[petsc-dev] Right-preconditioned GMRES

Pierre Jolivet pierre.jolivet at enseeiht.fr
Sun Oct 13 10:07:28 CDT 2019

> On 13 Oct 2019, at 3:48 PM, Mark Adams <mfadams at lbl.gov> wrote:
> 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.

Yeah, of course: I switched back from P_3 to P_1 and now the first iterate is OK.
I am indeed most definitely messing up with the RHS at some point before the KSPSolve.


> 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 <mailto: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 <mailto: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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