[Users] Synchronization and MoL

[Erik Schnetter]

On Sep 19, 2006, at 12:34:07, Ian Hinder wrote:

> Hi,
>
> When using MoL with Carpet, in which situations should I 'SYNC' my
> grid functions?  In the MoL documentation, it says that MoL handles
> all SYNCing across processors,

Does it?  I don't think so.  I don't see any synchronisation  
happening in MoL.  I think the real statement is:

MoL does the correct thing, independent of whether you sync the state  
vector or the RHS of the state vector.  But the user has to sync at  
least one of these.

> so I don't need to worry about it.  So
> I take this to mean that when I compute my RHS quantities on the
> interior of the grid in MoL_CalcRHS, I do not need to SYNC them in the
> schedule file.  This agrees with what I find in the various example
> thorns.
>
> I currently apply my boundary conditions to the RHS variables in
> MoL_RHSBoundaries; do I need to SYNC them?  I am not interested in the
> RHS variables being correct at the end of a time step, just that the
> evolution is correct.  All the examples I can find in Cactus and
> Carpet use the Boundary interface, rather than setting the RHS
> variables directly.  These seem to SYNC the evolved variables in the
> routine they call in MoL_PostStep.

You have two choices; you can either sync the state vector or its  
RHS.  This is somewhat independent of where you apply the physical  
(outer) boundary condition.  In our multi-block code, we apply the  
boundary conditions to our RHS via penalties, and we also sync the RHS.

Using the boundary interface is another issue; the boundary interface  
also applies symmetry and inter-patch boundary conditions.  We  
therefore also use the boundary interface to apply boundary  
conditions onto the RHS.

In principle, you can mix and match applying the triple  
(synchronisation, boundary interface, physical boundaries) to either  
the state vector or the RHS.  We chose to apply all to the RHS for  
consistency reasons, but you can do otherwise.

-erik

-- 
Erik Schnetter <schnetter at cct.lsu.edu>

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