[Dune] communication patterns in ISTL

[Markus Blatt]

Hi,

On Wed, Dec 20, 2017 at 09:35:51PM +0100, Christian Engwer wrote:
> can someone enlighten me?
> 
> What is the meaning of copyCopyToAll::copyCopyToAll(...)?
>

exactly what the name says it sends the values marked as copy to all
others.

> It seems strange to me. We start with a partition of indices according
> to owner and then we might have copies of indices owned by an other
> process.
>
> Now, copyCopyToAll is supposed to communicate those indices tagged as
> copy to all other processes having this index. But what is this
> supposed to do?
> a) Assume that the copies don't contain consistent values. Now every
>    process is sending around his own value. Depending on which message
>    arrives last the receiving process gets an arbitrary value. This
>    seems to be wrong and we consequently we should not be allowed to
>    call copyCopyToAll, if the copies are not consistent.



Well, you are missing some patterns. Just assume the partitioning is
not static, but you repartition from N to n processors (n>N). For an
entity that was previously marked as copy on two adjacent processors
but will only be on one process after the repartitioning you might
need this. Maybe the entity had less visible neighbours on each
process before the repartitioning than before. If you e.g. repartition
a matrix, then the some row entries might even need to up added up.

Concerning "not allowed". This is C++ and you are allowed to shoot
yourself in the food.

> b) assume we have consistent copies. Why should be communicate at all?
>    The copies are sent around, but we retrieve the value that we
>    previously sent to an other process.
>

Again: Due to repartitioning to fewer processors.

Markus

> It seems I misunderstood something dramatically.
> 
> Best
> Christian
> 
> PS: the two functions
> OwnerOverlapCopyCommunication::addOwnerOverlapToAll and
> OwnerOverlapCopyCommunication::addOwnerCopyToAll are not used in ISTL
> at all. Is anybody using them outside of the core modules?
>

The were used in downstream modules in the past and if somebody
decides to do additive domain decomposition solvers the might be
again.

Cheers,

Markus
-- 
Dr. Markus Blatt - HPC-Simulation-Software & Services http://www.dr-blatt.de
Pedettistr. 38, 85072 Eichstätt, Germany
Tel.: +49 (0) 160 97590858
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