[Users] Failing AHFinderDirect test cases.

[Steven R. Brandt]

Can this explain why the Schmortal was unable to find horizons?

Cheers,
Steve

On 05/31/2010 04:32 PM, Peter Diener wrote:
> Hi,
>
> I have been looking at why some of AHFinderDirect test cases fail.
>
> First I checked the tests running on 1 processor and found that the cause
> of the failing test cases was a bug fix made in 2005 where the tests had
> not been updated afterwards. For those tests I regenerated the output.
>
> One additional test failed by not producing output because it didn't
> converge to the specified tolerance. I wasn't able to find a version of
> the code (after the date that the test case had been created) that I was
> able to compile and that passed the test. For that reason (after
> discussing with Erik) I disabled that test by renaming the parameter file
> to someting not ending in .par.
>
> After that I realized that a lot of the tests failed when running on 2
> procs. After some investigating, I realized that the cause was to be find
> in Exact. When going from 1 to 2 procs, the coordinates setup by CoordBase
> and CartGrid3d can differ at the roundoff level. In the particular case I
> was looking at in one case the y-coordinate was
>
> -3.3999999999999999
>
> while in the other it was:
>
> -3.4000000000000004
>
> so a difference only in the last digit.
>
> This introduced a difference of the computed 4-metric in the 2 last
> digits. Now Exact uses 2nd order finite differencing with an epsilon of
> 1.e-6 to calculate time derivatives of the 3-metric and and the spatial
> derivatives of the shift that is needed to calculate the extrinsic
> curvature. In this process there is a significant cancelletion of digits
> leading to a loss of 6-7 digits so the extrinsic curvature only agreed
> to 9-10 digits. In other grid points the coordinates were identical and
> so was the extrinsic curvature. So essentially the process of constructing
> the extrinsic curvature iteself lead to some arbitrary noise at the 9-10
> digit level. When this was fed into AHFinderDirect the differences were
> amplified even more leading to differences in the mass at the 6th digit
> level.
>
> Currently Erik is working on improving this (by using higher order finite
> differencing with a larger epsilon), but the main conclusion is that we
> have to be very careful when making tests based on initial data from the
> current version of Exact and may have to reduce the accuracy criteria for
> pass/fail significantly.
>
> Cheers,
>
>     Peter
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