[petsc-dev] Using PCFieldSplitSetIS

Matthew Knepley knepley at gmail.com
Wed Mar 16 08:41:52 CDT 2011 

On Wed, Mar 16, 2011 at 8:27 AM, Thomas Witkowski <
thomas.witkowski at tu-dresden.de> wrote:

> Jed Brown wrote:
>
>  On Wed, Mar 16, 2011 at 07:37, Thomas Witkowski <
>> thomas.witkowski at tu-dresden.de <mailto:thomas.witkowski at tu-dresden.de>>
>> wrote:
>>
>>    Thanks for explanations! It works fine in my code. But I have two
>>    questions about it, maybe you can help me with them:
>>    - To the first, is the LU factorization on block A_00 done only once?
>>
>>
>> Yes, one way to confirm this is to run with -log_summary and check
>> MatLUFactor{Sym,Num}.
>>
>>    - I have run the code with -fieldsplit_1_ksp_monitor to get some
>>    more information about the internal solves. I expected to get
>>    information about one iterative solver (for solving the Schur
>>    complement system), but I got three, all of them need around 20
>>    iterations for my example. Could you explain to me what is
>>    actually solved there?
>>
>>
>> I'm going to guess that you still have an outer KSP that (in the global
>> norm, rather than the partitioned norm used inside of splits) has a tighter
>> tolerance, therefore it takes a few outer iterations. If you use loose inner
>> tolerances then the preconditioner becomes nonlinear and you'll need to use
>> FGMRES for the outer. When in doubt, run with -ksp_view and show us the
>> results if you don't understand. Additionally, monitoring inner solves
>> separately can be useful, e.g. -fieldsplit_0_ksp_converged_reason
>> -fieldsplit_1_ksp_monitor -ksp_monitor_true_residual.
>>
> Okay, I run my code with the options "-pc_fieldsplit_type schur
> -fieldsplit_interior_ksp_type preonly -fieldsplit_interior_pc_type bjacobi
> -fieldsplit_interior_sub_pc_type lu -fieldsplit_boundary_ksp_monitor
> -ksp_monitor_true_residual -fieldsplit_interior_ksp_converged_reason". The
> splits are named "interior" and "boundary". The ksp output is as follows:
>
>   Linear solve converged due to CONVERGED_ITS iterations 1
>   Residual norms for fieldsplit_boundary_ solve.
>   0 KSP Residual norm 1.790059331071e-04
>   Linear solve converged due to CONVERGED_ITS iterations 1
>   1 KSP Residual norm 1.237356212928e-04
>   Linear solve converged due to CONVERGED_ITS iterations 1
>   2 KSP Residual norm 7.952220245101e-05
>   Linear solve converged due to CONVERGED_ITS iterations 1
>   3 KSP Residual norm 3.502858665398e-05
>   Linear solve converged due to CONVERGED_ITS iterations 1
>   4 KSP Residual norm 1.601085190129e-05
>   Linear solve converged due to CONVERGED_ITS iterations 1
>   5 KSP Residual norm 8.491161243057e-06
>   Linear solve converged due to CONVERGED_ITS iterations 1
>   6 KSP Residual norm 4.778227395989e-06
>   Linear solve converged due to CONVERGED_ITS iterations 1
>   7 KSP Residual norm 2.571919352633e-06
>   Linear solve converged due to CONVERGED_ITS iterations 1
>   8 KSP Residual norm 1.353979940120e-06
>   Linear solve converged due to CONVERGED_ITS iterations 1
>   9 KSP Residual norm 7.408377337496e-07
>   Linear solve converged due to CONVERGED_ITS iterations 1
>  10 KSP Residual norm 3.810694345834e-07
>   Linear solve converged due to CONVERGED_ITS iterations 1
>  11 KSP Residual norm 1.955889542056e-07
>   Linear solve converged due to CONVERGED_ITS iterations 1
>  12 KSP Residual norm 9.938906113920e-08
>   Linear solve converged due to CONVERGED_ITS iterations 1
>  13 KSP Residual norm 4.893445675773e-08
>   Linear solve converged due to CONVERGED_ITS iterations 1
>  14 KSP Residual norm 2.395128458062e-08
>   Linear solve converged due to CONVERGED_ITS iterations 1
>  15 KSP Residual norm <1.2464425896>1.246442589657e-08
>   Linear solve converged due to CONVERGED_ITS iterations 1
>  16 KSP Residual norm 6.316454504022e-09
>   Linear solve converged due to CONVERGED_ITS iterations 1
>  17 KSP Residual norm 3.088668836241e-09
>   Linear solve converged due to CONVERGED_ITS iterations 1
>  18 KSP Residual norm 1.593454221996e-09
>   Linear solve converged due to CONVERGED_ITS iterations 1
> [0]  Petsc-Iteration 0: 0.574504
>  0 KSP preconditioned resid norm 5.745043818120e-01 true resid norm
> 1.584249437360e-01 ||r(i)||/||b|| 1.000000000000e+00
>   Linear solve converged due to CONVERGED_ITS iterations 1
>   Residual norms for fieldsplit_boundary_ solve.
>   0 KSP Residual norm 1.790059331000e-04
>   Linear solve converged due to CONVERGED_ITS iterations 1
