[Developers] Re: behavior of HorizonRadiusInDirection

Jonathan Thornburg jthorn at aei.mpg.de
Mon Jun 13 11:09:25 CDT 2005


Hi, Yosef, sorry for the delay in responding to your E-mail.
[I think this issue might be of more general interest, so I'm cc-ing
the mailing list on this reply.]

You asked:

>  I would like to know how to use the aliased function 
> HorizonRadiusInDirection from AHFinderDirect. I would like to get the
> radius of the horizon along the x,y,z axes of the origin of the horizon. I 
> intend to construct a ellipsoid based on these radii. I
> tried to get these radii by providing the points 
> (x,y,z)=(1,0,0),(-1,0,0),(0,1,0,0),(0,-1,0,0),(0,0,1),(0,0,-1). This seemed 
> to work on one processor but failed on two processors. In the latter case the 
> returned radii were zero on the second processor (which did not contain the 
> horizon). Should I take the maximum radius over all processors to get the 
> radius that I want?

I think you've found a bug in AHFinderDirect: to improve performance,
it finds different horizons on different processors, and doesn't broadcast
horizon shapes to all processors unless it thinks they'll be needed...
and I neglected to update the "they'll be needed" computation when I
introduced  HorizonRadiusInDirection() .  :(

I think the best way to fix this problem is to introduce a new parameter
to (optionally) force the code to always broadcast this information.
I'll do this...

ciao,

-- 
-- Jonathan Thornburg <jthorn at aei.mpg.de>
    Max-Planck-Institut fuer Gravitationsphysik (Albert-Einstein-Institut),
    Golm, Germany, "Old Europe"     http://www.aei.mpg.de/~jthorn/home.html
    "Washing one's hands of the conflict between the powerful and the
     powerless means to side with the powerful, not to be neutral."
                                       -- quote by Freire / poster by Oxfam




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