[petsc-dev] Right-preconditioned GMRES

Smith, Barry F. bsmith at mcs.anl.gov
Sun Oct 13 13:16:30 CDT 2019

  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.


> 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>

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