[petsc-dev] asm / gasm

[Barry Smith]

> On Jul 3, 2016, at 11:59 AM, Mark Adams <mfadams at lbl.gov> wrote:
> 
> Thanks, I saw that.  ASM converges slower than SOR :(

   You can try  -mg_levels_pc_asm_local_type multiplicative that Matt added, but you probably need to switch to GMRES since I don't think Matt did a symmetric version of it.


> 
> On Sun, Jul 3, 2016 at 6:50 PM, Barry Smith <bsmith at mcs.anl.gov> wrote:
> 
>   Here is the problem:
> 
>         if (!sz) {
>           IS       nis;
>           ierr = ISCreateGeneral(PETSC_COMM_SELF, 0, NULL, PETSC_COPY_VALUES, &nis);CHKERRQ(ierr);
>           ierr = PCASMSetLocalSubdomains(subpc, 1, NULL, &nis);CHKERRQ(ierr);
>           ierr = ISDestroy(&nis);CHKERRQ(ierr);
> 
> It was setting the first IS when it should be setting the second. When I run after this change I get reasonable results.
> 
>  0 KSP Residual norm 2.469493285658e+02
>   1 KSP Residual norm 1.335901394170e+01
>   2 KSP Residual norm 5.623002716761e-01
>   3 KSP Residual norm 5.201650164984e-02
>   4 KSP Residual norm 3.763050190737e-03
>   5 KSP Residual norm 3.024410926322e-04
> Linear solve converged due to CONVERGED_RTOL iterations 5
>   0 KSP Residual norm 2.469493285658e-03
>   1 KSP Residual norm 1.335901394170e-04
>   2 KSP Residual norm 5.623002716761e-06
>   3 KSP Residual norm 5.201650164983e-07
>   4 KSP Residual norm 3.763050190737e-08
>   5 KSP Residual norm 3.024410926322e-09
> Linear solve converged due to CONVERGED_RTOL iterations 5
>   0 KSP Residual norm 2.469493285658e-08
>   1 KSP Residual norm 1.335901394170e-09
>   2 KSP Residual norm 5.623002716761e-11
>   3 KSP Residual norm 5.201650164984e-12
>   4 KSP Residual norm 3.763050190737e-13
>   5 KSP Residual norm 3.024410926322e-14
> 
> 
> 
> > On Jul 3, 2016, at 10:40 AM, Mark Adams <mfadams at lbl.gov> wrote:
> >
> > Barry,
> >
> > GAMG must be setting up the ASM incorrectly, on empty processors, when using the aggregates.  In asm.c, I see that osm->is_local is NULL for empty processors, and this causes a problem when this branch is taken by less than 8 processes (of 8):
> >
> >       if (osm->is_local) { // asm.c:282
> >        ....
> >         ierr = VecScatterCreate(vec,osm->is_local[i],osm->y_local[i],isl,&osm->prolongation[i]);CHKERRQ(ierr);
> >
> > I get an error "MPI_Allreduce() called in different locations (code lines) on different processors ", because "vec" here is global. This gamg.c code must not be correct:
> >
> >         if (!sz) {
> >           IS       is;
> >           ierr = ISCreateGeneral(PETSC_COMM_SELF, 0, NULL, PETSC_COPY_VALUES, &is);CHKERRQ(ierr);
> >           ierr = PCASMSetLocalSubdomains(subpc, 1, &is, NULL);CHKERRQ(ierr);
> >           ierr = ISDestroy(&is);CHKERRQ(ierr);
> >         } else {
> >           ....
> >         }
> >
> > I will look into why osm->is_local seems to be NULL here when it should have an empty IS.
> >
> > Mark
> 
>