[Users] GRHydro Multipatch announcement

[Ian Hinder]

On 15 Sep 2011, at 17:10, Roland Haas wrote:

> Dear all,
> 
> as already mentioned in the meeting minutes and discussed in last week's
> CIGR call, Christian Reisswig has been working on adding Llama
> Multipatch compatibility to GRHydro. For those of you not familiar with
> Llama, this is the description from the Llama website (www.llamacode.org):
> --8<--
> The Llama code is a 3-dimensional multiblock infrastructure with
> adaptive mesh-refinement for Cactus based on Carpet. It provides
> different patch systems that cover the simulation domain by a set of
> overlapping patches. Each of these patches has local cooordinates with a
> well defined relation to global Cartesian coordinates. However, all
> computations are carried out using a global Cartesian tensor basis such
> that involved tensor transformations between patch systems can be
> avoided. Information between the different patches is communicated via
> interpolation in the overlap zones.
> --8<--
> 
> We would like to incorporate these changes into the public development
> version of GRHydro. Right now multipatch support is functional for
> pure hydro simulations but still incomplete (eg. only the HLLE solver
> is supported, others will currently fail with multipatch enabled). No
> (serious) attempt has been made to incorporate MP into MHD, though the
> changes required should be straightforward. The proposed patch is
> backward compatible in that old parameter files will continue to work
> without change.
> 
> However the patch touches a large number of code lines since all
> references to vector/tensor components have to be replaced with
> references to tensor components in a local tensor basis. In practice
> this means that all occurances of gxx, betax, vel[i] etc. have to be
> replaced by gaa, betaa, lvlel[i]. On top of that, the requirement to
> stay backwards compatible (and to not use extra memory) with old
> parameter files when MP is not actually employed adds another layer so
> that in the end gxx is replaced by g11 etc. This is all rather
> straightforward, but unfortunately tedious and makes for a large
> patch.
> 
> We have tested that the code runs the old GRHydro testsuites and passes
> the same testsuites that a current vanilla checkout passes.
> 
> We will begin adding multipatch support during the day.

Do you know if there is any performance penalty when multipatch is switched off?

-- 
Ian Hinder
ian.hinder at aei.mpg.de