No-sleep bugs

14 Pages·2012·0.63 MB·English

What is keeping my phone awake?
Characterizing and Detecting No-Sleep Energy Bugs in
Smartphone Apps
Abhinav Pathak Abhilash Jindal
PurdueUniversity PurdueUniversity
[email protected] [email protected]
Y. Charlie Hu Samuel P. Midkiff
PurdueUniversity PurdueUniversity
[email protected] [email protected]
ABSTRACT Keywords
Despitetheirimmensepopularityinrecentyears,smartphonesare Smartphones,Mobile,Energy,Energy-Bug,No-Sleep-Bug.
and will remain severely limited by their battery life. Preserv-
ing this critical resource has driven smartphone OSesto undergo 1. INTRODUCTION
aparadigmshiftinpower management: bydefault everycompo-
nent,includingtheCPU,staysofforinanidlestate,unlesstheapp
1.1 Motivation
explicitlyinstructstheOStokeepiton! Suchapolicyencumbers
app developers to explicitly juggle power control APIs exported Smartphoneshavesurpasseddesktopmachinesinsalesin2011
by the OS tokeep thecomponents on, during their activeuse by tobecomethemostprevalentcomputingplatforms[1]. Toenrich
theapp and off otherwise. Theresultingpower-encumbered pro- theuserexperience,moderndaysmartphonescomewithahostof
grammingunavoidablygivesrisetoanewclassofsoftwareenergy hardwareI/Ocomponents embeddedinthem. Thelistofcompo-
bugsonsmartphonescalledno-sleepbugs, whicharisefrommis- nentsbroadlyfallintotwocategories:traditionalcomponentssuch
handlingpowercontrolAPIsbyappsortheframeworkandresult asCPU,WiFiNIC,3Gradio,memory,screenandstoragethatare
insignificantandunexpectedbatterydrainage. alsofoundindesktopandlaptopmachines,andexoticcomponents
Thispapermakesthefirstadvancestowardsunderstandingand such as GPS, camera and various sensors. And they differ from
automatically detecting software energy bugs on smartphones. It theirdesktop/laptopcounterpartsinthatpowerconsumedbyindi-
makes the following three contributions: (1) we present the first vidualI/Ocomponentsisoftencomparableto,orhigherthan,the
comprehensivestudyofrealworldno-sleepenergybugcharacter- powerconsumedbytheCPU.
istics; (2) we propose the first automatic solution to detect these This, alongwiththefactthatsmartphones havelimitedbattery
bugsbasedontheclassicreachingdefinitionsdataflowanalysisal- life, dictatesthatenergyhasbecome themostcriticalresourceof
gorithm; (3) we provide experimental data showing that our tool smartphones. Preserving this crucial resource has driven smart-
accurately detected all 14 known instances of no-sleep bugs and phoneOSestoresorttoaparadigmshiftincomponentpowerman-
found30newbugsinthe86appsexamined. agement. On desktop machines, where the CPU accounts for a
majority of the energy consumption, the default energy manage-
mentpolicyisthattheCPUstayson(orrunsatahighfrequency)
unlessanextendedperiodoflowloadhasbeenobserved.Thepol-
Categories andSubject Descriptors
icyisconsistentwiththehistoricalnotionthatenergymanagement
D.2.5[SoftwareEngineering]:TestingandDebugging isasecondclasscitizensincemachinesarepluggedintoapower
source[2]. Smartphones,insharpcontrast,makepowermanage-
ment policy a first class citizen. In fact, the power management
General Terms policyonsmartphones hasgonetotheotherextreme: thedefault
powermanagement policyisthateverycomponent, includingthe
Design,Experimentation,Measurement CPU,staysofforinanidlestate,unlesstheappexplicitlyinstructs
theOStokeepiton!
