LAPACK Archives

[Lapack] lapack bug in divide-and-conquer: is anybody listening


Each iteration operates within a shrinking interval which contains the desired 
root, and the bisection is called whenever the iteration goes out of the 
interval; so the bisection is the fall back (I just double checked). 200 is 
extreme, but I think it should never get that far, otherwise I would be really 
worried. It would be helpful if someone could tell me what is the exact number 
of iterations needed to fix that bug.

best, -rencang

From: James Demmel [demmel@Domain.Removed]
Sent: Saturday, November 20, 2010 10:00 AM
To: julie langou
Cc: lapack@Domain.Removed; lapackers; beboppers; Li, Ren-cang
Subject: Re: [Lapack] lapack bug in divide-and-conquer: is anybody listening

And sorry for also not getting to this sooner. When you say that the
convergence bug in divide-and-conquer was fixed by raising MAXIT
from 40 to 200 in xlaed4 (written by Ren-Cang), then that makes me
think that some flavor of bisection would be a better fall-back when
convergence fails (since the maximum number of iterations could be
more like the number of bits in the floating point word, so 32 or 64).
This is very intricate code (and asking the to-be-hired programmer to fix
it would likely require a very long learning curve) so let me ask Ren-Cang
if he has any opinions.


julie langou wrote:
Sorry for not getting back to you earlier.
We indeed corrected the bug you found, thank you for the patch. It is
released in LAPACK 3.3.0 (released this week).
This bug was labelled bug0064.
If you could confirm that this issue is fixed, I will add a post on
the forum to let the users know.

On Nov 18, 2010, at 8:25 AM, Evan Drumwright wrote:


Please allow me to say how much I use and appreciate lapack.  It
makes my research *much* easier and I would be stuck without it.

... which is why the following problem is so disconcerting.  I am
experiencing a bug related to least squares solution via singular
value decomposition that at least one other person has experienced
and have gotten no response on the lapack user's forum from the
lapack maintainers.

Please see:

for a description of the bug, conditions to reproduce it, and a fix.
 I would greatly appreciate it if this fix made its way into lapack!

Thanks and regards,
Evan Drumwright

Lapack mailing list
Lapack@Domain.Removed <mailto:Lapack@Domain.Removed>

Julie Langou; Research Associate in Computer Science
Innovative Computing Laboratory;
University of Tennessee from Denver, Colorado ;-)


Lapack mailing list

<Prev in Thread] Current Thread [Next in Thread>

For additional information you may use the LAPACK/ScaLAPACK Forum.
Or one of the mailing lists, or