[petsc-users] Questions about PCMG

Wed Apr 4 13:48:14 CDT 2012

   Use the PCMG.

On Apr 4, 2012, at 1:40 PM, Yuqi Wu wrote:

> Can I also ask another questions related to the twolevel preconditioner? I have also try to use PCComposite to create two level ASM preconditioner instead of using PCMG. Since I am not able to use DA, which way is better to use in create a two level preconditioner?
> 
> If I use the PCMG approach, it seems that the LU factorization in the up smoother can be avoided, only the LU factorization in the down smoother are need to be computed. But if I have the following precondition created with PCComposite
> 
> ierr = PCSetType(finepc,PCCOMPOSITE);CHKERRQ(ierr);
> /* smooth down preconditioner */
> ierr = PCCompositeAddPC(finepc,PCASM);CHKERRQ(ierr);
> /* coarse solve */
> ierr = PCCompositeAddPC(finepc,PCSHELL);CHKERRQ(ierr);
> /* smooth up preconditioner */
> ierr = PCCompositeAddPC(finepc,PCASM);CHKERRQ(ierr);
> 
> How can I save the LU factorization in the smooth up stage, if I want to use the same precondition in smooth up and smooth down?
> 
> Thank you.
> 
> 
> Yuqi
> 
> ---- Original message ----
>> Date: Wed, 4 Apr 2012 13:15:45 -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>
>> 
>> 
>> On Apr 4, 2012, at 12:53 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?
>> 
>>  By default PETSc's multigrid uses ILU smoothing. You can change to SOR with -mg_levels_pc_type sor
>> 
>>  The factorization only needs to done once then triangular solves (MatSolve) are done for each smoothing step. Since there are only two different matrices for the fine level (from the first and second SNES iteration) there only need to be two factorizations. The smoother is run 9 times because there are 9 multigrid iterations.
>> 
>>  Barry
>> 
>>> 
>>> 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.
>>>>>> 
>>>> 
>> 