[petsc-users] Issue of mg_coarse_ksp not converge

[Matthew Knepley]

On Mon, Oct 2, 2017 at 8:30 AM, Wenbo Zhao <zhaowenbo.npic at gmail.com> wrote:

> Matt
>
> Because I am not clear about what will happen using 'preonly' for large
> scale problem.
>

The size of the problem has nothing to do with 'preonly'. All it means is
to apply a preconditioner without a Krylov solver.


> It seems to use a direct solver from below,
> http://www.mcs.anl.gov/petsc/petsc-current/docs/
> manualpages/KSP/KSPPREONLY.html
>

However, I still cannot understand why you would change the default?

  Matt


>
> Thanks!
> Wenbo
>
> On Mon, Oct 2, 2017 at 5:09 PM, Matthew Knepley <knepley at gmail.com> wrote:
>
>> On Sun, Oct 1, 2017 at 9:53 PM, Wenbo Zhao <zhaowenbo.npic at gmail.com>
>> wrote:
>>
>>> Matt,
>>> Thanks for your reply.
>>> It DOES make no sense for this problem.
>>> But I am not clear about the 'preonly' option. Which solver is used in
>>> preonly? I wonder if 'preonly' is suitable for large scale problem such as
>>> 400,000,000 unknowns.
>>> So I tried 'gmres' option and found these error messages.
>>>
>>
>> I mean, why are you setting this at all. Just do not set the coarse
>> solver. The default should work fine.
>>
>>   Thanks,
>>
>>     Matt
>>
>>
>>> Could you give me some suggestions?
>>>
>>> Thanks.
>>>
>>> Wenbo
>>>
>>>
>>> On Mon, Oct 2, 2017 at 12:34 AM, Matthew Knepley <knepley at gmail.com>
>>> wrote:
>>>
>>>> On Sun, Oct 1, 2017 at 6:49 AM, Wenbo Zhao <zhaowenbo.npic at gmail.com>
>>>> wrote:
>>>>
>>>>> Hi,
>>>>>
>>>>> I met some questions when I use PETSC/SLEPC to solve two-group neutron
>>>>> diffusion equations with finite difference method. The grid is 3*3*3, when
>>>>> DOF on each points is 2. So the matrix size is 54*54.
>>>>> It is generalized eigenvalue problem Ax=\lamda Bx, where B is
>>>>> diagonally dominant matrix but not symmetry.
>>>>> EPS is set as below,
>>>>>  ierr = EPSSetProblemType(eps,EPS_GNHEP);CHKERRQ(ierr);¬
>>>>>  ierr = EPSSetWhichEigenpairs(eps,EPS_LARGEST_REAL);CHKERRQ(ierr);¬
>>>>>
>>>>> Krylovschur is used as eps sovler. GAMG is used as PC.
>>>>> I tried agg_nsmooths and mg_coarse_ksp_type. Only non-smooths and
>>>>> preonly is OK.
>>>>>
>>>>
>>>> Why are you setting the coarse solver. This makes no sense.
>>>>
>>>>    Thanks,
>>>>
>>>>     Matt
>>>>
>>>>
>>>>>
>>>>> Test 1
>>>>> $ make NCORE=1 runkr_nonsmooth
>>>>> mpirun -n 1 ./step-41 \
>>>>>    -st_ksp_type gmres  \
>>>>>    -st_pc_type gamg -st_pc_gamg_type agg -st_pc_gamg_agg_nsmooths 0 \
>>>>>    -st_ksp_view  -mata AMAT.dat -matb BMAT.dat \
>>>>>    -st_mg_coarse_ksp_type preonly   -st_mg_coarse_ksp_monitor  \
>>>>>    -eps_nev 1 -eps_ncv 10  -eps_monitor  -log_view > log_nonsmooth 2>&1
>>>>>
>>>>> Test 2
>>>>> $ make NCORE=1 runkr_smooth
>>>>> mpirun -n 1 ./step-41 \
>>>>>    -st_ksp_type gmres  \
>>>>>    -st_pc_type gamg -st_pc_gamg_type agg -st_pc_gamg_agg_nsmooths 1 \
>>>>>    -st_ksp_view  -mata AMAT.dat -matb BMAT.dat \
>>>>>    -st_mg_coarse_ksp_type preonly   -st_mg_coarse_ksp_monitor  \
>>>>>    -eps_nev 1 -eps_ncv 10  -eps_monitor -log_view > log_smooth 2>&1
>>>>> makefile:43: recipe for target 'runkr_smooth' failed
>>>>> make: *** [runkr_smooth] Error 91
>>>>>
>>>>> Test 3
>>>>> $ make NCORE=1 runkr_gmres
>>>>> mpirun -n 1 ./step-41 \
>>>>>    -st_ksp_type gmres  \
>>>>>    -st_pc_type gamg -st_pc_gamg_type agg -st_pc_gamg_agg_nsmooths 0 \
>>>>>    -st_ksp_view  -mata AMAT.dat -matb BMAT.dat \
>>>>>    -st_mg_coarse_ksp_type gmres  -st_mg_coarse_ksp_monitor
>>>>> -st_mg_coarse_ksp_rtol 1.0e-6 \
>>>>>    -eps_nev 1 -eps_ncv 10  -eps_monitor  -log_view > log_gmres 2>&1
>>>>> makefile:59: recipe for target 'runkr_gmres' failed
>>>>> make: *** [runkr_gmres] Error 91
>>>>>
>>>>> Log files were attched.
>>>>> The matrix file were also attched as AMAT.dat and BMAT.dat.
>>>>>
>>>>> Is it correct? Or something wrong with my code or commad-line?
>>>>>
>>>>> Thanks!
>>>>>
>>>>> Wenbo
>>>>>
>>>>
>>>>
>>>>
>>>> --
>>>> 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
>>>>
>>>> https://www.cse.buffalo.edu/~knepley/ <http://www.caam.rice.edu/~mk51/>
>>>>
>>>
>>>
>>
>>
>> --
>> 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
>>
>> https://www.cse.buffalo.edu/~knepley/ <http://www.caam.rice.edu/~mk51/>
>>
>
>


-- 
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

https://www.cse.buffalo.edu/~knepley/ <http://www.caam.rice.edu/~mk51/>
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