[petsc-users] Termination criteria for solving linear system

Markus Bürg buerg at kit.edu
Tue Feb 2 07:01:16 CST 2010

Hello Thomas,

I do not think that you can say this a priori, because you cannot 
predict how much "gain" the iteration step will produce. You can try to 
reduce the solver precision further, but then it can happen that the 
precision of your solution explodes.

Best Regards,

Am 02.02.10 13:41, schrieb Thomas Witkowski:
> Hallo Markus,
> I just want to reduce the precision of the solution of the system of 
> linear equations! To do this it is not required (and in my simulations 
> also not possible) to reduce the number of degrees of freedom. It is a 
> question of how to define the solver parameters to cancel the solution 
> process at the right moment.
> Regards,
> Thomas
> Markus Bürg wrote:
>> Hello Thomas,
>> the precision of the solver does not correspond to the precision of 
>> the solution for your problem. If you do not want such a small 
>> residual in your problem, you have to reduce the number of degrees of 
>> freedom.
>> Best Regards,
>> Markus
>> Am 02.02.10 13:05, schrieb Thomas Witkowski:
>>> In my computations I use "bicg" preconditioned by "bjacobi" with an 
>>> absolute tolerance of 1^e-8 as termination criteria. After PETSc has 
>>> computed the solution, the code computes the residual. It is usually 
>>> in the magnitude of 1^-13. I'm not really sure what happens here. Is 
>>> it, because PETSc checks the preconditioned residual? If so, is 
>>> there a way to tune the termination criteria to the true residual? I 
>>> would like to save some computational time, since I do not need to 
>>> solve the system with this high precision.
>>> Regards,
>>> Thomas

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