Hi Bjørn, On Mon, 2004-02-02 at 10:04, Bjorn Tore Sund wrote:
On Mon, 2 Feb 2004, Odon Farkas wrote:
Hmm, I would be interested in some example code showing such behaviour. Because on AMD64 (in 64bit mode) we have twice as many registers and use SSE2 by default. This means that in theory (and mostly in practice too) floating point on AMD64 should outperform anything on 32bit quite a bit. This has nothing to do with 64bit or not, but rather with the much better instruction set available in 64bit mode.
I was also surprised, since I expected the same. Gaussian uses the PGI fortran compiler and the code was linked with AMD's ACML. It is very likely a current handicap of the PGI compiler. Also a possible reason is that the cache can store more 32 bit integers than the default 64-bit ones when compiled with i8. Gaussian is not open source (see www.gausssian.com), so I am not allowed to give example code, the whole program is cca. 1M lines in Fortran. I could not find any SpecFP benchmark which was done on Opteron/Athlon64 in 64/bit mode and could outperform the same machine in 32-bit mode. However, I really hope this will happen...
This is surprising. We've been using the PGI 64 bit compilers on Opteron since this summer with very good results. Haven't tried Gaussian, but various self-written mathematical (including floating point) applications have run with the expected speed. SLES 8 for AMD64 and PGI 5.0, now up to PGI 5.1. With this said, we've mostly been using the C++ compiler.
I did not say that the performance in the 64-bit environment is not good, just that the floating point performance is currently somewhat better in the 32-bit environment.
I would contemplate the possibility of a weakness in the Gaussian code, though.
I wouldn't blame Gaussian at all, since it has practically the same code compiled on a wide range of machines, with a mature support for 64-bit machines. Anyway, we will see when Gaussian provides official support Opterons. I did what I could, however, my port was not an "official" one.
Bjørn
Best wishes, Ödön