[petsc-users] Question PETSc

[Mark F. Adams]

On Feb 19, 2013, at 12:53 PM, Marcelo Xavier Guterres <m.guterres at gmail.com> wrote:

> Esteemed colleague,
> 
> thanks for the previous answer.  But still can not understand one aspect.
> 
> 
> The GMRES behaves well up to a point, but then the quality of the final solution is bad, compared to analytical solution.
> 
> 
> It is a problem in the method or truncation error?

No, it is an approximate solver and it gets worse as the problem size increases.  CG gets worse also.  CG is just better for this problem.

> 
> 
> m	n	m x n	error ( KSPCG x PCJACOBI )	error ( KSPGMRS x PCJACOBI )
> 3	3	9	1,92E−16	4,42E−16	
> 
> 4	4	16	2,08E−16	6,46E−16	
> 
> 5	5	25	4,41E−16	9,63E−16	
> 
> 6	6	36	8,77E−16	8,26E−16	
> 
> 7	7	49	2,37E−06	2,52E−06	
> 
> 8	8	64	1,17E−05	1,33E−05	
> 
> 9	9	81	9,32E−06	1,26E−05	
> 
> 10	10	100	7,33E−06	9,93E−06	
> 
> 20	20	400	4,22E−05	3,16E−04	
> 
> 30	30	900	1,06E−04	2,37E−02	
> 
> 40	40	1600	2,14E−04	3,77E−02	
> 
> 50	50	2500	3,37E−04	9,36E−03	
> 
> 100	100	10000	1,27E−03	6,53E−02	
> 
> 200	200	40000	4,32E−03	3,67E−01	
> 
> 300	300	90000	9,53E−02	1,02E+00	
> 
> 400	400	160000	1,68E−02	2,10E+00	
> 
> 500	500	250000	2,61E−02	3,66E+00	
> 
> 
> 
> 
> -- 
> Saudações Pampeanas
> -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
> 
> Phd. student Marcelo

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