[petsc-users] CG fails to converge in parallel

[Bojan Niceno]

Thanks a lot Pierre, I use PCGAMG as I type this answer.  Not only that it
works, but converges much faster than PCASM :-)

    Have a great day,

    Bojan

On Thu, Apr 20, 2023 at 8:07 AM Pierre Jolivet <pierre.jolivet at lip6.fr>
wrote:

>
> > On 20 Apr 2023, at 7:53 AM, Bojan Niceno <
> bojan.niceno.scientist at gmail.com> wrote:
> >
> > Dear all,
> >
> >
> > I am solving a Laplace equation with finite volume method on an
> unstructured grid with a Fortran code I have developed, and PETSc 3.19
> library.
> >
> > I first used cg solver with asm preconditioner, which converges nicely
> when executed sequentially, but fails for MPI parallel version.  I believed
> that there must be an error in which I set up and assemble parallel
> matrices for PETSc, but I soon noticed that if I use bicg with asm,
> everything works fine, the parallel bicg/asm shows almost the same
> convergence as sequential version.
>
> KSPCG requires a symmetric PC.
> By default, PCASMType is PC_ASM_RESTRICT, which yields a non-symmetric
> preconditioner.
> With a single process, this does not matter, but with more than one
> process, it does.
> If you switch to -pc_asm_type basic, KSPCG should converge.
> That being said, for Laplace equation, there are much faster alternatives
> than PCASM, e.g., PCGAMG.
>
> Thanks,
> Pierre
>
> > I could carry on with bicg, but I am still worried a little bit.  To my
> knowledge of Krylov solvers, which is admittedly basic since I am a
> physicist only using linear algebra, the convergence of bicg should be very
> similar to that of cg when symmetric systems are solved.  When I run my
> cases sequentially, I see that's indeed the case.  But in parallel, bicg
> converges and cg fails.
> >
> > Do you see the above issues as an anomaly and if so, could you advise
> how to search for a cause?
> >
> >
> >     Kind regards,
> >
> >
> >     Bojan
> >
> >
> >
> >
> >
>
>
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