[petsc-users] Store and reuse the factor of matrix

[Matthew Knepley]

On Thu, Aug 18, 2016 at 9:22 PM, Jinlei Shen <jshen25 at jhu.edu> wrote:

> ​Hi,
>
> I'm trying to implement modified newton method to solve the nonlinear
> finite element using petsc.
>
> As well known,  the advantage of modified newton is the Jacobian matrix is
> always same during the iteration, which means once the J is factorized at
> the first iteration, we can store the factors and avoid the factorization
> for next iteration if we use direct solver, e.g. super_lu. Therefore, the
> option FACTORED in SUPER_LU is quite useful.
>
> However, it looks like the option FACTORED is not available in
> SUPER_LU_DIST in petsc. I tried, and it shows 'unknown option'.
>
> Is there alternative way to use the same idea of FACTORED in petsc for
> super_lu?
>
> Also, I'm wondering whether iterative solver in PETSC is also able to
> apply the same strategy.
>
> In other words, in the problem where Jacobian is constant, only residue
> and solution vectors need to be updated, is there any way to take advantage
> of such same Jacobian pattern to expedite the computation using iterative
> solver?
>
> Thank you
>
> BTW, though using modified newton will increase the iteration number,
> however, in the case which is much more expensive to factorize the
> jacobian, more iterations will probably be worthwhile.
>

You can use


http://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/SNES/SNESSetLagJacobian.html

http://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/SNES/SNESSetLagPreconditioner.html#SNESSetLagPreconditioner

to get fine-grained control over this without writing any code.

   Matt

Bests,
> Jinlei
>



-- 
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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