NAPman: Network-Assisted Power Management for WiFi Devices

Transcription

1 NAPman: Network-Assisted Power Management for WiFi Devices Eric Rozner Presented by University of Texas at Austin Andy Pyles November 10th,

2 Introduction 2 WiFi-enabled mobile devices increasingly popular WiFi radio increases power by 3x (HTC Tilt) Power (mw) WiFi WiFi off on Base Base HTC Tilt Power Consumption 2

3 3 Power Save Mode Clients use Power Save Mode (PSM) to power down Power (mw) interface AP buffers data on client s behalf Saves power while client idles WiFi off WiFi on WiFi PSM Base Base Base HTC Tilt Power Consumption 3

4 PSM Problems in the Wild 4 1. Scheduling PSM data with competing background traffic Up to 4x increase in client energy usage Contention! Unfair to PSM & background traffic Decrease in capacity from unnecessary retx s 4

5 PSM Problems in the Wild 5 2. Multiple s s contend in current schemes Causes up to 45% increase in energy consumption Contention! 5

6 PSM Problems in the Wild 6 3. Heterogenous client PSM implementations Clients implement variety of schemes AP s don t distinguish clients Amplifies problems 6

7 NAPman 7 1. Energy-efficient fair scheduling algorithm mitigates effect of competing background traffic Up to 70% energy savings Fair to PSM & background traffic Eliminate unnecessary retx s to avoid wasting capacity 2. Virtualized APs for scheduling competing PSM nodes Avoiding contention leads to energy savings 3. AP-based adaptation to support heterogenous clients No changes to pre-existing clients or standards! 7

8 Roadmap 8 Power Save Mode (PSM) in standard Static s Adaptive s PSM scheduling & problems w/ competing traffic NAPman design and implementation Evaluation Related Work & Conclusion 8

9 Static PSM 9 zzz 9

10 Static PSM 9 zzz AP buffers packets for sleeping client 9

11 Static PSM 9 1 Periodic beacon messages indicate buffered data 9

12 Static PSM 9 1 Periodic beacon messages indicate buffered data 9

13 Static PSM 9 PS-Poll Client notifies AP it is awake 9

14 Static PSM 9 AP enqueues data and sets MORE bit 9

15 Static PSM 9 Client retrieves data until buffer is empty 9

16 Static PSM 9 zzz Client returns to sleep after retrieving data 9

17 Adaptive PSM 10 adaptive PSM 10

18 Adaptive PSM 10 null:awake adaptive PSM Client sends NULL frame with power bit set 10

19 Adaptive PSM 10 adaptive PSM AP enqueues all buffered & newly arriving data 10

20 Adaptive PSM 10 zzz adaptive PSM Client informs AP of pending sleep after inactivity timeout 10

21 Client Implementations 11 WiFi Client PSM Mode HP ipaq hw6945 iphone 3GS gphone HTC Magic HTC Tilt 8900 Static PSM Adaptive PSM Aggressive timeout Adaptive PSM Conservative timeout Static PSM 11

22 Roadmap 12 Power Save Mode (PSM) in standard PSM scheduling & problems w/ competing traffic Normal Scheduling High Priority Scheduling NAPman design and implementation Evaluation 12

23 AP s in the Wild 13 Access Point Linksys BEFW11S4 v4 Madwifi based D-Link DIR n DD-WRT v23 SP2-based Linksys WRT54GL Linksys WRT310N n Netgear WGR614v7 PSM Scheduling Normal Normal High Priority High Priority Normal High Priority Normal 13

24 Roadmap 14 Power Save Mode (PSM) in standard PSM scheduling & problems w/ competing traffic Normal Scheduling Energy Wastage Unnecessary Retransmissions Unfairness to PSM traffic 14

25 Roadmap 15 Power Save Mode (PSM) in standard PSM scheduling & problems w/ competing traffic Normal Scheduling Energy Wastage Unnecessary Retransmissions Unfairness to PSM traffic refer to paper 15

26 Normal Scheduling 16 Background client 16

27 Normal Scheduling 16 PS-Poll Background client 16

28 Normal Scheduling 16 Normal Scheduling appends to queue Background client 16

29 Normal Scheduling 16 Normal Scheduling appends to queue Background client 16

30 Normal Scheduling 16 Client keeps highpowered WiFi on while queue drains! Normal Scheduling appends to queue Background client 16

