[petsc-users] Adapting MatMult and PCMG functions in matrix-free method.

[domenico lahaye]

Dear PETSc developers, 

  Thank you for your support to Abdul. 


   We are in the process of developing a multilevel 

Krylov solver for the Helmholtz equation. Abdul 

has implemented Algorithm I for this purpose. 

We next would like to implement Algorithm II. 

Algorithm || amounts to replacing every occurrence 

of the system matrix $A$ in Algorithm I by 

$M^{-1} A$. This replacement should occur on all 

levels. We thought of two ways to realize this 

replacement. 


1) We thought of adopting a matrix-free approach, 

and to plug in the operation with  $M^{-1}$ there. 

This would require a ksp context inside MatMult.    

We wonder whether this a approach is feasible to 

take. 


2) The other approach would by to implement a 

customized pcmg preconditioner that we can adapt 

to our needs. This could be a more elegant approach, 

at the cost of doing more work. Is the assumption 

that the second approach is more elegant correct 

and would you be able to give advice on how to tackle 

this approach? 


  Kind wishes, Domenico. 




________________________________




----- Forwarded Message -----
>From: Matthew Knepley <knepley at gmail.com>
>To: Abdul Hanan Sheikh <hanangul12 at yahoo.co.uk>; PETSc users list <petsc-users at mcs.anl.gov> 
>Sent: Thursday, 5 July 2012, 15:45
>Subject: Re: [petsc-users] Adapting MatMult and PCMG functions in matrix-free method.
> 
>
>On Thu, Jul 5, 2012 at 7:34 AM, Abdul Hanan Sheikh <hanangul12 at yahoo.co.uk> wrote:
>
>Dear developers and users,
>>Summer greetings.
>>We have few question listen below: 
>>
>>
>>1. 
>>
>>The first question is about adapting " MatMult " function in matrix-free method.
>>We intend to incorporate a KSP context inside "MatMult" . The immediate question is how to
>>provide more than one matrices as input. 
>
>
>You provide extra data through the context for the MATSHELL
> 
>Is this idea of incorporating a KSP context inside "MatMult" function workable ? Does it make any confrontation 
>>
>>with philosophy of development of Petsc. ? 
>
>
>I am not sure you want this. Do you think PCKSP can do what you want? There is not enough information here to help us answer.
> 
>2. 
>>
>>An other advance level feedback is needed. 
>>
>> Re-implementing PCMG function {mg.c } will lead any violation of philosophy of Petsc-development ??
>
>
>Again, there is not enough information. Can you do what you want by just replacing the monitors?
>
>
>  Thanks,
>
>
>     Matt
> 
>3. 
>>
>>Which one of the above both is more elegant and feasible to work on ? 
>>
>>
>>
>>
>>
>>Thanking in anticipation, Abdul 
>>
>>
>>
>>
>>
>
>
>
>-- 
>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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