[petsc-dev] asm / gasm

[Matthew Knepley]

On Wed, Jun 22, 2016 at 5:51 AM, Mark Adams <mfadams at lbl.gov> wrote:

> I'm trying to get block smoothers to work for gamg.  We (Garth) tried
> this and got this error:
>
>
>  - Another option is use '-pc_gamg_use_agg_gasm true' and use
>> '-mg_levels_pc_type gasm'.
>>
>>
> Running in parallel, I get
>
>      ** Max-trans not allowed because matrix is distributed
>  ----
>
> First, what is the difference between asm and gasm?
>

ASM is very simple Additive Schwarz which can have multiple blocks per
process, but cannot have blocks spanning
processes. GASM was meant to generalize this, but Dmitry wrote it and (to
my knowledge) the support situation is
uncertain. Maybe Barry know better.


> Second, I need to fix this to get block smoothers. This used to work.  Did
> we lose the capability to have blocks that span processor subdomains?
>

Did we ever have it?


> gamg only aggregates across processor subdomains within one layer, so
> maybe I could use one layer of overlap in some way?
>

ASM can do arbitrary overlap.

  Matt


> Thanks,
> Mark
>
>


-- 
What most experimenters take for granted before they begin their
experiments is infinitely more interesting than any results to which their
experiments lead.
-- Norbert Wiener
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