[petsc-users] Using MUMPS as a preconditioner for KSPSolve()

[Evan Um]

Dear Matt,

Thanks for your quick reply. I mean avoiding MUMPS's internal back/forward
solvers (JOB=3). Does KSPSOLVE() have its own back/forward routines?

Evan

On Fri, Dec 5, 2014 at 12:20 PM, Matthew Knepley <knepley at gmail.com> wrote:

> On Fri, Dec 5, 2014 at 2:11 PM, Evan Um <evanum at gmail.com> wrote:
>
>> Dear PETSC Users,
>>
>> I tried to use a Cholesky factor (MUMPS results) as a preconditioner for
>> KSPSolve(). An example code is pasted below. When the code runs, the log
>> file indicates that Job=3 (i.e. backward/forward substitution) of MUMPS is
>> called every time inside the loop. Is there anyway to avoid JOB=3 of MUMPS
>> and use the factor as a pure preconditioner for the CG solver inside
>> KSPSOLVE()? On my cluster, JOB=3 shows unexpected slow performance (see
>> Vol72, Issue 35) and should be avoided. In advance, thanks for
>>
>
> What do you mean by the above sentence. When you Cholesky factor a matrix
> A, you get
>
>   A = L^T L
>
> When you use this factorization as a preconditioner, you modify your
> original problem to
>
>   (L^T L)^{-1} A x = (L^T L)^{-1} b
>
> When you use a Krylov method like CG, this means that at every iterate you
> must apply
> (L^T L){-1} which entails a forward and backward solve.
>
> If you mean, you only want to apply (L^T L)^{-1} and not run CG, then use
> -ksp_type preonly
>
>   Matt
>
>
>> your help.
>>
>> Regards,
>> Evan
>>
>>
>> Code:
>> KSPCreate(PETSC_COMM_WORLD, &ksp_fetd_dt);
>> KSPSetOperators(ksp_fetd_dt, A_dt, A_dt);
>> KSPSetType (ksp_fetd_dt, KSPPREONLY);
>> KSPGetPC(ksp_fetd_dt, &pc_fetd_dt);
>> MatSetOption(A_dt, MAT_SPD, PETSC_TRUE);
>> PCSetType(pc_fetd_dt, PCCHOLESKY);
>> PCFactorSetMatSolverPackage(pc_fetd_dt, MATSOLVERMUMPS);
>> PCFactorSetUpMatSolverPackage(pc_fetd_dt);
>> PCFactorGetMatrix(pc_fetd_dt, &F_dt);
>> MatMumpsSetIcntl(F_dt, 4, 1); // Turn on MUMPS's log file.
>> KSPSetType(ksp_fetd_dt, KSPCG);
>> KSPSetTolerances(ksp_fetd_dt, 1e-9, 1.0e-50, 1.0e10, ksp_iter);
>> for (int i=0; i<1000; i++) {
>> // Create a new RHS vector B_dt
>> KSPSolve(ksp_fetd_dt,B_dt,solution);
>> // Output solution time=time2-time1;
>> }
>>
>
>
>
> --
> 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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