Seven Grams Revisited
By rolfk on Jan 16, 2009
After the "7 gram story" not only did hit Techcrunch, but also my favourite german-language news site, let's revisit my original calculation:
In 2005, Prof. Paul A. Strassmann George Mason University, held a presentation “Google - A Model for the Systems Architecture of the Future”, pdf here. Slide 11 says that in those days, 31654 2-socket x86 servers worked in parallel to serve 40 million searches a day.
So how much energy does a 2-socket x86 server at Google consume? Google engineer Luiz André Barroso gave an answer in his 2007 paper "Power Provisioning for a Warehouse-sized Computer". They measured 143 Watts.
These 143 Watts have to be multiplied with a power factor. In 2007 I used a factor of 2 (the industry standard, meaning: "For every Watt fed into the computer you feed another Watt in Cooling etc.). In 2008 Google reported a PUE factor of 1.21. With this, every server actually draws 173 watts for power and cooling combined.
So 31654 servers times 173 watts times 24 hours gives us 131427 KWh energy a day. Divided by 40mln searches, that's 3.3 Wh or 2 grams CO2 per search, using 600 grams of CO2 for every KWh generated with the german energy mix of nuclear, carbon-based and renewable sources
In 2005, a dual socket x86 server meant two cores. Today we have eight cores and roughly four times the throughput per watt, bringing us down to 0.8 Wh or 0.5 grams CO2 per search
So the difference between the "7 grams" from 2007 and this calculation today is caused by: Quad Core CPUs, lower measured power draw of the servers, PUE of 1.21, carbon factor calculated with energy mix instead of coal only. Net result: A factor of 12
And the real number Google disclosed this week is even lower: 0.3Wh or 0.2 grams CO2
That is really not much: If you start a search every 30 seconds on a typical Laptop (30 Watts), your client uses more energy triggering than Google executing the search.