[petsc-dev] Right-preconditioned GMRES

[Mark Adams]

Oh I see you have established that it is the overlap and got something that
looks like the load on a homogenous BC and the processor boundary ... very
odd.

Does the bad version converge to the correct solution if you iterate? It
looks like your BC is wrong.

It sure looks like a bug. Can you valgrind this?


On Sun, Oct 13, 2019 at 9:34 AM Mark Adams <mfadams at lbl.gov> wrote:

> Try -pc_asm_overlap 0 with ASM.
>
> And I trust the KO run works with 1 processor (direct solve)
>
> On Sun, Oct 13, 2019 at 3:41 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
>>
>>
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