energy costs
Josip Loncaric
josip at nianet.org
Tue Mar 11 07:40:10 PST 2003
Helms, Scott A wrote:
> [...] I asked about kilowatts per gigaflops [...]
Your concern about KW/Gflop is completely justified. Both IBM and Cray
salesmen will happily advertise their power efficiency. Today's CPUs are
primarily constrained by their thermal limits, and about half of the time
spent in CPU design workshops is spent on CPU power management. As a result,
we get interesting "features" such as thermal throttling on CPUs, memory, etc.
In theory, one could use reversible physical processes for computing and thus
recover all of the energy by reversing the computation at the end. This kind
of computer would require no net power, although some energy would be needed
to reach the desired result. However, this computer would not be allowed to
forget any of the states it went through. In some sense, power dissipation is
the penalty we pay for forgetting -- and thanks to speculative evaluation,
today's CPUs evaluate then forget even more than is commonly known.
I can't seem to find my KW/Gflop data right now, but I believe that in that
regard a machine such as Cray X1 compares rather favorably with Beowulf
clusters. Both kinds of systems will improve power efficiency each year, but
their relative merit will probably evolve quite slowly...
Sincerely,
Josip
--
Dr. Josip Loncaric, Research Fellow mailto:josip at nianet.org
National Institute of Aerospace http://research.nianet.org/~josip/
144 Research Drive mailto:j.loncaric at larc.nasa.gov
Hampton, VA 23666, USA Tel. +1-757-766-1395 Fax +1-757-766-1401
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