[Beowulf] Re: OpenMP wierdness on dual AMD 2350 box w/ SL5.2 x86_64

David Mathog mathog at caltech.edu
Wed Nov 26 11:03:34 PST 2008

"Nathan Moore" <ntmoore at gmail.com> wrote

> The program scales beautifully up to OMP_NUM_THREADS~6 or 7, but when
I set
> the number of threads to 8, something weird happens, and instead of taking
> the normal 241 iterations to converge, the program converges after 1 step.
> This reeks of numerical instability to me, but my programming
experience in
> x86_64 is very limited.

I did not read through your code very carefully, but are you by any
chance running out of memory somewhere, without the code in place to
catch and report the error?   In other words, it might be a coincidence
that this happens at 8 threads.  If you increase the size of the arrays
you may find the bug moves to a smaller number of threads.


David Mathog
mathog at caltech.edu
Manager, Sequence Analysis Facility, Biology Division, Caltech

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