>   1 KSP Residual norm 1.237356237744e-04
>   Linear solve converged due to CONVERGED_ITS iterations 1
>   2 KSP Residual norm 7.952220078415e-05
>   Linear solve converged due to CONVERGED_ITS iterations 1
>   3 KSP Residual norm 3.502859285561e-05
>   Linear solve converged due to CONVERGED_ITS iterations 1
>   4 KSP Residual norm 1.601086485810e-05
>   Linear solve converged due to CONVERGED_ITS iterations 1
>   5 KSP Residual norm 8.491169384185e-06
>   Linear solve converged due to CONVERGED_ITS iterations 1
>   6 KSP Residual norm 4.778229157872e-06
>   Linear solve converged due to CONVERGED_ITS iterations 1
>   7 KSP Residual norm 2.571917112191e-06
>   Linear solve converged due to CONVERGED_ITS iterations 1
>   8 KSP Residual norm 1.353977280183e-06
>   Linear solve converged due to CONVERGED_ITS iterations 1
>   9 KSP Residual norm 7.408352317360e-07
>   Linear solve converged due to CONVERGED_ITS iterations 1
>  10 KSP Residual norm 3.810710583336e-07
>   Linear solve converged due to CONVERGED_ITS iterations 1
>  11 KSP Residual norm 1.955898169394e-07
>   Linear solve converged due to CONVERGED_ITS iterations 1
>  12 KSP Residual norm 9.938892788465e-08
>   Linear solve converged due to CONVERGED_ITS iterations 1
>  13 KSP Residual norm 4.893428546840e-08
>   Linear solve converged due to CONVERGED_ITS iterations 1
>  14 KSP Residual norm 2.395120857135e-08
>   Linear solve converged due to CONVERGED_ITS iterations 1
>  15 KSP Residual norm <1.2464367927>1.246436792784e-08
>   Linear solve converged due to CONVERGED_ITS iterations 1
>  16 KSP Residual norm 6.316409779118e-09
>   Linear solve converged due to CONVERGED_ITS iterations 1
>  17 KSP Residual norm 3.088649355374e-09
>   Linear solve converged due to CONVERGED_ITS iterations 1
>  18 KSP Residual norm 1.593449602302e-09
>   Linear solve converged due to CONVERGED_ITS iterations 1
>   Linear solve converged due to CONVERGED_ITS iterations 1
>   Residual norms for fieldsplit_boundary_ solve.
>   0 KSP Residual norm 1.593449603155e-09
>   Linear solve converged due to CONVERGED_ITS iterations 1
>   1 KSP Residual norm 1.011857676662e-09
>   Linear solve converged due to CONVERGED_ITS iterations 1
>   2 KSP Residual norm 5.660892669788e-10
>   Linear solve converged due to CONVERGED_ITS iterations 1
>   3 KSP Residual norm 3.046799343903e-10
>   Linear solve converged due to CONVERGED_ITS iterations 1
>   4 KSP Residual norm <1.6004713993>1.600471399329e-10
>   Linear solve converged due to CONVERGED_ITS iterations 1
>   5 KSP Residual norm 8.083216601016e-11
>   Linear solve converged due to CONVERGED_ITS iterations 1
>   6 KSP Residual norm 4.308671648345e-11
>   Linear solve converged due to CONVERGED_ITS iterations 1
>   7 KSP Residual norm 2.381959817139e-11
>   Linear solve converged due to CONVERGED_ITS iterations 1
>   8 KSP Residual norm 1.264839790114e-11
>   Linear solve converged due to CONVERGED_ITS iterations 1
>   9 KSP Residual norm 6.435098853282e-12
>   Linear solve converged due to CONVERGED_ITS iterations 1
>  10 KSP Residual norm 3.184769579797e-12
>   Linear solve converged due to CONVERGED_ITS iterations 1
>  11 KSP Residual norm <1.5022019430>1.502201943016e-12
>   Linear solve converged due to CONVERGED_ITS iterations 1
>  12 KSP Residual norm 7.099486208425e-13
>   Linear solve converged due to CONVERGED_ITS iterations 1
>  13 KSP Residual norm 3.443222176388e-13
>   Linear solve converged due to CONVERGED_ITS iterations 1
>  14 KSP Residual norm <1.6478434989>1.647843498912e-13
>   Linear solve converged due to CONVERGED_ITS iterations 1
>  15 KSP Residual norm 8.208508173926e-14
>   Linear solve converged due to CONVERGED_ITS iterations 1
>  16 KSP Residual norm 4.376604355287e-14
>   Linear solve converged due to CONVERGED_ITS iterations 1
>  17 KSP Residual norm 2.596069152755e-14
>   Linear solve converged due to CONVERGED_ITS iterations 1
>  18 KSP Residual norm 1.335744434252e-14
>   Linear solve converged due to CONVERGED_ITS iterations 1
>  1 KSP preconditioned resid norm 2.608069232839e-13 true resid norm
> 1.449869994655e-13 ||r(i)||/||b|| 9.151778504469e-13
>
> I have no idea how to interpret this output! Could you help me with it?


Get rid of -fieldsplit_interior_ksp_converged_reason. It is confusing
things. Then you will see
the outer iteration and the inner boundary iteration which is indented.

   Matt


>
> Thomas
>



-- 
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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