[petsc-users] How to investigate the reason for slow convergence rate?

[Barry Smith]

http://www.mcs.anl.gov/petsc/documentation/faq.html#kspdiverged


On Jul 21, 2012, at 4:30 AM, Bao Kai wrote:

> 
> HI, all, 
> 
> I am still suffering from the slow convergence rate of the KSP solution. 
> 
> I changed the code to use Petsc3.3 and then try the gamg precoditioner, the convergence rate is better, while it took more total time because it took much more time for each iteration and some extra time for pre-processing. 
> 
> I am wondering if there are some ways that can help me to investigate the slow convergence rate for KSP solution so that I can do some improvement.  Is DMMG will be a good solution?
> 
> Thank you very much. 
> 
> Best Regards, 
> Kai  
> 
> Message: 2
> Date: Wed, 11 Jul 2012 15:17:15 -0500
> From: Matthew Knepley <knepley at gmail.com>
> To: PETSc users list <petsc-users at mcs.anl.gov>
> Subject: Re: [petsc-users] Does this mean the matrix is
>         ill-conditioned?
> Message-ID:
>         <CAMYG4Gk7T=q+w1PKO7G_TW07iDzux90Sncbv9K7d0FD-MDrLRg at mail.gmail.com>
> Content-Type: text/plain; charset="iso-8859-1"
> 
> On Wed, Jul 11, 2012 at 12:40 PM, Bao Kai <paeanball at gmail.com> wrote:
> 
> > Hi, all,
> >
> > The following is the output from the solution of a Poisson equation
> > from Darcy's law.
> >
> > To compute the condition number of matrix, I did not use PC and use
> > GMRES KSP to do the test.
> >
> > It seems like that the condition number keep increasing during the
> > iterative solution. Does this mean the matrix is ill-conditioned?
> >
> 
> Generally yes. Krylov methods take a long time to resolve the smallest
> eigenvalues, so this approximation is not great.
> 
> 
> > For this test, it did not achieve convergence with 10000 iterations.
> >
> > When I use BJOCABI PC and BICGSTAB KSP, it generally takes about 600
> > times iteration to get the iteration convergent.
> >
> > Any suggestion for improving the convergence rate will be much
> > appreciated.  The solution of this equation has been the bottleneck of
> > my code, it takes more than 90% of the total time.
> >
> 
> Try ML or GAMG.
> 
>     Matt
> 
> 
> > Thank you very much.
> >
> > Best Regards,
> > Kai
> >
> 
>