Rapid PHY Selection (RPS): A Performance Evaluation of Control Policies
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1 Rapid PHY Selection (RPS): A Performance Evaluation of Control Policies Ken Christensen Department of Computer Science and Engineering University of South Florida Tampa, FL christen@cse.usf.edu (813) This material is based upon work funded by the National Science Foundation under grant CNS of 45 IEEE EEE Study Group January 15, 2007 ieeetalk07.ppt (January 6, 2007)
2 Acknowledgments Much of the work was done by Chamara Gunaratne as a graduate student at USF Much of the work was done in collaboration with Bruce Nordman of LBNL 2of 45 IEEE EEE Study Group January 15, 2007
3 Agenda Motivation and introduction Control policies Effects on higher layers Summary and future directions 3of 45 IEEE EEE Study Group January 15, 2007
4 Agenda Motivation and introduction Control policies Effects on higher layers Summary and future directions 4of 45 IEEE EEE Study Group January 15, 2007
5 Motivation Goal is Energy Efficient Ethernet Many Ethernet links are idle most of the time This applies especially to desktop links Does not apply to all Ethernet links Match link rate with link utilization for efficiency Lower link rate consumes less power Must always have highest link rate available if/when needed Savings potential (at the wall socket) 2 to 4 W per link for 1 Gb/s versus 100 Mb/s 10 to 20 W per link for 10 Gb/s versus 1 Gb/s 5of 45 IEEE EEE Study Group January 15, 2007
6 Motivation continued Existing laptop NICs drop link rate to save power When entering sleep or off state When on battery power Existing auto-negotiation is used Requires 3 to 4 seconds to switch link data rate Not acceptable for real time use What is needed A fast method to switch link rate for real time use 6of 45 IEEE EEE Study Group January 15, 2007
7 Introduction to RPS RPS is Rapid PHY Selection A possible mechanism for fast switching of link rate Supported on both ends of a link RPS mechanism could be a MAC frame handshake Desktop LAN switch ALR Request MAC frame ALR ACK MAC frame Resynchronize link Switching time Time 7of 45 IEEE EEE Study Group January 15, 2007
8 Introduction to RPS continued RPS is a mechanism only Need a control policy to determine when to switch link rate Outside the scope of any possible RPS standard Multiple control policies are possible This can be a competitive advantage for vendors We investigate technical feasibility The control policy determines possible energy savings 8of 45 IEEE EEE Study Group January 15, 2007
9 Agenda Motivation and introduction Control policies Effects on higher layers Summary and future directions 9of 45 IEEE EEE Study Group January 15, 2007
10 Control policy trade-off Fundamental trade-off is Time in low data rate versus packet delay Want lowest possible packet delay? Use only highest link rate at all times Want lowest possible energy use? Use only lowest link rate at all times Seek a balance 10 of 45 IEEE EEE Study Group January 15, 2007
11 Control policy goals A good control policy is 1) Simple 2) Responsive to changes in link utilization 3) Does not cause oscillation of link rate Resulting in Low and bounded packet delay Maximized energy savings 11 of 45 IEEE EEE Study Group January 15, 2007
12 Dual-threshold policy Use thresholds in output buffers If queue is above qhigh then switch to high rate If queue is below qlow then switch to low rate switch link desktop PC qlow qhigh qhigh qlow packets packets queue thresholds in the NIC queue thresholds in the switch port 12 of 45 IEEE EEE Study Group January 15, 2007
13 Dual-threshold policy continued FSM for dual-threshold policy HIGH LOW 1 (reqlow = true) (tretry expired) (qlen < qlow) send goto low MAC frame, reset tretry 2 3 receive a NACK reqlow = false (receive an ACK) (detect link resynchronization) resynchronize link 4 (receive goto low MAC frame) (qlen < qlow) send ACK, resynchronize link 5 (receive goto low MAC frame) (qlen < qlow) send NACK, reqlow = true receive goto high MAC frame send ACK, resynchronize the link 6 WAIT (receive an ACK) (detect link resynchronization) resynchronize link 7 qlen > qhigh 8 send goto high MAC frame, reset tretry receive goto high MAC frame 9 10 tretry expired send ACK, resynchronize the link 13 of 45 IEEE EEE Study Group January 15, 2007
14 Dual-threshold policy continued A pseudocode process description Handshake details omitted Executes on receiving a frame if (link rate is low) if (buffer length greater than qhigh) handshake for high link rate Executes on transmitting a frame if (link rate is high) if (buffer length less than qlow) handshake for low link rate 14 of 45 IEEE EEE Study Group January 15, 2007
15 Dual-threshold policy continued This policy can cause oscillation of link rate Will occur with smooth traffic at medium utilization Can be dampened with timers up timer for high link rate down timer for low link rate But, there may be a better policy 15 of 45 IEEE EEE Study Group January 15, 2007
