[Users] Phone Call Mon, Nov 8th

[Eloisa Bentivegna]

On Nov 9, 2010, at 8:30 AM, Peter Diener wrote:

> Hi,
> 
> A bit more information to add to the confusion. Using the git version of Carpet, I was not able to get the same results on 1 and 2 processors. Not even going to a full grid as Erik did. However, with the added interpolation debug output built into AHFinderDirect I do notice a somwehat different behavior. With PUGH I see differences (within 11-12 decimal places) in the interpolation results before the first call to the linear solver. With Carpet I don't. There the first differences appear in the horizon coordinates after the first linear solve. So with Carpet it appears that the linear solver gets the same data on 1 and 2 procs but finds a different solution.

Peter,

by debug output, do you mean that you're also checking mean curvature and expansion, iteration by iteration? The linear solver also needs these quantities, along with the shape h, to compute the next surface guess. Since both mean curvature and expansion require interpolation, this could explain the differences between different processor counts, even if the initial guesses for h are identical.

> I get the same results using PUGH and Carpet on 1 processor. The results on 2 processors using PUGH and Carpet differs from the 1 processor results and with each other.
> 
> Also the results differ depending on whether bitant or full mode is used
> even on 1 processor.

I think here it is important to state whether the differences are compatible with roundoff or not. Identical results are guaranteed only for identical operations, and turning symmetries on and off does not generally satisfy this criterion.

I've checked my tests and I do find that, with the git version of Carpet, the differences reduce from about 1e-06 to 1e-14 by enabling full mode. The latter is arguably acceptable, the former obviously not.

Eloisa