ell1 norm convergence test

[Ryan Yan]

Sorry, FGMRES does not need to compute residual expilictly. Please forget
about what I just asked.

Yan

On Mon, Sep 21, 2009 at 3:04 PM, Ryan Yan <vyan2000 at gmail.com> wrote:

> Hi Barry,
> Thanks for the suggestion.
>
> I am using FGMRES as the krylov solver.  Is there a direct access to the
> residual vector for FGMRES. I guess the answer is yes, since that's the
> Krylov method which supports right preconditioning and provides the true
> residual 2_Norm monitor, right?
>
> If so, which header should I include to fetch the residual directly from
> KSP_FGMRES?
>
> Yan
>
>
>
>
> On Mon, Sep 21, 2009 at 2:43 PM, Barry Smith <bsmith at mcs.anl.gov> wrote:
>
>>
>>  Yan,
>>
>>    This depends on the Krylov method being used. For example with GMRES
>> the residual vector is NOT available at each iteration (the 2-norm of the
>> residual is approximated via a recurrence relationship).
>>
>>    You can call KSPBuildResidual() to have the true residual computed for
>> you. Note: it is expensive because it actually builds the current solution
>> and computes r = b - A*x
>>
>>   What we intended is that if you want the residual efficiently for
>> example for the CG method, you determine what Krylov method you want to use
>> the include the appropriate private include file and access the residual
>> directly from the data structure. This would be efficient. (but like I said
>> does not work for all methods). For cg include src/ksp/ksp/impls/cg/cgctx.h
>>
>>
>>   Barry
>>
>>
>> On Sep 21, 2009, at 12:44 PM, Ryan Yan wrote:
>>
>>  Hi All,
>>> The following question is only for test and comparison reason.
>>>
>>> Is there a command line option to set up using 1-Norm(sum of absolute
>>> value of residuals)  as convergence monitor and test.
>>>
>>> Alternatively, if I can get the residual "r" out at the end of each
>>> iteration, then it is going to be much simpler, since I can just call
>>> VecNorm(r, NORM_1, &r_ell1) and pass the r_ell1 into the convergence monitor
>>> and test. Can anyone inform me if PETSc has an interface for the resuidual
>>> r?
>>>
>>> Or one has to call the convergence test:
>>> MyKSPConverged(ksp,n,rnorm,flag,dummy);
>>> and following the steps:
>>> 1. fetch the exact solution:
>>>
>>> KSPBuildSolution(ksp,PETSC_NULL_OBJECT,x,ierr)
>>>
>>> 2. calculate the residual:
>>>
>>> pass the Right Hand side and the Matrix in to calculate the 1-Norm. But
>>> this step is *not* obvious, since the matrix and rhs is already distributed
>>> over each processes.
>>>
>>> 3. set up the convergence test based on the 1_Norm residual.
>>>
>>>
>>> Thank you very much,
>>>
>>>
>>> Yan
>>>
>>
>>
>
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