31 Normal Scheduling 16 Client keeps highpowered WiFi on while queue drains! Normal Scheduling appends to queue Background client 16

32 Normal Scheduling 16 Client keeps highpowered WiFi on while queue drains! zzz Background client Normal Scheduling appends to queue 16

33 Normal Scheduling 16 Normal Scheduling Client keeps highwastes energy when powered WiFi on while there s competing queue drains! network traffic zzz Background client Normal Scheduling appends to queue 16

34 Roadmap 17 Power Save Mode (PSM) in standard PSM scheduling & problems w/ competing traffic Normal Scheduling Energy Wastage Unnecessary Retransmissions Unfairness to PSM traffic refer to paper 17

35 Unnecessary Retx s 18 adaptive PSM wakes for buffered data Background client 18

36 Unnecessary Retx s 18 adaptive PSM null:awake wakes for buffered data Background client 18

37 Unnecessary Retx s 18 adaptive PSM waits for its data Background client 18

38 Unnecessary Retx s 18 adaptive PSM Some inactivity time-outs are aggressive! Background client 18

39 Unnecessary Retx s 18 adaptive PSM Some inactivity time-outs are aggressive! Background client 18

40 Unnecessary Retx s 18 zzz adaptive PSM null:sleep Background client Inactivity timer fires and client sleeps 18

41 Unnecessary Retx s 18 zzz adaptive PSM Background client 18

42 Unnecessary Retx s 18 zzz adaptive PSM Background client Client asleep, data unnecessarily retx d & lost! 18

43 Unnecessary Retx s 18 Limits capacity: Unnecessary retx s waste airtime and also can lower data rate zzz adaptive PSM Background client Client asleep, data unnecessarily retx d & lost! 18

44 Roadmap 19 Power Save Mode (PSM) in standard PSM scheduling & problems w/ competing traffic Normal Scheduling High Priority Scheduling NAPman design and implementation Evaluation 19

45 Roadmap 20 Power Save Mode (PSM) in standard PSM scheduling & problems w/ competing traffic Normal Scheduling High Priority Scheduling Unfairness to background traffic NAPman design and implementation Evaluation 20

46 High Priority Scheduling 21 Background client 21

47 High Priority Scheduling Background client 21

48 High Priority Scheduling Background client 21

49 High Priority Scheduling PS-Poll Background client 21

50 High Priority Scheduling High Priority bypasses Background client 21

51 High Priority Scheduling Saves energy & eliminates unnecessary retx s at expense of fairness High Priority bypasses zzz Background client 21

52 High Priority Starvation 22 Throughput (Kbps) Static, saturated CBR demands, 1Mbps PHY PSM throughput Background average Number of Background Clients A high-priority unfairly takes 1/2 capacity 22

53 Roadmap 23 Power Save Mode (PSM) in standard PSM scheduling & problems w/ competing traffic NAPman design and implementation Evaluation Related Work & Conclusion 23

54 Quick Recap 24 Problems of current PSM scheduling techniques Energy wastage due to competing traffic Unfairness Unnecessary retransmissions waste capacity Solution goals Energy-efficient, but fair to network traffic Support multiple s in network Compatible w/ adaptive & static s 24

55 Roadmap 25 Power Save Mode (PSM) in standard PSM scheduling & problems w/ competing traffic NAPman design and implementation Problems with first-come, first-served (FCFS) NAPman design and examples Evaluation 25

56 FCFS Scheduling Background client 26

57 FCFS Scheduling Normal Scheduling unfair to PSM traffic Background client 26

58 FCFS Scheduling High Priority Scheduling unfair to background Background client 26

59 FCFS Scheduling Fair Scheduling would waste energy Background client 27

60 FCFS Scheduling Fair Scheduling would waste energy Background client Need scheduler that s fair and energy efficient 27

61 Roadmap 28 Power Save Mode (PSM) in standard PSM scheduling & problems w/ competing traffic NAPman design and implementation Problems with first-come, first-served (FCFS) NAPman design and examples Evaluation 28

62 NAPman Design 29 Scheduling algorithm for PSM traffic 1. NAPman scheduler: energy-efficient & fair 2. Virtualized APs for scheduling competing PSM nodes 3. AP-based adaptation to support heterogenous clients 29

63 NAPman Design NAPman scheduler Energy efficient: send PSM data with high-priority Fair to background traffic Selectively advertise buffered data Advertise buffered data only if its immediate high-priority scheduling doesn t violate FCFS 2. Virtualized APs for scheduling competing PSM nodes 3. AP-based adaptation to support heterogenous clients 30

