[petsc-users] Termination criteria for solving linear system

Thomas Witkowski thomas.witkowski at tu-dresden.de
Tue Feb 2 06:41:23 CST 2010

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.



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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