Skilled in the Art of Being Idle: Reducing Energy Waste in Networked Systems

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).

One line summary: This paper examines the value of using proxies to handle idle-time traffic for sleeping hosts with the goal of reducing wasted energy consumption in networked end-systems; it does this by analyzing and classifying traffic to see what can be ignored or automatically handled and by examining several potential proxy designs.

Summary

This paper examines the problem of reducing wasted energy consumption in powered-on but idle networked end-systems such as desktops in home and office environments. It discusses various solutions, and in particular examines the value of using a proxy to handle idle-time traffic on behalf of sleeping hosts. The main problem is that while vendors have built in hardware support for sleep (S-states) to reduce power consumption while idle, surveys of office buildings indicate that the vast majority of machines are fully on while idle, instead of taking advantage of these sleep states. One reason for this is that a sleeping machine loses its network presence i.e. it cannot send or receive network messages, and another reason is that users or administers occasionally want to be able to schedule tasks to run during these idle times. Both of these reasons cause users to not make use of sleep states. This paper thus tries to answer the following questions: (1) Is the problem worth solving? (2) What network traffic do idle machines see? (3) What is the design space for a proxy? (4) What implications does proxying have for future protocol and system design.

To begin to answer these questions, the authors first collect network and user-level activity traces from 250 client machines belonging to Intel employees and attempt to classify the traffic. They first classify each packet as being either broadcast, multicast, or unicast, then whether it is incoming or outgoing. They find outgoing traffic tends to be dominated by unicast while incoming traffic is made of significant proportions of all three. They estimate the potential for sleep in four scenarios: (a) ignore broadcast and wake for the rest, (b) ignore multicast and wake for the rest, (c) ignore both broadcast and multicast, and (d) wake for all packets. They find that broadcast and multicast are mainly responsible for reducing the amount of potential sleep time and that doing away with just one of broadcast or multicast is not effective. The authors next classify the traffic based on protocol type, and evaluate each protocol on two metrics: total volume of traffic, and something the authors call half-sleep time. A high half-sleep time means that protocol’s packets could be handled by waking the machine up, whereas a low half-sleep time means that to achieve useful amounts of potential sleep time the proxy would have to handle them. They find that the bulk of broadcast traffic is for address resolution and service discovery, and a lot of other broadcast traffic is from router-specific protocols. Broadcast traffic allows for very little sleep in the office, but significantly more in the home. A proxy could easily handle most of these broadcast protocols. Multicast traffic is mostly caused by router protocols and is often absent or extremely reduced in homes as compared to offices. All router traffic is ignorable. From their analysis of unicast traffic, they speculate that it might be possible to ignore or eliminate much of unicast traffic. Finally, they classify traffic into one of the following three categories regarding the need to proxy that traffic: don’t wake, don’t ignore, and policy-dependent, and into one of the following three categories regarding the difficulty in proxying that traffic: ignorable/drop, handle via mechanical responses, and require specialized processing.

Next, they present four proxy designs. The first ignores all traffic classified as ignorable and wakes the host for the rest. The second ignores all traffic classified as ignorable, responds to traffic listed as capable of being handled by mechanical responses, and wakes the machine for the rest. The third does the same as the second except that it wakes up for traffic belonging to a certain set and drops any other incoming traffic. Lastly, the fourth does the same as the third except that it also wakes up for a certain set of scheduled tasks. They find that the simplest proxy, proxy 1, is inadequate for office environments and nearly inadequate for home environments, but that proxy 3 achieves a good amount of sleep time in all scenarios – more than 70% of the idle time. They also find that the effectiveness of proxy 2 depends a great deal on the environment. Given this, the best trade-off between complexity of design and power savings depends on the environment. Also, the authors also note that since scheduled wake-ups are infrequent, the impact on sleep is minimal, so proxy 4 performs practically the same as proxy 3. Finally, they offer a basic proxy architecture that could serve as a framework to building the different proxies designs they considered, and to demonstrate the feasibility of building a proxy, they implemented a simple proxy prototype in Click. The authors end by speculating about how systems could be redesigned to make them more power-aware, thereby simplifying the implementation of proxies, making proxies more effective, or eliminating the need for proxies altogether.

Critique

One thing I really liked about this paper is how the authors analyzed and classified network traffic before considering proxy design. In retrospect it seems absolutely necessary to guiding the design of a proxy. It was also just informative in general to look at the traffic traces from the perspective of which packets are ignorable, which can be handled automatically, and which are actually “important”. I also liked how they examined several points in the proxy design space and compared them. Overall I thought this was a very thoughtful and well organized paper and I think it should stay in the syllabus.