mpg and perf/watt are misleading
Monday Jun 23, 2008
Last friday I blogged about an article on Duke University's Larrick & Soll's research:
Posting a vehicle’s fuel efficiency in “gallons per mile” (GPM) rather than “miles per gallon” (MPG) would help consumers make better decisions about car purchases and environmental impact, researchers from Duke University’s Fuqua School of Business report in the June 20 issue of Science magazine.The main issue is that people usually make comparisons by linear improvement in miles/gallon, but this leads most to errors. Switching to gallons/mile (and as I said for servers watt/performance) avoids these problems.
If one does the calculations correctly of course it doesn't matter, but on a quick look one can be mislead. For example, (do this quickly!) if one climbs 10 miles a hill and gets 10mpg and then coasts down the hill for 10 miles getting 100 mpg, how many mpg does one average? If you didn't come up with an answer of 18mpg (or nearly double the uphill rate), then you should consider looking at the reciprocal calculation.
If on that same hill that same car gets 1 gal/10 miles (10mpg) uphill and 0.1gal/10miles (100mpg), then it is easy to that coasting downhill can only come close doubling your fuel efficiency. Even if you doubled your fuel efficiency on the downhill section to 200mpg (0.05gal/10 miiles) you can see that your average fuel efficiency doesn't change much.
As I've said before on servers it is also critical to understand watt/performance on a wide variety of benchmarks, Sun understands this. This way you avoid benchmarks were vendors only highlight small-memory and low-GHz configurations.
Finally increase your server utilisation (even a small amount) and closely look at power-performance (watt/perf).
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