[petsc-dev] Right-preconditioned GMRES

[Matthew Knepley]

On Wed, Nov 6, 2019 at 1:32 PM Pierre Jolivet via petsc-dev <
petsc-dev at mcs.anl.gov> wrote:

> On 6 Nov 2019, at 7:30 PM, Fande Kong <fdkong.jd at gmail.com> wrote:
>
> Even if both the linear operator and the preconditioner  operator are
> symmetric, the preconditioned operator by RAS will  NOT be symmetric so
> that CG is not applicable.
>
> RAS keeps overlapping elements during restriction but discards them when
> doing interpolation, which leads to an unsymmetric preconditioned operator.
>
>
> Of course.
> What’s your point?
>

I guess Fande's point is that we should consider switching to CG or MINRES,
since we are being careful about the PC.

  Thanks,

     Matt


> Thanks,
> Pierre
>
> Fande,
>
>
>
> On Wed, Nov 6, 2019 at 11:13 AM Pierre Jolivet via petsc-dev <
> petsc-dev at mcs.anl.gov> wrote:
>
>> I need to figure this out myself first.
>> In the meantime, here is another (PCASM related) question for you: why is
>> PCASMType switched to BASIC when using a symmetric Pmat? (I guess I don’t
>> have to tell you about the performance of RAS vs. ASM)
>> To me, that would make sense if the default KSP was also switched to CG
>> instead of GMRES if the Pmat and Amat were symmetric, but I don’t think
>> this is the case.
>>
>> Thanks,
>> Pierre
>>
>> > On 24 Oct 2019, at 5:40 PM, Smith, Barry F. <bsmith at mcs.anl.gov> wrote:
>> >
>> >
>> >  Send the code and exact instructions to run a "good" and a "bad" ASM
>> >
>> >   Barry
>> >
>> >
>> >> On Oct 14, 2019, at 10:44 AM, Pierre Jolivet <
>> pierre.jolivet at enseeiht.fr> wrote:
>> >>
>> >> Here are the three logs.
>> >> FGMRES also gives a wrong first iterate.
>> >> I think Mark was right in the sense that the problem is _most likely_
>> in my RHS.
>> >> But I need to figure out why I only get this problem with
>> right-preconditioned KSPs with restrict or none.
>> >>
>> >> Thanks,
>> >> Pierre
>> >>
>> >>
>> >>
>> >>> On 13 Oct 2019, at 8:16 PM, Smith, Barry F. <bsmith at mcs.anl.gov>
>> wrote:
>> >>>
>> >>>
>> >>> Is this one process with one subdomain? (And hence no meaningful
>> overlap since there is nothing to overlap?) And you expect to get the
>> "exact" answer on one iteration?
>> >>>
>> >>> Please run the right preconditioned GMRES with -pc_asm_type [restrict
>> and basic and none]  -ksp_monitor_true_solution  and send the output for
>> the three cases.
>> >>>
>> >>> For kicks you can also try FGMRES (which always uses right
>> preconditioning) to see if the same problem appears.
>> >>>
>> >>> Barry
>> >>>
>> >>>
>> >>>> On Oct 13, 2019, at 2:41 AM, Pierre Jolivet via petsc-dev <
>> petsc-dev at mcs.anl.gov> wrote:
>> >>>>
>> >>>> Hello,
>> >>>> I’m struggling to understand the following weirdness with PCASM with
>> exact subdomain solvers.
>> >>>> I’m dealing with a very simple Poisson problem with Dirichlet +
>> Neumann BCs.
>> >>>> If I use PCBJACOBI + KSPPREONLY or 1 iteration of GMRES either
>> preconditioned on the right or on the left, I get the expected result, cf.
>> attached screenshot.
>> >>>> If I use PCASM + KSPPREONLY or 1 iteration of GMRES preconditioned
>> on the left, I get the expected result as well.
>> >>>> However, with PCASM + 1 iteration of GMRES preconditioned on the
>> right, I don’t get what I should (I believe).
>> >>>> Furthermore, this problem is specific to -pc_asm_type restrict,none
>> (I get the expected result with basic,interpolate).
>> >>>>
>> >>>> Any hint?
>> >>>>
>> >>>> Thanks,
>> >>>> Pierre
>> >>>>
>> >>>> $ -sub_pc_type lu -ksp_max_it 1 -ksp_type gmres -pc_type bjacobi
>> -ksp_pc_side right -> bjacobi_OK
>> >>>> $ -sub_pc_type lu -ksp_max_it 1 -ksp_type gmres -pc_type asm
>> -ksp_pc_side left -> asm_OK
>> >>>> $ -sub_pc_type lu -ksp_max_it 1 -ksp_type gmres -pc_type asm
>> -ksp_pc_side right -> asm_KO
>> >>>>
>> >>>> <bjacobi_OK.png><asm_OK.png><asm_KO.png>
>> >>>
>> >>
>> >> <dump-basic><dump-none><dump-restrict>
>> >
>>
>>
>

-- 
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.cse.buffalo.edu/~knepley/>
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