[petsc-users] direct solver question

[Matthew Knepley]

On Tue, Aug 7, 2012 at 1:53 PM, Tabrez Ali <stali at geology.wisc.edu> wrote:

>  Matt
>
> So if I want to use a direct solver I should really use (2) instead of (3).
>

To guarantee robustness, yes.

   Matt


> Tabrez
>
> On 08/07/2012 01:05 PM, Matthew Knepley wrote:
>
> On Tue, Aug 7, 2012 at 12:48 PM, Tabrez Ali <stali at geology.wisc.edu>wrote:
>
>> Barry
>>
>> So which one of the two gives the correct solution? And yes the condition
>> number is high.
>>
>> Actually I am trying to solve a trivial quasi-static problem with the
>> following flags
>>
>> (1) -ksp_type gmres -pc_type asm
>> (2) -ksp_type gmres -pc_type lu -pc_factor_shift_type nonzero
>> (3) -ksp_type preonly -pc_type lu -pc_factor_shift_type nonzero
>>
>
>  This does no iterative refinement, so you are stuck with the initial
> accuracy of the LU
> factorization, which can be bad for an ill-conditioned matrix.
>
>     Matt
>
>
>> The results at t=0 for all three cases are equivalent.
>>
>> For the time dependent part (linear system is solved again), results from
>> (1) and (2) are the same. But (3) gives a very different answer.
>>
>> I also solve the same quasi-static problem using a different code (does
>> not use PETSc) and the results obtained are similar to (1) and (2).
>>
>> So basically I am trying to understand why '-ksp_type preonly' gives me a
>> (potentially) wrong solution.
>>
>> Thanks
>> Tabrez
>>
>>
>>
>> On 08/07/2012 12:01 PM, Barry Smith wrote:
>>
>>> On Aug 7, 2012, at 12:06 PM, Tabrez Ali<stali at geology.wisc.edu>  wrote:
>>>
>>>  Hello list
>>>>
>>>> By any chance is using
>>>>
>>>> -ksp_type gmres -pc_type lu
>>>>
>>>> similar to
>>>>
>>>> -ksp_type preonly -pc_type lu?
>>>>
>>>> Should the results be equivalent?
>>>>
>>>> T
>>>>
>>>     The should be similar but will not be identical necessarily, in
>>> particular when the matrix is ill conditioned they can be very different
>>> and in fact GMRES may iterate a few times.
>>>
>>>     Barry
>>>
>>>
>>
>>   --
>> No one trusts a model except the one who wrote it; Everyone trusts an
>> observation except the one who made it- Harlow Shapley
>>
>>
>
>
>  --
> 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
>
>
>


-- 
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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