This paper begins with three observations:
- Energy-related costs are an increasingly large portion of total data center operating expenses.
- The cost of electricity can vary significantly between different times and between different regions at the same time.
- Many distributed systems already have the ability to dynamically route requests to different hosts and physical regions (for example, most CDNs try to minimize client latency and bandwidth costs by routing requests to a data center “close” to the client.)
Therefore, the paper investigates the feasibility of shifting load among replicas of a service that are located in different geographic regions, according to the current price of electricity in each region. For this to be effective, several things must be true:
- There must be significant variation in the price of electricity available in different regions at the same time.
- Data centers must be energy proportional: as the load on a data center is decreased by a factor of k, its energy usage should decrease by the same factor.
- Routing traffic to minimize the cost of electricity may result in increasing client latency and using more bandwidth (since cheap power might be far away from the client); the additional routers traversed might also use additional energy.
To answer the first question, the authors conduct a detailed empirical study of the cost of energy in different regions across the US, and compare that information with traffic logs from Akamai’s CDN. The authors use the Akamai traffic data to estimate how much the cost of electricity could be reduced by routing requests to the cheapest available electricity source, subject to various additional constraints.
The authors don’t do much to address the second question: they admit that the effectiveness of this technique depends heavily on energy-proportionality, but most computing equipment is not very energy-proportional (idle power consumption of a single system is typically ~60% of peak power usage, for example). Since energy-proportionality is the subject of much recent research, they express the hope that future hardware will be more energy-proportional. Finally, they carefully consider the impact of electricity-price-based routing on other optimization goals: for example, they consider only changing routes in a way that doesn’t result in any increased bandwidth charges (due to the “95-5” pricing scheme that most bandwidth providers use). A realistic implementation of this technique would consider electricity cost as one factor in a multi-variable optimization problem: we want to simultaneously minimize electricity cost, minimize client-perceived latency and minimize bandwidth charges, for example.
Summary of Results
The authors found significant asymmetry in electricity prices between geographic areas; furthermore, this asymmetry was dynamic (different regions were cheaper at different times). These are promising results for dynamic routing of requests based on electricity prices.
When cluster energy usage is completely proportional to load and bandwidth cost is not considered, price-sensitive routing can reduce energy costs by ~40%. The savings drop to only 5% if the energy-proportionality of current hardware is used, and the savings drop to a third of that if we are constrained to not increase bandwidth costs at all (assuming 95-5 pricing). Hence, this technique is only really effective if energy-proportional data centers are widely deployed.
I thought this was a great paper. The basic idea is simple, but their empirical study of the US electricity market was carefully done, and the results are instructive.
One interesting question is what would happen to the electricity market if techniques like these were widely deployed. Essentially, electricity consumption would become more price-elastic. When a given region offers a lower price, demand could move to that region quite quickly, which might act to drive up the price. Conversely, it would lower demand in higher-priced regions, lowering the price — and hence benefiting more inelastic energy consumers in that region.