[petsc-users] Questions about PCMG

Wed Apr 4 13:24:28 CDT 2012

I would expect 4 calls to MatLUFactorSym here.  It looks like the coarse grid is not getting refactored in the second SNES solve.

Are you using Galerkin coarse grids?  Perhaps you are not setting a new coarse grid with KSPSetOperator and so MG does not bother refactoring it.

Mark

On Apr 4, 2012, at 1:53 PM, Yuqi Wu wrote:

> Thank you.
> 
> Can I ask another question? 
> 
> In my log summary output, it shows that although there are two SNES iteration and total 9 linear iterations. The functions MatLUFactorSym and MatLUFactorNum are only called for three times. 
> 
> MatLUFactorSym         3 1.0 1.4073e+00 1.0 0.00e+00 0.0 0.0e+00 0.0e+00 1.5e+01  1  0  0  0  2   1  0  0  0  2     0
> MatLUFactorNum         3 1.0 3.2754e+01 1.0 9.16e+09 1.0 0.0e+00 0.0e+00 0.0e+00 31 97  0  0  0  32 97  0  0  0   280
> 
> I checked the -info output. It shows that One MatLUFactorSymbolic_SeqAIJ() is called in down smoother of the first SNES, one MatLUFactorSymbolic_SeqAIJ() is called in the coarse solve of the first SNES, and one MatLUFactorSymbolic_SeqAIJ() is called in the down smoother of the second SNES.
> 
> Do you have any ideas why there are 9 multigrid iterations, but only 3 MatLUFactorSymbolic calls in the program?
> 
> Best
> 
> Yuqi
> 
> 
> 
> 
> ---- Original message ----
>> Date: Tue, 3 Apr 2012 20:08:27 -0500
>> From: petsc-users-bounces at mcs.anl.gov (on behalf of Barry Smith <bsmith at mcs.anl.gov>)
>> Subject: Re: [petsc-users] Questions about PCMG  
>> To: PETSc users list <petsc-users at mcs.anl.gov>
>> 
>> 
>>  There are two linear solves (for 1 SNES and 2 SNES) so there are two MGSetUp on each level. Then a total of 9 multigrid iterations (in both linear solves together) hence 9 smoother on level 0 (level 0 means coarse grid solve). One smooth down and one smooth up on level 1 hence 18 total smooths on level 1.  9 computation of residual on level 1 and 18 MgInterp because that logs both the restriction to level 0 and the interpolation back to level 1 and 18 = 9 + 9.
>> 
>>  Barry
>> 
>> On Apr 3, 2012, at 7:57 PM, Yuqi Wu wrote:
>> 
>>> Hi, Barry,
>>> 
>>> Thank you. If my program converges in two SNES iteration,
>>> 0 SNES norm 1.014991e+02, 0 KSP its (nan coarse its average), last norm 0.000000e+00
>>> 1 SNES norm 9.925218e-05, 4 KSP its (5.25 coarse its average), last norm 2.268574e-06.
>>> 2 SNES norm 1.397282e-09, 5 KSP its (5.20 coarse its average), last norm 1.312605e-12.
>>> 
>>> And -pc_mg_log shows the following output
>>> 
>>> MGSetup Level 0        2 1.0 3.4091e-01 2.1 0.00e+00 0.0 3.0e+02 6.0e+04 3.0e+01  1  0  3 11  2   1  0  3 11  2     0
>>> MGSmooth Level 0       9 1.0 1.2126e+01 1.0 9.38e+08 3.2 2.8e+03 1.7e+03 6.4e+02 33 71 28  3 34  35 71 28  3 35   415
>>> MGSetup Level 1        2 1.0 1.3925e-01 2.1 0.00e+00 0.0 1.5e+02 3.1e+04 2.3e+01  0  0  1  3  1   0  0  1  3  1     0
>>> MGSmooth Level 1      18 1.0 5.8493e+00 1.0 3.66e+08 3.1 1.5e+03 2.9e+03 3.6e+02 16 28 15  3 19  17 28 15  3 19   339
>>> MGResid Level 1        9 1.0 1.1826e-01 1.4 1.49e+06 2.4 2.0e+02 2.7e+03 9.0e+00  0  0  2  0  0   0  0  2  0  0    70
>>> MGInterp Level 1      18 1.0 1.2317e-01 1.3 7.74e+05 2.2 3.8e+02 1.1e+03 1.8e+01  0  0  4  0  1   0  0  4  0  1    37
>>> 
>>> What are the MGSmooth, MGResid, MGInterp represent for?
>>> 
>>> Best
>>> 
>>> Yuqi
>>> 
>>> ---- Original message ----
>>>> Date: Tue, 3 Apr 2012 19:19:23 -0500
>>>> From: petsc-users-bounces at mcs.anl.gov (on behalf of Barry Smith <bsmith at mcs.anl.gov>)
>>>> Subject: Re: [petsc-users] Questions about PCMG  
>>>> To: PETSc users list <petsc-users at mcs.anl.gov>
>>>> 
>>>> 
>>>> -pc_mg_log doesn't have anything to do with DA or DMMG it is part of the basic PCMG. Are you sure you are calling SNESSetFromOptions()?
>>>> 
>>>> Barry
>>>> 
>>>> On Apr 3, 2012, at 6:56 PM, Yuqi Wu wrote:
>>>> 
>>>>> Hi, Mark,
>>>>> 
>>>>> Thank you so much for your suggestion.
>>>>> 
>>>>> The problem 1 is resolved by avoiding calling PCMGSetNumberSmoothUp. 
>>>>> 
>>>>> But since I am using the unstructured grid in my application, I didn't use DA or dmmg, so -pc_mg_log didn't give any level information. I try to run my code using -info with 1 processor, and I find out some interesting issues.
>>>> 
>> 
> 