[petsc-users] Query regarding convergence of KSP solver

[Rachit Prasad]

Hello,

I am working to solve a linear system of equations using a KSP solver. The
size of the matrix is approximately ~370,000. The following settings have
been used to set up the solver"

[image: Inline image 1]

The L2 norm of the RHS for this problem is ~365, which has been calculated
using the subroutine *VecNorm*. When I solve this system, I am finding that
irrespective of the number of maximum iterations the L2 norm of final
residual is pretty much constant around ~37860. The L2 norm of the residual
has been found using the subroutine *KSPGetResidualNorm. *The following is
how the residual is varying for different maximum iterations.
*[image: Inline image 2]*

With such being the case, I have the following doubts to clarify:

   - Are the settings used by me for the KSP solver adequate enough? Or am
   I missing something?
   - What could be reasons for my KSP solver to not converge below the
   permissible tolerance?
   - For a linear system of given size, how can one determine the number of
   maximum iterations that should be set for the KSP solver? Is there a thumb
   rule?


Regards,
Rachit Prasad
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.mcs.anl.gov/pipermail/petsc-users/attachments/20150908/fc6618ce/attachment-0001.html>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: KSP residual convergence.JPG
Type: image/jpeg
Size: 30079 bytes
Desc: not available
URL: <http://lists.mcs.anl.gov/pipermail/petsc-users/attachments/20150908/fc6618ce/attachment-0002.jpe>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: KSP settings.JPG
Type: image/jpeg
Size: 34260 bytes
Desc: not available
URL: <http://lists.mcs.anl.gov/pipermail/petsc-users/attachments/20150908/fc6618ce/attachment-0003.jpe>