[gmx-users] bug in 3.1.5_beta Altivec loops?
lindahl at stanford.edu
Fri Jan 17 17:39:58 CET 2003
Choon Peng wrote:
> Anyone came across funny numerical results obtained from version
>3.1.5_beta on G4 using Altivec?
>For a NPT simulation (NVT is OK), I actually got a value of200 bar for
>pressure while the same beta installed on a PIII clustergave the more
>acceptable result of 3.8 (correct result is 2.3)!Also, quantities like
>total energy, temperature, etc are also quiteway off.
>Initially I suspected there's general bugs in the beta release but
>after confirming with a PIII (using SSE) installation, the prime
>suspect is now the Altivec calculations. I know they're faster than in
>3.1.4 due to more tuning and will inaccuracies creep in as a result of
>too aggressive tuning for performance?
Pressure is *extremely* sensitive to minor numerical differences, but
an example if the error in energies and a small input file to reproduce it.
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