Dear Dr. Bangerth,
  I tried a few levels of the tolerance, 1e-3, 1e-4 ,1e-6 and 1e-8. 1e-3
converged because that is in the same level with the solution. Error occurs
below 1e-4.  I have tried the Jacobi preconditioner instead of SSOR in the
single thread CG version, it converged as well.
Best regards,
Yuesu


On Fri, Aug 14, 2020 at 10:14 PM Wolfgang Bangerth <bange...@colostate.edu>
wrote:

> On 8/14/20 5:53 PM, yuesu jin wrote:
> >
> > I also wrote a single thread CG version, which converges well. The
> > preconditioner in the parallel version is blockJacobi but the single
> thread
> > version I used is SSOR. Does it matter to the convergence? Thank you!
>
> Yes, in exactly the ways the error message shows: If you are using a
> different
> preconditioner, you should expect different convergence behavior.
>
> Whether you should expect one or the other method to work (or fail)
> depends on
> the properties of the matrix you are trying to solve. I'd also check
> whether
> the residual 0.00295038 is large or small compared to the initial
> residual.
> What is the tolerance you are using in the SolverControl object?
>
> Best
>   W.
>
> --
> ------------------------------------------------------------------------
> Wolfgang Bangerth          email:                 bange...@colostate.edu
>                             www: http://www.math.colostate.edu/~bangerth/
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-- 
Yuesu Jin,
Ph.D student,
University of Houston,
College of Natural Sciences and Mathematics,
Department of Earth and Atmospheric Sciences,
Houston, Texas 77204-5008
346-404-2062

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