Inparticular,allsmartphoneOSes,e.g.,Android,IOS,andWin-
dowsMobile,employanaggressivesleepingpolicywhichputthe
componentsofthephonetosleep,i.e.,putsthemintoanidlestate
Permission tomake digital orhardcopies ofall orpartofthis workfor
personalorclassroomuseisgrantedwithoutfeeprovidedthatcopiesare immediatelyfollowingabriefperiodofuserinactivity. Intheidle
notmadeordistributedforprofitorcommercialadvantageandthatcopies state, the smartphone as a whole draws near zero power, since
bearthisnoticeandthefullcitationonthefirstpage.Tocopyotherwise,to nearlyallthecomponents, includingCPU,areputtosleep. Such
republish,topostonserversortoredistributetolists,requirespriorspecific asleepingpolicyislargelyresponsibleforprolonged smartphone
permissionand/orafee.
standbytimes–smartphonescanlastdozensofhourswhenidle.
MobiSys’12,June25–29,2012,LowWoodBay,LakeDistrict,UK.
Theaggressivesleepingpolicy,however,severelyimpactssmart-
Copyright2012ACM978-1-4503-1301-8/12/06...$10.00.
267
phoneapps,sinceanappmaybeperformingcriticaltasksbyinter- work, including popular apps (e.g., Facebook) and built-in (i.e.,
mittentlyinteractingwiththeexternalworldusingvarioussensors. shippedwith)appsandservices(e.g.,theAndroidemailapp).The
Forexample,anappsyncingwitharemoteserveroverthenetwork bugsarecollectedbycrawlingInternetmobileforums,bugrepos-
mayappear toperformnoactivitywhenwaitingfor theserver to itories,commitlogsofopensourceAndroidappsandbyrunning
send itsreply, andthesystemmaybeput tosleepbytheaggres- our no-sleep bug detector developed inthispaper. Foreach bug,
sivesleepingpolicy,leavingtheremoteserverwithaviewoflost wecarefullyexamineitsreportedsymptoms,correspondingsource
connectivity. codeandrelatedpatches(whenavailable),anddeveloper’sdiscus-
Toavoidsuchdisruptionsduetotheaggressivesleepingpolicy, sions (when available), or the analysis performed by our bug de-
smartphoneOSesprovideasetofmechanismsforappdevelopers tector. Ourstudyrevealsataxonomyofthreemajorcausesofno-
toexplicitlynotifytheOSoftheirintentiontocontinueusingeach sleep energy bugs, which provide useful guidelines and hints to
component. Inparticular, theOSexportsexplicitpower manage- designingeffectivedetectiontechniques. Ourstudyalsoconfirms
menthandlesandAPIs,typicallyintheformofpowerwakelocks thesignificantburdenpower-encumbered programmingplaceson
andacquireandreleaseAPIs[3],forusebytheappdeveloper to appdevelopers. Forexample,makingasingleoutgoingorincom-
specify when aparticular component needs tostay on, or awake, ingphonecallinAndroidinvolvesabout40invocationsofpower
untilitisexplicitlyreleasedfromduty. control APIs in the Dialer app and the framework’s Radio Inter-
Wearguesuchexplicitmanagementofsmartphonecomponents face Layer services to dynamically manage the power control of
by appdevelopers haspresented totheapp developer aprofound theCPU,screen,andothersensors!
paradigmshiftinsmartphoneprogrammingthatwecallaspower- (2) The first solution to automatically detect no-sleep energy
encumbered programming. This new programming paradigm bugs: We make the key observation that power control APIsare
placesasignificantburdenondeveloperstoexplicitlymanipulate explicitlyembeddedintheappsourcecodebytheappdevelopers,
thepowercontrolAPIs(§4.1detailsonesuchexampleofthebur- andtwooutofthethreecausesforno-sleepenergybugsfromour
den placed on developer due to power encumbrance). This ma- characterizationstudyarebecauseaturn-onAPIcallismissinga
nipulation isrequired toensurethecorrect operation oftheapps. matching turn-off API call before the end of the program execu-
Consequently,power-encumberedprogrammingunavoidablygives tion.Wethusproposeacompile-timesolutionbasedontheclassic
risetoanewclassofsoftwareenergybugsonsmartphones,called reachingdefinitionsdataflowanalysisproblem[6]toautomatically
no-sleepbugs. No-sleepbugsaredefinedasenergybugsresulting inferthepossibilityofano-sleepbuginagivenapp. Oursolution
from mis-handling power control APIs in an app or framework,1 detectsno-sleepbugsinsingle-threaded andmulti-threadedapps,
resultinginthesmartphonecomponentsstayingonforanunneces- aswellasevent-basedappswhichhavemultipleentrypoints.Like
sarilylongperiodoftime. No-sleepbugsformoneimportantcat- all static analysis based tools, our detection tool can suffer false
egoryofthefamilyofsmartphoneenergybugswhicharedefined positivesbuthasthetremendousadvantageofnoruntimeoverhead
in[4]aserrorsinthesmartphonesystem(anapportheframework, and no falsenegatives (to the best of our abilitiesof establishing
the OS,or the hardware) that cause an unexpectedly high energy thegroundtruth).
