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

[Alexander Grayver]

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

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 <mailto: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

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