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

[Matthew Knepley]

On Sat, Jul 21, 2012 at 4:30 AM, Bao Kai <paeanball at gmail.com> 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.
>

Try ML to see if it has better defaults for your problem. If not, you will
have to start experimenting with the solver
parameters.


> 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?
>

No.

    Matt


> 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
>> >
>>
>>
>>


-- 
What most experimenters take for granted before they begin their
experiments is infinitely more interesting than any results to which their
experiments lead.
-- Norbert Wiener
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