get the preconditioner matrix

[Barry Smith]

   Again, this does not make sense. Prometheus as a dense matrix
will 1) require much much to much memory and 2) take much to
long to apply O(n^2) flops. The whole idea behind multilevel
methods is to be roughly order O(n) to apply.

   If Mark has provided a PCView() and PCLoad() for Prometheus
then it could be saved and reused (but it would not be saved
as a dense matrix), BUT I don't think Mark has done this (plus
it would very likely require rerunning on the same number of
processors).

   You just need to calculate the preconditioner for each time
you run the program.

    Barry

On Thu, 13 Apr 2006, Letian Wang wrote:

> Barry:
>
> Thank you for your reply. What I want to do is to use PETSc for
> optimization. I use Prometheus pre-conditioner to solve the initial problem.
> Usually it spends much time on getting the pre-conditioner, then the
> iterations are relatively going faster. I'm thinking to save the
> pre-conditioner matrix for the initial problem (Now I know I can do that by
> PCComputeExplicitOperator), then for other very similar problems, I can pass
> the saved pre-conditioner and apply them directly to the new problem. Do you
> think it will work?
>
> Is there any routine to apply the saved matrix as pre-conditioner? Or I have
> to program an user-defined PC routine?
>
> Thanks.
>
> Letian
>
> -----Original Message-----
> From: owner-petsc-users at mcs.anl.gov [mailto:owner-petsc-users at mcs.anl.gov]
> On Behalf Of Barry Smith
> Sent: Wednesday, April 12, 2006 7:56 PM
> To: petsc-users at mcs.anl.gov
> Subject: Re: get the preconditioner matrix
>
>
>   Letian,
>
>   What do you mean be "pre-conditioner matrix"? It is very rare
> that a preconditioner is explicitly represented as a matrix; it is almost
> always just some code that applies the operator. In general an explicitly
> represented preconditioner would actually be a dense matrix.
>
>   If you truly want this dense matrix you can call
> PCComputeExplicitOperator()
> and store the resulting matrix to a file.
>
>    Barry
>
>
> On Wed, 12 Apr 2006, Letian Wang wrote:
>
>> Dear all,
>>
>>
>>
>> Is it possible to obtain the PETSc pre-conditioner matrix and output it to
> a
>> file? How can I do that?  Thanks.
>>
>>
>>
>> Letian Wang
>>
>>
>>
>>
>
>
>