[petsc-users] Reusing ML preconditioner

[John Fettig]

On Wed, Jun 29, 2011 at 10:38 PM, Barry Smith <bsmith at mcs.anl.gov> wrote:
>   What do you mean a slower method? How can you verify that it is still a good preconditioner without actually running the old preconditioner on the new matrix.

I have two equivalent ways of building the matrix: 1) MatSetValues,
(standard route) 2) MatCreate*WithArrays.  Both routines produce
identical matrices and identical solutions.  However, if I run with
version 1), the preconditioner is pretty good for many solves.  If I
run with version 2), the preconditioner only works well for a couple
of solves.

>    BTW: You don't need that complicated stuff above. Just do
>
>    KSPSetOperators( ksp, Amat, Amat, SAME_PRECONDITIONER );
>    KSPSolve().
>    change Amat, or destroy it and make a new one
>    KSPSetOperators( ksp, Amat, Amat, SAME_PRECONDITIONER );
>    KSPSolve().
>
>    It will solve the new linear system using the old preconditioner built during the first KSPSolve(). There is no reason to pass in the old version of Amat you copied to Pmat

I tried this and it works ok for a couple of solves (a couple of
nonlinear iterations), but on the next timestep it takes a lot more
iterations than what I referenced above as version 1).  The total
KSPSolve time in version 1) strongly indicates to me that it is indeed
using the old preconditioner.

Thanks,
John