[petsc-users] snes + matrix free + coloring

panourg at mech.upatras.gr panourg at mech.upatras.gr
Wed Oct 10 17:13:59 CDT 2012

It makes sense because i want to compute the preconditioner only per some
time steps of my finite element code whereas in other times step
(non-linear systems) i dont compute the prec and only the mf works using
the same prec from previous step.

Finally, could you tell me a source with all these functions so that i can
solve my problem?


> On Wed, Oct 10, 2012 at 4:33 PM, <panourg at mech.upatras.gr> wrote:
>> What do you mean when you say that i do not use a matrix free method at
>> all?
>> With matrix-free method only the operator is "free". Not the
>> preconditioning matrix.
> Yes, but if you are assembling the Jacobian using coloring, but then only
> using it as a preconditioner and using a separate matrix-free finite
> differencing step to apply the operator, then you are just goofing off. Of
> course you can do it, but it doesn't make sense.
>> For this i always set -snes_mf_operator option.
>> Either way is any way to have preconditioner without forming the its
>> prec
>> matrix??
> Sure, but you have to understand the problem, e.g., multilevel structure,
> plane waves, an embedded discretization. If you want to use PETSc's
> automatic preconditioners instead of rolling your own, then you should
> assemble the operator.

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