consumptionbythesystemasawhole. We further present the complete implementation of our static
Discussions of energy bugs on numerous Internet forums have analysis detection tool for apps written for Android. The tool is
narrowedthecausestomis-handlingofpowercontrolAPIsbyapps capableofrunningdirectlyontheappinstallers(.apkfiles)and
andtheframeworkonsmartphonesOSes,includingAndroid,IOS, hence source code is not required. Our implementation handles
and WindowsMobile. Ourrecent survey[4] hasfound that 70% thespecificsofevent-drivenmobileprogrammingandoftheJava
ofallenergyproblemsinappsandframeworksreportedbymobile languagesuchasruntimenullpointerexceptionsandobjectrefer-
userswereduetono-sleepenergybugs. Theseandothertypesof ences.
energybugshavecausedagreatdealofuserfrustrations. Despite (3)Detectingnewno-sleepbugsinAndroidappsandframework:
theirseverity,i.e.,highbatterydrain,tothebestofourknowledge Wehaverunour no-sleepbugdetection toolon86Android apps
therehasbeen nostudyof anykindof smartphone energy bugs, andtheframeworkcollectedfromtheAndroidmarket.Experimen-
muchlessno-sleepenergybugs. Drawingparallelswithresearch talevaluation showsthatour toolaccurately detectedallreported
ontraditionalsoftwarebugs(e.g.,concurrencybugsinconcurrent instances of no-sleep bugs, as well as 30 instances of new previ-
programs[5]),acomprehensivetreatmentofenergybugsonsmart- ously unreported no-sleep bugs. These include no-sleep bugs in
phoneswillrequire(1)agoodunderstandingofrealworldenergy manypopular apps, e.g.,thedefault AndroidEmailapp. Ourno-
bug characteristics, learned fromcommon mistakesprogrammers sleepbugdetectionincurredfalsepositivesin13outofthe55apps
make in writing smartphone apps, to lead to effective debugging itreportedtocontainabug.
techniques; and (2) developing multi-faceted approaches toelim-
inating energy bugs, including avoiding energy bugs during app 2. POWER ENCUMBERED
development(e.g.,byprovidingbetterprogramminglanguagesup-
PROGRAMMING
portforpowermanagement)andcompileandruntimedetection.
In this section, we first describe the energy management APIs
1.2 Our Contributions
and theirsemanticsthat areexposed tothedevelopers intheAn-
Thispapertakesthefirststepstowardsunderstandingandauto- droidsmartphone OS,anddiscusstheburdentheyimposeonthe
maticallydetectingsoftwareenergybugsonsmartphones. Specifi- appdevelopers. WefirstdiscussprogrammingAPIsfortraditional
cally,ourpapermakesthreeconcretecontributions: components (e.g., screen, CPU) and then for exotic components
(1) The first characterization study of no-sleep energy bugs in (e.g.,GPS,Camera). Wealsodiscusstheissuesarisingfromevent
smartphone apps: We present the first comprehensive real world basedprogrammingmodelofsmartphoneapps.Wethenintroduce
no-sleepenergybugcharacterizationstudy. Ourstudyisbasedon theprominentclassofenergybugsstudiedinthispaper: no-sleep
no-sleep energy bugs in real world apps and the Android frame- bugs.
1Inthispaper,weusethetermframeworktorefertoboththeser-
vicesinandtheappsthatarebundledwith,theAndroidframework.
