[petsc-users] Convergence/accuracy degradation with increasing number of procs

[Matthew Knepley]

On Mon, Jan 9, 2012 at 10:37 AM, Alexander Grayver
<agrayver at gfz-potsdam.de>wrote:

> **
> I see this is due to preconditioner, but could you explain shortly what
> particularly causes this problem? It's not obvious to me.
>

The size of the block being factorized by ILU(0) decreases as p increases.
Thus, it is
a weaker preconditioner. I recommend Yousef Saad's book which explains all
this clearly.

   Matt


> Thanks.
>
> On 09.01.2012 17:36, Matthew Knepley wrote:
>
> On Mon, Jan 9, 2012 at 10:31 AM, Alexander Grayver <
> agrayver at gfz-potsdam.de> wrote:
>
>> I have tested default GMRES+ILU(0) solver for problem of size ~200000 and
>> I observed that both convergence rate and accuracy degrade when I increase
>> number of processes.
>> E.g., number of iterations almost doubles when going from 1 to 4
>> processors.
>>
>> I noticed that ILU(0) is somehow represented by sequential matrices
>> internally and feel that this might be a reason?
>>
>
>  The default is Block Jacobi+ILU(0) and GMRES(30) which will definitely
> degrade as the number of
> processes is increased. You would need to use an optimal preconditioner
> like Multigrid if you want
> a constant number of iterates.
>
>     Matt
>
>
>> Regards,
>> Alexander
>>
>
>
>
>  --
> 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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