64 1. NAPman Scheduler Background client 31

65 1. NAPman Scheduler 31? Background client Time to send beacon: advertise buffered data? 31

66 1. NAPman Scheduler 31? Can t skip without violating FCFS Background client Priority sending PSM data would violate FCFS 31

67 1. NAPman Scheduler 31 0? Background client Preserve fairness, don t advertise data! 31

68 1. NAPman Scheduler Background client 32

69 1. NAPman Scheduler 32? Background client Time to send next beacon: okay to advertise? 32

70 1. NAPman Scheduler Background client Priority sending pkt 3 maintains FCFS: Yes! 32

71 1. NAPman Scheduler Background client 32

72 1. NAPman Scheduler Background client Send buffered packet with high-priority 32

73 1. NAPman Scheduler 32 8 zzz NAPman is energyefficient while maintaining fairness Send buffered packet with high-priority Background client 32

74 NAPman Design NAPman scheduler 2. Virtualized APs for scheduling competing PSM nodes One per virtual AP (VAP) Each VAP has its own beacon Stagger beacons in time to avoid contention 3. AP-based adaptation to support heterogenous clients 33

75 2. Virtualized APs (VAPs) beacon interval time Clients contend at each beacon 34

76 2. Virtualized APs (VAPs) beacon interval time Each assoc. to own VAP Stagger VAP beacons in time Isolate competing PSM traffic to save power 34

77 NAPman Design NAPman scheduler 2. Virtualized APs for scheduling competing PSM nodes 3. AP-based adaptation to support heterogenous clients Determine if client is adaptive or static Individualized support for adaptive clients Maintain buffer even while client is awake Learn timeout values to avoid unnecessary retx s Energy-efficient & fair: check buffer after each xmit 35

78 3. AP-based Adaptation adaptive PSM 3 Background client 36

79 3. AP-based Adaptation adaptive PSM 3 null:awake Background client Client just sent null frame 36

80 3. AP-based Adaptation adaptive PSM Background client Send only fair buffered data 36

81 3. AP-based Adaptation adaptive PSM 2 Background client Check for fairness after each transmission 36

82 3. AP-based Adaptation adaptive PSM 2 Background client Send buffered data if fair & not close to timeout 36

83 3. AP-based Adaptation adaptive PSM 2 Background client Buffer PSM data: ensure fairness & avoid retx s 36

84 NAPman Implementation 37 Modified Madwifi Timestamp affixed to each incoming packet Interrupt generated after each transmission Beacon update and MORE bit functions altered PSM traffic enqueued in e high-priority queue Basic virtualization support already provided 37

85 Roadmap 38 Power Save Mode (PSM) in standard PSM scheduling & problems w/ competing traffic NAPman design and implementation Evaluation Related Work & Conclusion 38

86 Evaluation Methodology 39 1 Mbps (see paper for autorate) Madwifi-based AP, WinXP background node Static : HP ipaq hw900 Adaptive : iphone 3GS Monsoon Solutions power monitor Isolate power consumption of WiFi interface 39

87 Evaluation 40 Single 128 Kbps radio (adaptive & static ) Web workload Multiple s Controlled experiment See paper for more 40

88 Static PSM: Radio Power (mw) Normal High Priority NAPman 1.9x more power Latency (ms) Normal High Priority NAPman Background Traffic (Kbps) NAPman provides similar power as High Priority Background Traffic (Kbps) NAPman can provide better latency than normal scheduling 41

89 Adaptive PSM: Radio Awake Time Per Pkt (sec) Normal High Priority NAPman Background Traffic (Kbps) Retx s Per Pkt Normal High Priority NAPman Background Traffic (Kbps) NAPman provides similar performance to High Priority 42

90 Web Workload Consumed Energy (J) Normal High Priority NAPman 3x more energy <20% overhead Cumulative Fraction Pkts Normal could skip Pkts High Priority skips Background Traffic (Kbps) NAPman works well in web workloads Packets NAPman is fair for both types of traffic 43

91 Multiple PSM Clients 44 1 PSM pkt per beacon, saturated background demands 800 Power (mw) Normal High Priority NAPman 45% more energy s NAPman avoids contention to save power 44

92 Scheduling Overview 45 Problem Normal High Priority NAPman Energy Efficient Fair to PSM Fair to Background Eliminate U ncsry Retx Support Multiple PSM 45