16 Utilization-threshold threshold policy Make explicit utilization measurement part of policy Prevents drop to low rate when not warranted Executes on receiving a frame if (link rate is low) if (buffer length greater than qhigh) handshake for high link rate Executes on transmitting a frame if (link rate is high) if (buffer length less than qlow) if (measured utilization less than low target) handshake for low link rate 16 of 45 IEEE EEE Study Group January 15, 2007
17 Performance evaluation Implemented simulation model of RPS and policies Queuing model built using CSIM19 Used trace and synthetic traffic as input Varied the link rate switching time Control variables Utilization of input traffic Burstiness of input traffic Link rate switching time Response variables Packet (or burst) delay Time in low link rate 17 of 45 IEEE EEE Study Group January 15, 2007
18 Time scales of interest How long is a link idle between bursts of packets? Time the user leaves his or her PC Seconds, minutes, to hours Time between data transfer bursts (active use) Milliseconds to seconds For example, web surfing 18 of 45 IEEE EEE Study Group January 15, 2007
19 Trace traffic characteristics Collected packet level traces from 100 Mb/s links Note very low average utilization Trace Duration Description Avg util. USF #1 0.5 hours Link to busiest user in USF 4.11 % USF #2 0.5 Link to 10th busiest user 2.63 USF #3 0.5 Link to an average user 0.03 PSU #1 2.0 Link to a desktop PC 0.13 PSU #2 2.0 Link connecting two switches 1.01 PSU #3 2.0 Link connecting switch to router 1.03 PSU traces are from Suresh Singh at Portland State University 19 of 45 IEEE EEE Study Group January 15, 2007
20 Results with trace traffic Simulation results for dual-threshold policy Use 1 millisecond link rate switching time For 1 Gb/s the delays would be smaller Trace 10 Mb/s 100 Mb/s Delay Time in low rate USF # ms 0.09 ms 2.79 ms % USF # USF # PSU # PSU # PSU # Time in 10 Mb/s 20 of 45 IEEE EEE Study Group January 15, 2007
21 Results with trace traffic continued Simulation results for dual-threshold with timers Use 1 millisecond link rate switching time Lower delay is due to less oscillation Trace 10 Mb/s 100 Mb/s Delay Time in low rate USF # ms 0.09 ms 2.40 ms % USF # USF # PSU # PSU # PSU # Time in 10 Mb/s 21 of 45 IEEE EEE Study Group January 15, 2007
22 Results with trace traffic continued Simulation results for utilization-threshold policy Use 1 millisecond link rate switching time Even lower delay, but less time in low rate Trace 10 Mb/s 100 Mb/s Delay Time in low rate USF # ms 0.09 ms 1.47 ms % USF # USF # PSU # PSU # PSU # Time in 10 Mb/s 22 of 45 IEEE EEE Study Group January 15, 2007
23 Simulation with Poisson traffic Use Poisson to model smooth traffic Model a 1 Gb/s link Link switching time is 1 millisecond For utilization-threshold policy tutil is the time period used for sampling utilization Set utilization threshold to be 5% of 1 Gb/s 23 of 45 IEEE EEE Study Group January 15, 2007
24 Results for Poisson traffic Results for both policies Dual-threshold stays high due to oscillation! Mean response time (ms) tutil = 1ms tutil = 0.1ms tutil = 10ms tutil = 100 ms Dual-threshold Utilization (% of 1 Gb/s) tutil is the sample time for utilization 24 of 45 IEEE EEE Study Group January 15, 2007
25 Simulation with bursty traffic Synthetically generated 1 Gb/s bursty traffic Pareto burst size, exponential interburst time Mean burst size is small (about 8.4 KB) Small burst size is worst case for energy savings Link switching time set to 1 millisecond 25 of 45 IEEE EEE Study Group January 15, 2007
26 Results for bursty traffic Results for mean packet delay Again, dual-threshold stays high due to oscillation! Mean response time (ms) tutil = 100 ms tutil = 10ms Utilization (% of 1 Gb/s) Dual-threshold tutil = 1ms 0.5 ms delay at 5% utilization 26 of 45 IEEE EEE Study Group January 15, 2007
27 Results for bursty traffic continued Results for time in low data rate Time in low rate (%) tutil = 100 ms tutil = 10ms Utilization (% of 1 Gb/s) Dual-threshold tutil = 1ms 80% time in low rate at 5% utilization 27 of 45 IEEE EEE Study Group January 15, 2007
28 Simulation of a single burst Transfer time of a single burst Vary the burst size Vary the link switching time At what point can RPS be noticed by a user? 28 of 45 IEEE EEE Study Group January 15, 2007
29 Results for transfer time of a burst Results for transfer time of a single burst Note that human just noticeable time is 100 milliseconds Transmission time 1000 s 10 s 100 ms 1 ms Rate switching time = 100 ms 10 ms 1 ms RPS is probably not noticeable = 1 Gb/s = 100 Mb/s 0.01 ms 10KB 100KB 1MB 10MB 100MB 1GB Burst size 29 of 45 IEEE EEE Study Group January 15, 2007
