)
>> row 3: (0, 0)  (1, -59201.7)  (2, 0)  (3, 156037)  (4, 0)  (5, 93996.1)
>> row 4: (2, 13422.8)  (3, 0)  (4, -13422.8)  (5, 0)
>> row 5: (2, 0)  (3, -58000.3)  (4, 0)  (5, 59201.7)
>>
>>
> 1) Get rid of the entries that are 1e-300 or below
>
>

Is there any way to get rid of these entries automatically? Or we have to
do this explicitly? Thank you.

Que





>  the RHS vector:
>>
>> -9115.66
>> -17760.5
>> -425.926
>> -35400.9
>> 9541.58
>> -17520.2
>>
>> the solution I got was:
>>
>> -2.00943e+52
>> -1.17929e+18
>> -2.00943e+52
>> 7.121e+16
>> -2.00943e+52
>> -7.0943e+16
>>
>> I checked with matlab and I got the totally different solution. I guess
>> there is something wrong with the linear solver I set up as follow:
>>
>>     KSPSetOperators(ksp, mat_A, mat_A, SAME_NONZERO_PATTERN);
>>
>>     // set the linear solve options
>>     KSPSetType(ksp, KSPGMRES);
>>     KSPGMRESSetRestart(ksp, 100);
>>     // set preconditioning options
>>     KSPGetPC(ksp, &pc);
>>     PCSetType(pc, PCBJACOBI);
>>     KSPSetFromOptions(ksp);
>>     // Solve the linear system
>>     KSPSolve(ksp,vec_rhs,vec_sol);
>>
>> Could anyone give me a suggestion where should be the source of error?
>> Thank you!
>>
>
> 2) Always start with -pc_type lu -ksp_type preonly
>
> 3) It appears that your matrix is close to singular (LU will check)
>
>   Matt
>
>
>> Que
>>
>>
>
>
> --
> 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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