268
Table1:SummaryofpoweroperationsexportedbyAndroidAPIs.
Componentlock/managername Component(s) BatteryDrain Comments
(APItostart/stop) upto(%/hr)
TraditionalComponents
PARTIAL_WAKE_LOCK(acquire/release) CPU 5% CPUrunsdespiteanytimers
SCREEN_DIM_WAKE_LOCK(acquire/release) CPUandScreen(DIM) 12% Noilluminationifshutdown,else
SCREEN_BRIGHT_WAKE_LOCK(acquire/release) CPUandScreen(bright) 25% illuminatestilllockrelease(Flag
FULL_WAKE_LOCK(acquire/release) CPU, Screen (bright) and Key- 25% ACQUIRE_CAUSES_WAKEUP
boardbacklight forcesilluminationinallcases)
ExoticComponents
PROXIMITY_SCREEN_OFF_WAKE_LOCK(acquire/release) Screen,ProximitySensor 25% Screenshutsifsensoractivates
LocationManager(requestLocationUpdate/removeUpdates) GPS 15% Tracksuserlocation
SensorManager(registerListener/unregisterListener) Accelerometer,Gyro,Proximity Sensormanagerclasscontrols
MagneticField,etc.[8] 10% varioussensorsonphone
MediaRecorder(start/stop) Mic/Camera(forvideo) 20% Usuallystoresmediaonsdcard
Camera(startPreview/stopPreview) Camera(forstillpictures) 20% Oneappatatimeregisterscamera
thecorrespondingcomponent(theACQUIRE_CAUSES_WAKEUPflag
Listing1:Anexamplepowerwakelockusage.
wakesupthescreen),orkeepsthecomponentawakeifitisalready
1 PowerManager.WakeLock wl = pm.newWakeLock(PowerManager. so. Inotherwords,anacquire( )calledonanalreadyacquired
PARTIAL_WAKE_LOCK);
wakelock istreatedasa nop. Similarly, arelease()called on
2 wl.acquire(); //CPU should not go to sleep
3 net_sync(); //Perform critical task here anacquiredwakelocksetsthecomponentfreetosleepasfarasthe
4 wl.release(); //CPU is free to sleep perspectiveofthiswakelockisconcerned,irrespectiveofthenum-
ber of times an acquire() has been called on the lock. In this
sense, anon-reference counted wakelock islikeacondition vari-
2.1 Managing Traditional Components
able.
TheAndroidframeworkexportswakelockfunctionalitythrough In contrast, reference counted wakelocks are like semaphores.
PowerManager.Wakelock 2 class, with4 different options and Eachacquire()ofawakelockincrementstheinternalcounteras-
associatedAPIsformanagingseveraltraditionalcomponents:CPU, sociatedwiththe(instanceof)wakelock,andarelease()decre-
screen, and the keyboard backlight. A wakelock is an instance mentstheinternalcounter. Itonlyletsthecomponentsleep(from
(object in Java) of the wakelock class, instantiated using one of theperspectiveofthiswakelock)iftheinternalcountervaluereaches
4 options, and each option has adifferent effect on the hardware 0.Areleaseisanop3inothercases.
component, assummarizedinTable1. Forexample,optionFULL Tomakemattersevenmorecomplicated, anacquirecanalso
_WAKE_LOCKinstantiatesalockthatwhenacquiredbothkeepsthe becalledwithatimer[10],whichinstructsthesystemtoreleaseit
CPU and screen on at full brightness and turns on the keyboard automaticallyoncethetimeoutintervalexpires.
