[Users] using PETSc with the appropriate benchmark

[Erik Schnetter]

On Apr 14, 2008, at 10:04:19, Avi Purkayastha wrote:
> Hi all,
>
> I hope this is the appropriate list for the following question, or  
> please redirect if not.
>
> From the tutorial it seems like CactusElliptic/EllPETSc seems to be  
> the only thorn using the petsc lib. However the default Cactus thorn  
> list (see below) does not include CactusElliptic. So my newbie  
> question is the following:
>
> I have run the default build with the Bench_BSSN_PUGH_80l.par  
> parameter file. If I were to add the  CactusElliptic thorn and build  
> it, does the elliptic equations get solved when using the above  
> benchmark, or do I need a different benchmark?
>
> If there is documentation which explains the above i.e. benchmarks  
> with the appropriate thorn lists, that would be very helpful.
>
> Thanks for the help.
>
> 	-- Avi
>
> P.S. the default thorn list I was referring to has CactusBase,  
> CactusEinstein and CactusPUGH as the high level thorns.

Avi,

the thorn EllPETSc is a wrapper of the PETSc library.  It implements a  
certain interface which is defined in the thorn EllBase.  Another  
thorn implementing the same interface is EllSOR; this thorn implements  
an SOR solver, which is a much simpler algorithm, but obviously not  
suited for large problems.  EllPETSc is not part of the default thorn  
lists because it requires PETSc to be installed on the system, which  
is not always the case.  One particular problem you may come across  
with PETSc is that the API is updated from time to time, and we then  
have to update the PETSc wrapper in Cactus to follow these changes,  
while remaining backwards compatible.

The benchmark Bench_BSSN_PUGH_80l.par uses an explicit time  
integrator, which means that there are no elliptic equations which  
need to be solved.  Therefore adding EllPETSc to your thorn list won't  
actually change anything in the benchmark, and it won't use PETSc.

We have at the moment no benchmark for elliptic equations, but we have  
examples solving elliptic equations e.g. in the thorn CactusWave/ 
IDScalarWaveElliptic.  This example solves an elliptic equation for  
the initial condition, and then continues with a few time steps of  
explicit time stepping without solving an elliptic equation.  This  
example could be standardised into a benchmark, which would mostly  
entail choosing interesting and reasonable problem sizes.

The problem size is chosen by the parameter global_nsize e.g. in  
CactusWave/IDScalarWaveElliptic/par/source_petsc.par.

-erik

-- 
Erik Schnetter <schnetter at cct.lsu.edu>   http://www.cct.lsu.edu/~eschnett/

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