[Users] Meeting minutes

[Ian Hinder]

On 15 Nov 2010, at 17:46, Roland Haas wrote:

> Testsuites
> * AHFinderDirect/AEILocalInterp: Carpet/mercurial with full grid  
> gives identical results on 1 or 2 processors, PUGH with bitant grid  
> does not. This points to PUGHInterp to be th culprit.

> However Ian sees differences when using Multipole and Carpet as well  
> (for 32 processors). This points to the symmetry handling in  
> AEILocalInterp being the culprit.

Erik found the source of the Multipole problem, and it was unrelated.   
The problem was that Multipole uses Hermite polynomial interpolation  
by default which requires 3 ghost zones, but the tests were being run  
with 2.  This leads to AEILocalInterp not having enough points to  
interpolate with a centered stencil when close to an interprocessor  
boundary.  In such cases, AEILocalInterp silently off-centres the  
interpolation stencil, leading to a different interpolation error,  
though still of the correct order.

I have modified Multipole so that it disables this off-centering by  
default; this should allow such problems to be identified more  
easily.  I have also modified the test suites to use 3 ghost zones,  
and they now pass on 32 processes, in addition to the 1 and 2 process  
tests that were passing before.

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