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eh4

Beginner

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11-13-2011
11:58 PM

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Constant residual in GMRES

I am trying to solve a sparse system of linear equations using GMRES (7600 x 7600). Ihave not done any preconditioning yet. The problem I am facing is that after a certain number of iterations, the residual between two consecutive steps does not change. Well, it does change but very very slowly. I am talking about 1d-12 or smaller per iteration.

The system of equations solved nicely with PARDISO. As I need to solve a large system later on, I really need to get GMRES to work. Does anyone know any remedy to this problem? Another thing I should mention is that the iterative solver does not stop even when the maximum number of iterations specified has been exceeded. It just kept on iterating. Why is this so?

One final thing, does the arrangement of the system of equations affect the convergence of the solver? I have tried arranging it both ways and both of them gave the same outcome.

Thank you.

Best wishes,

EH

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mecej4

Black Belt

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11-14-2011
04:56 AM

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If you are able to post a small example code that displays the problematic behavior that you described, that code would facilitate identifying the sources of the problems.

The first suspect in such cases, where a complex library is used, is errors in the subroutine argument types/values and proper sequencing of calls.

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