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

[Matthew Knepley]

On Fri, Dec 5, 2014 at 2:31 PM, Evan Um <evanum at gmail.com> wrote:

> 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?
>

1) MUMPS stores these in its own format, so code from other packages like
PETSc is not useful here

2) I do not think the MUMPS has the wrong algorithm or implementation. This
process is slow in parallel,
    and should be avoided in favor of more scalable methods.

  Matt


> 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
>>
>
>


-- 
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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