[petsc-users] which is the best PC for GMRES Poisson solver?

[Matthew Knepley]

On Mon, Aug 26, 2013 at 2:33 AM, Zhang <zyzhang at nuaa.edu.cn> wrote:

> Hi, Matt
> BTW,
>
> Could you leave me any suggestion of those direct solver PC, such as
> superLU_dist? Thanks


For simple Poisson, I do  not expect direct methods to be competitive with
MG, however you can
easily try by configuring with --download-superlu and -pc_type lu
-pc_factor_mat_solver_package superlu

   Matt


> Zhenyu
>
>
> -----原始邮件-----
> *发件人:* "Matthew Knepley" <knepley at gmail.com>
> *发送时间:* 2013-08-24 18:20:00 (星期六)
> *收件人:* Zhang <zyzhang at nuaa.edu.cn>
> *抄送:* petsc-users at mcs.anl.gov
> *主题:* Re: [petsc-users] which is the best PC for GMRES Poisson solver?
>
> On Sat, Aug 24, 2013 at 12:07 AM, Zhang <zyzhang at nuaa.edu.cn> wrote:
>
>> Hi,
>>
>> Recently  I wrote a code of projection method for solving the
>> incompressile flow. It includes a poisson solution of pressure.
>> I transfered the code to be based on Petsc .
>>
>
> Use -pc_type gamg
>
>   Matt
>
>
>> However, in the case of 3D lid-driven flow case, the speed of petsc
>> version is not advantageous yet.
>>
>> I tried different combination of preconditioner with  GMRES solver. Among
>> them GMRES+PCILU or GMRES+SOR are both the fastest.
>> For a grid 80x80x80, GMRES+SOR serial version used 185.816307 secs.
>> However, for case 120x120x120, it diverged. So is GMRES+PCILU.
>>
>> Then  I tried a parallel comparison, as follows,
>>
>>
>>
>> #############################################################################################
>> # with Petsc-3.4.2, time comparison (sec)
>> # size (80,80,80), 200 steps, dt=0.002
>> #debug version  177.695939 sec
>> #opt   version  106.694733 sec
>> #mpirun -np 8 ./nsproj  ../input/sample_80.zzy.dat -ksp_type gmres -p
>> c_type hypre -ksp_rtol 1.e-2
>> #debug version  514.718544 sec
>> #opt   version  331.114555 sec
>> #mpirun -np 12 ./nsproj  ../input/sample_80.zzy.dat -ksp_type gmres
>> -pc_type hypre -ksp_rtol 1.e-2
>> #debug version  796.765428 sec
>> #opt   version  686.151788 sec
>> #mpirun -np 16 ./nsproj  ../input/sample_80.zzy.dat -ksp_type gmres
>> -pc_type hypre -ksp_rtol 1.e-2
>>
>> I do know sometimes problem with speed is not due to petsc, but my own
>> code, so please you are welcome for any suggestion
>> about which combination is the best for such a computation. I know
>> solving Poiison and Helmholtz is so common to see in numerical work.
>> Thank you first.
>>
>> BTW, I also tried to use superLU_dist as PC.
>> #mpirun -np 16 ./nsproj  ../input/sample_20.zzy.dat -ksp_type gmres
>> -pc_type lu -pc_factor_mat_solver_package superlu_dist -ksp_rtol 1.e-2
>>
>> But with 16 nodes, except case of 20x20x20 grids, all larger grids run e
>> xtremly slow.
>>  Since I never use the direct PC before, is it true that a good usage of
>> direct LU as Preconditioner
>> requires that the amount of procedures be much larger so that for each
>> node the calculation of direct solver is smalled enough to use it?
>>
>> Cheers,
>>
>> Zhenyu
>>
>
>
>
> --
> 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
>
>


-- 
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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