[petsc-users] Strange GAMG performance for mixed FE formulation

[Lawrence Mitchell]

On 02/03/16 22:28, Justin Chang wrote:
...


>         Down solver (pre-smoother) on level 3
> 
>           KSP Object:          (solver_fieldsplit_1_mg_levels_3_)    
>             linear system matrix = precond matrix:
...
>             Mat Object:             1 MPI processes
> 
>               type: seqaij
> 
>               rows=52147, cols=52147
> 
>               total: nonzeros=38604909, allocated nonzeros=38604909
> 
>               total number of mallocs used during MatSetValues calls =2
> 
>                 not using I-node routines
> 
>         Down solver (pre-smoother) on level 4
> 
>           KSP Object:          (solver_fieldsplit_1_mg_levels_4_)    
>             linear system matrix followed by preconditioner matrix:
> 
>             Mat Object:            (solver_fieldsplit_1_)            

...
> 
>             Mat Object:             1 MPI processes
> 
>               type: seqaij
> 
>               rows=384000, cols=384000
> 
>               total: nonzeros=3416452, allocated nonzeros=3416452


This looks pretty suspicious to me.  The original matrix on the finest
level has 3.8e5 rows and ~3.4e6 nonzeros.  The next level up, the
coarsening produces 5.2e4 rows, but 38e6 nonzeros.

FWIW, although Justin's PETSc is from Oct 2015, I get the same
behaviour with:

ad5697c (Master as of 1st March).

If I compare with the coarse operators that ML produces on the same
problem:

The original matrix has, again:

        Mat Object:         1 MPI processes
          type: seqaij
          rows=384000, cols=384000
          total: nonzeros=3416452, allocated nonzeros=3416452
          total number of mallocs used during MatSetValues calls=0
            not using I-node routines

While the next finest level has:

            Mat Object:             1 MPI processes
              type: seqaij
              rows=65258, cols=65258
              total: nonzeros=1318400, allocated nonzeros=1318400
              total number of mallocs used during MatSetValues calls=0
                not using I-node routines

So we have 6.5e4 rows and 1.3e6 nonzeros, which seems more plausible.

Cheers,

Lawrence

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