[petsc-users] Is this a bug? MatMultAdd_SeqBAIJ_11

[Barry Smith]

   Daniel,

     Thanks for the update. We can patch the release version of PETSc (normally we don't have a way to patch previous release versions)

     There is no direct option to always use the _N version. (For smaller blocks using _N is very slow).

    Barry


> On Sep 21, 2021, at 9:51 AM, Daniel Stone <daniel.stone at opengosim.com> wrote:
> 
> I seem to have confirmed that making the change suggested by Lawrence fixes things in our 
> case. Some alternate runs by a colleague that result in a blocksize 12 matrix also work
> fine - I think in that case MAtMultAdd_SeqBAIJ_N must be being used as I can't find
> a blocksize 12 analogue.
> 
> Is there by any chance a setting somewhere that can tell petsc to override the use of 
> blocksize specific routines like this and always go to, e.g., MatMultAdd_SeqBAIJ_N
> etc? Might be useful as a short term fix.
> 
> Thanks,
> 
> Daniel
> 
> On Tue, Sep 21, 2021 at 11:50 AM Lawrence Mitchell <wence at gmx.li <mailto:wence at gmx.li>> wrote:
> Hi Daniel,
> 
> > On 21 Sep 2021, at 11:19, Daniel Stone <daniel.stone at opengosim.com <mailto:daniel.stone at opengosim.com>> wrote:
> > 
> > Hello,
> > 
> > If we look at lines 2330-2331 in file baij2.c, it looks like there are some 
> > mistakes in assigning the `sum..` variables to the z array, causing
> > the function MatMultAdd_SeqBAIJ_11() to not produce the correct
> > answer.
> > 
> > I don't have a good example program to demonstrate this yet - it's
> > currently causing problems in a dev branch of pflotan_ogs that
> > can produce blocksize 11 matrices. When in parallel, a standard
> > matrix-vector multiplication calls MatMultAdd for the off-proc
> > contributions, and the result is wrong when this is redirected
> > to MatMultAdd_SeqBAIJ_11. Seems to be the root cause of
> > several solvers failing such as fgmres.
> > 
> > Can anyone confirm that these two lines seem incorrect?
> 
> Looks wrong to me, I guess this patch is correct?
> 
> diff --git a/src/mat/impls/baij/seq/baij2.c b/src/mat/impls/baij/seq/baij2.c
> index 2849ef9051..65513c8989 100644
> --- a/src/mat/impls/baij/seq/baij2.c
> +++ b/src/mat/impls/baij/seq/baij2.c
> @@ -2328,7 +2328,7 @@ PetscErrorCode MatMultAdd_SeqBAIJ_11(Mat A,Vec xx,Vec yy,Vec zz)
>        v    += 121;
>      }
>      z[0] = sum1; z[1] = sum2; z[2] = sum3; z[3] = sum4; z[4] = sum5; z[5] = sum6; z[6] = sum7;
> -    z[7] = sum6; z[8] = sum7; z[9] = sum8; z[10] = sum9; z[11] = sum10;
> +    z[7] = sum8; z[8] = sum9; z[9] = sum10; z[10] = sum11;
>      if (!usecprow) {
>        z += 11; y += 11;
>      }
> 
> 
> Lawrence

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