30 Wrap-up of control policy topic Multiple control policies are possible Three have been shown here Each side is independent and will interoperate Trade-off in packet delay versus energy saved Performance summary: About 0.5 millisecond increase in packet delay About 80% time in low data rate Is increase in delay noticeable to a user? 30 of 45 IEEE EEE Study Group January 15, 2007
31 Agenda Motivation and introduction Control policies Including humans (users) Effects on higher layers Summary and future directions 31 of 45 IEEE EEE Study Group January 15, 2007
32 Effects of RPS on users Users will disable power management if annoying Yet some users may tolerate slight annoyance in order to be green RPS with suitable policy should be entirely invisible To most users 32 of 45 IEEE EEE Study Group January 15, 2007
33 Effects of RPS on users continued Need to consider Effect of increased packet delay If packet loss can occur (and its effect) due to buffer overflows Human perception time is about 100 milliseconds Delays below this threshold are unnoticeable 33 of 45 IEEE EEE Study Group January 15, 2007
34 Effects of RPS on desktop applications Data transfer applications Web surfing File downloading Playback streaming application YouTube and etc. Realtime streaming applications VoIP telephony Try it! Surprising result: Video players can buffer about six seconds of video in the first second of play. So even a four second link outage causes no effect. 34 of 45 IEEE EEE Study Group January 15, 2007
35 Effects of RPS on TCP/IP Will a switching delay cause packet loss? What is the effect of packet loss? We have some preliminary work Using the ns-2 simulator 35 of 45 IEEE EEE Study Group January 15, 2007
36 Simulation of TCP/IP Preliminary results Use ns2 simulator to evaluate effects of RPS on TCP Study LAN and WAN environments FTP between two nodes via an intermediate node All default except window_ set to 1000 (default is 20) This 100 Mb/s link is disabled and then re-enabled at 1 Gb/s (switching time = 1 ms) desktop PC server link switch Delay is 1ms (LAN) or 15ms (WAN) Data rate = 1 Gb/s always 36 of 45 IEEE EEE Study Group January 15, 2007
37 Simulation of TCP/IP continued Preliminary results Important note on ns2 All buffers are destroyed in an ns2 model when a link is disabled/enabled. So, results here are much worse than realistic. We plan to develop a realistic model. 37 of 45 IEEE EEE Study Group January 15, 2007
38 Simulation of TCP/IP continued Preliminary results ns2 LAN environment Shows TCP congestion window (transition at Time = 5 s) fig7.xls Congestion window (pkts) Tahoe Reno NewReno Time (s) 38 of 45 IEEE EEE Study Group January 15, 2007
39 Simulation of TCP/IP continued Preliminary results ns2 WAN environment Shows TCP congestion window fig8.xls Congestion window (pkts) Tahoe Reno NewReno Time (s) 39 of 45 IEEE EEE Study Group January 15, 2007
40 Simulation of TCP/IP continued Preliminary results ns2 WAN environment Shows TCP congestion window (with 10 ms switching time) fig9.xls Congestion window (pkts) Tahoe Reno NewReno Time (s) 40 of 45 IEEE EEE Study Group January 15, 2007
41 Agenda Motivation and introduction Control policies Effects on higher layers Summary and future directions 41 of 45 IEEE EEE Study Group January 15, 2007
42 Summary Simple RPS control policies are possible Utilization-threshold control policy works well Added packet delay is very small Energy savings can be considerable Energy efficiency should be invisible Can be achieved with good RPS control policy 42 of 45 IEEE EEE Study Group January 15, 2007
43 Future work Further work is possible in control policies Context driven Adaptive based on user input Further work is possible in evaluating RPS effects Build a h/w device to kill link to emulate RPS switching Currently experimenting with a Soft-ALR ALR = Adaptive Link Rate ALR is RPS + control policy Use auto-negotiation to switch link rate via a Linux script Experience a 4 second link switch delay 43 of 45 IEEE EEE Study Group January 15, 2007
44 Future work continued Soft-ALR is now on SourceForge By Matt Landau (USF student) 44 of 45 IEEE EEE Study Group January 15, 2007
45 The End Any questions? Much of the material in this talk comes from C. Gunaratne, K. Christensen, B. Nordman, and S. Suen, Reducing the Energy Consumption of Ethernet with Adaptive Link Rate (ALR), submitted to IEEE Transactions on Computers in January C. Gunaratne, K. Christensen, and S. Suen, Ethernet Adaptive Link Rate (ALR): Analysis of a Buffer Threshold Policy, Proceedings of IEEE GLOBECOM 2006, November C. Gunaratne and K. Christensen, Ethernet Adaptive Link Rate: System Design and Performance Evaluation, Proceedings of the 31st IEEE Conference on Local Computer Networks, pp , November of 45 IEEE EEE Study Group January 15, 2007
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