bang for buck?
josip at icase.edu
Fri Mar 8 06:52:34 PST 2002
"Geoffrey D. Jacobs" wrote:
> Generally, SMP should help performance compared w/ 2 single CPUs (by
> Amdahl's rule) as average communcation speed is much higher. All in
> all, though, it depends on the application.
The phrase "it depends" is more accurate than the word "generally." For
a comparison of NAS benchmarks on singles/duals using different MPI
libraries and comparing TCP (fast ethernet) to VIA (Giganet cLAN)
Contention for shared resources (e.g. memory) and juggling different
connections (network/internal) on duals has significant effects. In my
tests, all 16-process NAS benchmarks run faster and more stably using
16x1 CPUs than 8x2 CPUs, regardless of the MPI library or network
The moral: Don't confuse high intra-node bandwidth with application
performance. Other bottlenecks often dominate.
Dr. Josip Loncaric, Research Fellow mailto:josip at icase.edu
ICASE, Mail Stop 132C PGP key at http://www.icase.edu./~josip/
NASA Langley Research Center mailto:j.loncaric at larc.nasa.gov
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