[petsc-users] KSPGMRESOrthog costs too much time

[Rongliang Chen]

Hi Jed,

Thank you for your suggestions.

Best,
Rongliang


> ------------------------------
>
> Message: 2
> Date: Thu, 17 Nov 2011 12:50:23 -0600
> From: Jed Brown <jedbrown at mcs.anl.gov>
> Subject: Re: [petsc-users] KSPGMRESOrthog costs too much time
> To: PETSc users list <petsc-users at mcs.anl.gov>
> Message-ID:
>        <CAM9tzSkqexpY9RD4Z=huP25P1yA4viKqSKFTb5hxZQcyZrU1rg at mail.gmail.com
> >
> Content-Type: text/plain; charset="utf-8"
>
> On Thu, Nov 17, 2011 at 12:18, Rongliang Chen <rongliang.chan at gmail.com
> >wrote:
>
> > I am using the composite pc now not the PCMG:
> >
> >       ierr = PCCompositeAddPC(finepc,PCSHELL);CHKERRQ(ierr);
> >       ierr = PCCompositeAddPC(finepc,PCASM);CHKERRQ(ierr);
> >
> >       ierr = PCCompositeGetPC(finepc,0,&coarsesolve);CHKERRQ(ierr);
> >       ierr = PCShellSetContext(coarsesolve,ctx);CHKERRQ(ierr);
> >       ierr = PCShellSetApply(coarsesolve,
> > CoarseSolvePCApply);CHKERRQ(ierr);
> >
> >       ierr = PCCompositeGetPC(finepc,1,&asmpc);CHKERRQ(ierr);
> >       ierr = PCSetOptionsPrefix(asmpc,"fine_");CHKERRQ(ierr);
> >       ierr = PCSetFromOptions(asmpc);CHKERRQ(ierr);
> >
> >       ierr = PCSetType(asmpc,PCASM);CHKERRQ(ierr);
> >       ierr = PCASMSetOverlap(asmpc,0);CHKERRQ(ierr);
> >       ierr = PCASMSetLocalSubdomains(asmpc,1,&grid->df_global_asm,
> > PETSC_NULL);CHKERRQ(ierr);
> >
>
> You can make your own event for the coarse level solve. Using PCMG instead
> of PCComposite would make your code more flexible, so you may want to
> consider doing it at some point.
>
>
> >
> > I just use two level method now and it is not very easy to try more
> levels
> > since I am using unstructure meshes.
> > I tried to solve the coarse level exactly by LU and it works well if the
> > coarse problem is small. When the coarse problem is large, LU is also
> very
> > slow (when the fine level problem is large, I can not use very small
> coarse
> > level problem). I found that nearly 90% of the time is spent on the
> coarse
> > level when the number of processor is large (np >512), so I want to know
> > which step of the coarse level solver costs the most of the time. Thanks.
> >
>
> This is a common problem. Depending on your equations, you might be able to
> solve the coarse-level problem using algebraic multigrid. Try
> -coarse_pc_type gamg (or however you set up the options prefixes; maybe
> also "hypre" or "ml" if you have those installed).
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