[petsc-dev] Right-preconditioned GMRES

[Pierre Jolivet]

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

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> 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>
>