S. Nedevschi, J. Chandrashekar, J. Liu, B. Nordman, S. Ratnasamy, N. Taft, “Skilled in the Art of Being Idle: Reducing Energy Waste in Networked Systems,” NSDI’09, (April 2009). [PDF]

Summary

This paper argues that putting networked end-systems into low-power sleep modes, instead of keeping them in higher-power-consuming idle states, can result in significant energy savings. However, a sleeping device loses its network presence and can prevent running scheduled tasks. There are two main approaches that address these issues: first, wake-on-lan (WoL) mechanisms wake machines up at the arrival of specific packets; and second, the use of a proxy that handles packets for the sleeping machine unless it is absolutely necessary to wake the machine up. The authors study the pros and cons of the two approaches based on data collected from 250 enterprise machines and present a proxy architecture with a narrow API interface to support different idle states.

From their measurement data, the authors found that the machines under study are active for only 10% of the time and on the average 50% of the time they are idle; only a small fraction are put to sleep at all. This observation suggests that there is enormous opportunity of saving energy by exploiting the sleep states. The authors also notice that there is always a steady flow of packets, which makes WoP (wake-on-packet) an infeasible choice.

In the proxy-based solution, packets destined for a sleeping host are intercepted by its proxy. The proxy decides whether to ignore/drop the packet or to respond somehow or to wake up the machine it is representing. To make judicious decisions, the authors identified different classes of packets (e.g., incoming/outgoing, unicast/broadcast/multicast) and found that both broadcast and multicast are largely responsible for poor sleep (80% more sleep time in home environment and 50% more in office environment). They deconstructed different classes of traffic and ended up with some ground rules about what to do when a proxy sees different types of packets.

Idle-time traffic have been differentiated along two different dimensions. The first classifies traffic based on the need to proxy the traffic (protocol) in question into three categories (don’t-wake protocols, don’t ignore protocols, and policy-dependent protocols). The second one identifies the complexity of decision making (ignorable(drop), handled via mechanical response, and require specialized processing). The end result is a firewall like architecture, where the power-proxy table consists of a list of rules with each rule consisting of a <trigger, action, timeout> tuple. For incoming packets, the proxy matches the rules and performs necessary actions. The authors also built a simple Click-based prototype of their proposed solution.

Comments

This is yet another very good measurement paper. Specially, the study of various types of traffic in different network environment is an eye-opener.

The solution presented in the paper is very much similar to firewall; so an intelligent guess would be that a lot of firewall literature might come in handy in solving the complexities of matching rules.