[petsc-users] Enquiry regarding log summary results

[Jed Brown]

On Sat, Oct 6, 2012 at 4:19 PM, TAY wee-beng <zonexo at gmail.com> wrote:

> May I know where can I find all the options for GAMG? I can't find it in
> the manual.
>

As always, run with -help. The options for GAMG specifically are controlled
under the prefix -pc_gamg_*. The smoothers  and coarse grid solvers are
configured under -mg_levels_*.


>
> I also used -info for the GAMG and what does these mean:
>
> 1. [9] MatAssemblyBegin_MPIAIJ(): Stash has 14396 entries, uses 1 mallocs
>

> How serious is this? Does it mean there's a lot of mallocs going on? Do I
> correct this by using higher d_nz and o_nz?
>

No, the stash holds entries that are stored on a different process from
where you assemble. Assembling some values off-process is common for finite
element discretizations, for example. Less than one percent of the values
are assembled off-process, which is fine.


>
> 2. [4] MatAssemblyEnd_SeqAIJ(): Matrix size: 112785 X 112785; storage
> space: 0 unneeded,3628127 used
> [4] MatAssemblyEnd_SeqAIJ(): Number of mallocs during MatSetValues() is 0
> [4] MatAssemblyEnd_SeqAIJ(): Maximum nonzeros in any row is 59
>
> When I use MatCreateAIJ, my d_nz and o_nz are both 7. If the row is 59,
> does it mean that my d_nz and o_nz are too small. Must I set it to the
> maximum nonzeros given by -info?
>

Either this line is not the first assembly or you didn't pass 7. If you
don't want to use extra memory, you should preallocate exactly, otherwise
it is better to over-estimate than to under-estimate.


>
> 3. [0] MatSetOption_SeqAIJ(): Option STRUCTURALLY_SYMMETRIC ignored
>
> My Poisson matrix is symmetrical, so why is it ignored?
>

This is a bad info message. That option is handled at a higher level. I'll
fix it.
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