Yosef,
in order for the bug to show, the world tube had to be at a distance A * dt *1/4 from the nearest Bondi-frame evolution point,where 'A' is some geometric factor, dependent on the metric. What we had not realized the last time this code was looked at, is that we were computing one of the corner points of the null parallelogram by an interpolation stencil that divided by a quantity that can vanish when the world tube was at this 'wrong' distance.
I rewrote the interpolation (and redefined the 'inner' end of the null parallelogram) such that this type of condition cannot occur anymore.
What we noticed was that in certain runs of ours there would be spikes in the waveform at scri+. We managed to correlate this problem to the world-tube having moved across this point where the interpolation became singular. After the fix the spikes went away.
Bela.
On Tue, Jul 31, 2012 at 12:06 PM, Yosef Zlochower yosef@astro.rit.edu wrote:
I updated the trac ticket. btw, these changes do not affect the testsuite. I'm actually surprised by this. Does the bugfix only affect the code under certain circumstances?
On 07/31/2012 02:49 PM, Bela Szilagyi wrote:
Yosef,
I have made several attempts to commit the changes, over the course of the past couple of months. Somehow every time I figure out how to re-download the code from the latest incarnation of the repository, I figure I no longer have write access or no longer know my password or some combination of the above. This simply because I am not an active Cactus user.
I had asked Roland to just push these changes as they are essential for the correctness of the code. If you rely on me finding the time to figure out how to (re-)gain checking access to these thorns, this will delay the application of the patches by further months.
The start-up related change could be logged as
Improve the start-up algorithm of the characteristic marching scheme from the inner boundary towards scri+.
All other changes are related to either improving IO (will correctly truncate/create diagnostic files) or allowing for flexible tuning of IO between checkpoints/restarts (by changing the parameters into steerable ones).
I'd like to ask that someone with working write access to the arrangement apply the patches.
Bela.
On Mon, Jul 30, 2012 at 12:05 PM, Yosef Zlochoweryosef@astro.rit.edu wrote:
On 07/30/2012 11:47 AM, Bela Szilagyi wrote:
Yosef, and all
'start-up' here refers to the way the null parallelogram algorithm (marching out along a characteristic slice and starting from the inner boundary), gets its integration constants set by the boundary data. This is not a t=0 issue, but rather a \lambda=0 issue in CCE language. I believe it is absolutely essential for the correctness of the code.
As far as changing parameters from non-steerable to steerable goes, I also believe they are useful as one can legitimately want to start a new diagnostic for a run when restarted from a particular checkpoint. Or even from the middle of a run, checkpointed or not. Those are, indeed, independent from the 'start-up' patch.
I do understand that the Einstein Toolkit maintenance team may not be familiar with the Pitt code. Please regard the 'start-up' patch as an amendment to the 1st release of the code, by those same people who worked on it before releasing it. If you need any details of why have I changed things in the particular way I have, I will be glad to explain.
Bela et al, It would probably be a good idea to have some kind of informative commit message. Since you checked that these changes are correct and necessary, can you commit them? I'll then update the testsuite, if needed.
Bela.
-- Dr. Yosef Zlochower Center for Computational Relativity and Gravitation Assistant Professor School of Mathematical Sciences Rochester Institute of Technology 85 Lomb Memorial Drive Rochester, NY 14623
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