# [Trilinos-Users] Belos relative residual question

Hoemmen, Mark mhoemme at sandia.gov
Wed Jan 30 13:11:50 MST 2013

```On Jan 30, 2013, at 12:00 PM, <trilinos-users-request at software.sandia.gov>
wrote:
> I've been using the iterative solvers in Belos (without preconditioning), and I have a question.  I'm trying to solve a linear system with a matrix from the Tim Davis collection (n~1.5 million, SPD, infinity norm~1e13, condition number unknown).  I get the following output:
>
> Belos::StatusTestGeneralOutput: Passed
>  (Num calls,Mod test,State test): (1001, 1, Passed)
>   Passed.......OR Combination ->
>     Failed.......Number of Iterations = 1000 == 1000
>     Unconverged..(2-Norm Imp Res Vec) / (2-Norm Res0)
>                  residual [ 0 ] = 3.529270e+01 > 1.000000e-09
>                  residual [ 1 ] = 3.315928e+01 > 1.000000e-09
>                  residual [ 2 ] = 3.379295e+01 > 1.000000e-09
>                  residual [ 3 ] = 3.545595e+01 > 1.000000e-09
>                  residual [ 4 ] = 3.886068e+01 > 1.000000e-09
>                  residual [ 5 ] = 3.623544e+01 > 1.000000e-09
>                  residual [ 6 ] = 3.766880e+01 > 1.000000e-09
>                  residual [ 7 ] = 3.573129e+01 > 1.000000e-09
>                  residual [ 8 ] = 3.265649e+01 > 1.000000e-09
>                  residual [ 9 ] = 3.407210e+01 > 1.000000e-09
>                  residual [ 10 ] = 3.412164e+01 > 1.000000e-09
>                  residual [ 11 ] = 3.268996e+01 > 1.000000e-09
>                  residual [ 12 ] = 3.239391e+01 > 1.000000e-09
>                  residual [ 13 ] = 3.618865e+01 > 1.000000e-09
>                  residual [ 14 ] = 3.389451e+01 > 1.000000e-09
>                  residual [ 15 ] = 3.418135e+01 > 1.000000e-09
>
> Any ideas as to why I might be getting a relative residual of 35?  As always, code is available on request.

Which iterative solver is it?  Also, could you let us know what solver options you are using?  Hopefully we can diagnose that.  It could be that you have restarting turned on; Belos' GMRES does restarting by default.  (1000 is the total number of iterations, but it's restarting every 40 iterations or so by default.)

I've been having some problems on my own end regarding your Ifpack question.  Those are not related to your code, which builds just fine.  I just need to fix something with my setup.  I haven't forgotten though, don't worry :-)

mfh
```