Cutting the Electric Bill for Internet-Scale Systems

Summary

The key idea of this paper is to shift load to data centers with lower energy costs. The authors make two observation that enable the proposed design:

1. Electricity prices vary. The price of electricity varies both in time and by location. For example, prices at night are generally less than during the day and prices for locations closer to generators are less than far away locations.
2. Large distributed systems already incorporate request routing and replication. Most internet services can easily route requests to one of multiple locations.

The savings in energy cost depends on the "energy elasticity" (also known as power proportionality) of clusters. In other words, how much does the energy consumption drop in response to a drop in load. The authors estimate that current servers use about 60% of peak power when idle, but they expect this number to improve.

The authors examine the price variations for energy over time and in various locations. They show that prices often vary by a factor of 2 with a day. They also show that prices are not correlated across different locations thus enabling savings by redirecting load.

Overall, the authors estimate that they can save 2% of energy costs now and that savings increase rapidly with system energy elasticity. In a fully elastic system they expect over 30% cost savings.

Thoughts

This paper presents a neat idea for cost savings, however, as the authors point out it hinges on the energy elasticity of servers. Without improvements in elasticity the savings would be fairly negligible. I suspect that this may be part of the reason companies don't bother to do this right now.