93 Roadmap 46 Power Save Mode (PSM) in standard Problems with buffered data scheduling schemes in face of competing traffic NAPman design and implementation Evaluation Related Work & Conclusion 46

94 Related Work 47 Client-centric Sleep between frames, forced idle mode [Liu08, Biswas04] Heuristics for adaptive PSM [Anand03,Krashinsky02,Qiao05,...] Side-channel information [Shih02, Agarwal07, Ananthanarayanan09,...] Complementary to NAPman Support via traffic shaping/proxies [Armstrong06,Chandra02,Tan07] NAPman isolates & prioritizes PSM, again complementary Competing PSM traffic [He07,Stine02,Xie09,Lin06,..] Changes standard, no discussion of fairness, adaptive PSM, &/or background traffic 47

95 Conclusions 48 Competing network traffic leads to the following problems in today s Power Save Mode (PSM) schedulers: Energy inefficiency Unfairness Unnecessary retransmissions that impact capacity NAPman: energy-efficient, fair scheduling, AP virtualization & adapt to s 70% energy savings, eliminate unnecessary retx s No changes to clients or standards! 48

96 Questions? 49 Thanks! 49

97 Trace Analysis 50 Simulated 192Kbps radio client in UCSD traces Up to 75% more energy Background traffic can be problematic in real networks 50

98 Managing Clients 51 DenseAP [Murty08] Beacons advertise hidden SSID Forces clients to send Probe Request packet to associate AP can then pick Virtual AP for client in the Probe Response 51

99 VAPs Exhausted 52 Typically some limit to number of VAPs Madwifi has at most 4 When more clients than VAPs Multiple clients to one VAP: least loaded, etc Possible to advertise only 1 client per beacon Could use PHY rate info to avoid having nodes wait longer than necessary 52

100 Managing VAP Beacons 53 Could just pre-allocate slots and fill them up as PSM clients associate to network Try to keep as much separation as possible If necessary, could reallocate all beacons for better spacing and just re-associate clients Some overhead, but gives all clients more equal support In either case: can use MORE bit to stop clients from contending 53

101 Different Fairness Constratins 54 Possible to use a more coarse-grained notion of fairness? Let PSM node get temporary control, but limit long term access Possible, need to be careful about multiple PSM clients impacting background FCFS is very fine-grained Easy to maintain fairness to background Very straight-forward to implement 54

102 U-APSD 55 Part of e standard, but a lot of current phones don t support it Any frame from client acts as trigger for buffered data Supports interactive traffic, VoIP, etc Same concepts from NAPman can be applied: fairness, energy-efficiency, multiple contention, etc 55

103 Energy-consumption 56 HTC Tilt Base: mw WiFi CAM: 1120 mw, PSM 72mW Other studies WiFi 60% of device s power (CoolSpots paper) 3G 4-6x more power than WiFi transfer (TailEnder) We show up to 4x more power for WiFi transfers when competing background traffic 56

104 Unfairness to PSM Traffic 57 Background client Normal Scheduling can be unfair to PSM traffic 57

105 Unfairness to PSM Traffic Background client Normal Scheduling can be unfair to PSM traffic 57

106 Unfairness to PSM Traffic Background client Normal Scheduling can be unfair to PSM traffic 57

107 Unfairness to PSM Traffic Background client Normal Scheduling can be unfair to PSM traffic 57

108 Unfairness to PSM Traffic PS-Poll Background client Normal Scheduling can be unfair to PSM traffic 57

109 Unfairness to PSM Traffic Background client Normal Scheduling can be unfair to PSM traffic 57

110 Unfairness to PSM Traffic PSM data enqueued after newer packets first-come, first-serve violated Background client Normal Scheduling can be unfair to PSM traffic 57

111 PSM Client Contention s buffer 2 s buffer 2 Both s have data to receive 58

112 PSM Client Contention 58 PS-Poll 1 1 s buffer 2 s buffer PS-Poll 2 Both s have data to receive 58

113 PSM Client Contention 58 1 PS-Poll 1 s buffer 2 s buffer PS-Poll 2 Client 1 wins contention, client 2 must wait 58

114 PSM Client Contention 58 zzz 1 1 s buffer 2 s buffer PS-Poll 2 Client 1 wins contention, client 2 must wait 58

115 PSM Client Contention 58 zzz 1 1 s buffer 2 s buffer PS-Poll 2 Client 2 wastes energy due to contention 58