[Trilinos-Users] Jacobi and Gauss-Seidel smoothers

[Martin Vymazal]

On 2016-09-13 18:28, Hoemmen, Mark wrote:
> On 9/13/16, 10:48 AM, "Trilinos-Users on behalf of
> trilinos-users-request at trilinos.org"
> <trilinos-users-bounces at trilinos.org on behalf of
> trilinos-users-request at trilinos.org> wrote:
>> Message: 13
>> Date: Tue, 13 Sep 2016 16:43:54 +0100
>> From: Martin Vymazal <martin.vymazal at vki.ac.be>
>> To: trilinos-users at trilinos.org
>> Subject: [Trilinos-Users] Jacobi and Gauss-Seidel smoothers
>> Message-ID: <90ae33366e927b5754371601289fe0e5 at vki.ac.be>
>> Content-Type: text/plain; charset=US-ASCII; format=flowed
>> 
>> Hello,
>> 
>>  I'm trying to understand how to use Jacobi and Gauss-Seidel methods 
>> in
>> Ifpack2 as smoothers. The documentation says that since Ifpack2 
>> derives
>> from Tpetra::Operator, it can also be used as a smoother. I found an
>> example in Doxygen docs for Ifpack2::Relaxation which has the 
>> following
>> code:
>> 
>> // Prepare an Ifpack2 preconditioner called 'prec'
>> // ...
>> // Prepare the relaxation instance for use.
>> prec.initialize ();
>> prec.compute ();
>> // Now prec may be used as a preconditioner or smoother,
>> // by calling its apply() method, just like any Tpetra::Operator.
>> 
>> Tpetra::Operator::apply(X, Y, ...) takes two Tpetra::MultiVectors, so 
>> I
>> assume I can't update the values of the input vector in place and must
>> store the updated entries of X in vector Y. Can X and Y be the same
>> multivector?
> 
> No.  X and Y may not be the same MultiVector, and they may not alias
> one another.
> 
>> Could you please provide a simple example how to use
>> Ifpack2 for relaxation? Thank you.
> 
> Trilinos/packages/ifpack2/example/ex1.cpp
> 
> mfh

Hi Mark,

   doesn't the example use the object 'M' as preconditioner? I'm sorry 
but I don't see any calls to apply(). What's the difference between the 
usage of M as preconditioner and smoother?

  Martin
> 
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