backlight. Third,theabovepowercontrolsemanticsarefromtheperspec-
Listing1illustratesabasicwakelockusage: howtoensurethat tive of one wakelock. Unlike traditional mutual exclusion locks,
the CPU does not sleep during some critical phase. The app de- differentwakelocks(eveninstantiatedwithdifferentoptions)onthe
claresawakelock (pmisaninstanceofPowerManager) andthen same component can be held by multipleentities(e.g., processes
acquires it, which instructs the OS not to put the CPU to sleep, andthreads) inthesystematthesametime. Evenasingleentity
irrespectiveofuseractivity,sinceitintendstoperformsomecriti- may hold multiple (instances of) wakelocks on the same compo-
caltask. Oncethecriticaltask(inthiscasearemotenetworksync nent. Thepowercontroleffectonacomponentmusttakeintoac-
net_sync())iscompletedtheappreleasesthewakelock, indi- count the state of all wakelocks. The component is switched on
catingtotheOSthatCPUcannowsleepaccordingtoitssleeping whenthefirstwakelockisheld.Onlywhenallthewakelocksfrom
policies. alltheentitiesforthecomponentarereleased,takingintoaccount
Semantics: The above simple usage of a wakelock is just like a thereferencecounting semanticsforeach, canthecomponent go
conventionalmutualexclusionlock,i.e.,anappexplicitlyacquires tosleep,subjecttohigherlevelsleepingpolicies(administeredby
andreleasesittoinstructtheOStoswitchthecomponent onand frameworkprocesses),e.g.,sleepafter5secondsofuserinactivity.
off,respectively. Likeanobjectawakelockcanbesharedamong Thisdemonstratesthenewprogrammingburdeninflictedonapp
several threads of a process. The semantics of wakelocks, how- developers: powermanagementisnolongerjustatransparentOS
ever, arequitedifferentfromthoseofconventional mutualexclu- ordrivertask;thedevelopersnowneedtoperformexplicitpower
sionlocks. managementintheapplayer.
First,asshowninTable1,asinglewakelock(instantiatedwith
one of the four options) controls one or more components. Sec- 2.2 Managing ExoticComponents
ond,thepowercontroleffectofasinglewakelockdependsonthe Inadditiontotraditionalcomponents,modernsmartphonescome
configurationofthatwakelock:awakelockcanbeconfiguredtobe with several “exotic” components embedded in them. These in-
referencecounted[9]. Inanalmostpeculiarsense, itismorelike cludeGPS,camera,severalsensorssuchasanaccelerometer,prox-
aconditionvariablewhenconfigurednottobereferencecounted, imity sensor, and gyroscope. Some of these components are the
andasemaphorewhenconfiguredtobereferencecounted. biggest energyconsumers insmartphones, e.g., GPSandcamera,
We first consider non-reference counted wakelocks. An anddrainthebatteryatahighrate[11,12,13].
acquire()onareleasedorfreshinstanceofwakelockwakesup
3NewerAndroidAPIsthrowanexceptionwhenrelease()iscalled
2Apps need android.permission.WAKE_LOCK permission from onanunacquiredwakelock.Hence,areleaseisusuallycalledafter
userstousethisclass. testingifthelockiscurrentlyheld(usingAPIwakelock.isHeld()).
269
Unlikesomeofthetraditionalcomponents, e.g.,WiFiNIC,the outgoingorincomingcallinAndroidresultedin30-40distinctin-
newexoticcomponentsareusedinanexpliciton-offfashion. For stancesofwakelockacquiresandreleases!
example, the GPS is explicitly turned on, using the OS exported
API, to acquire the smartphone location and in this state it con- 2.4 No-Sleep Bugs
sumesbatteryatahighrate. Oncethelocationisdetermined, the Thenewburdenofexplicitcomponentpowermanipulationfrom
componentisexplicitlyturnedoff,triggeringthecomponenttore- power-encumbered programming, combined with the complexity
turntoalowpowerstate. ofhandling eventsintheappbehavior, caneasilyoverwhelmde-
Likewakelocks,theexplicitpowermanagementofexoticcom- velopers and lead to programming mistakes in manipulating the
ponentsplacesasignificantprogrammingburdenonappdevelop- powercontrolAPIs.IncorrectorinefficientusageofsuchAPIscan
ers. An incorrect or inefficient use of these APIscan easilylead leadtoanunexpecteddrainofthephonebattery,knownasno-sleep
topoorutilizationofthesecomponents,wastingsignificantbattery bugs[4].
energy(see§2.4). A“no-sleepbug”isaconditionwhereatleastonecomponentof
Semantics:Table1liststheAPIsexportedbyAndroidforaccess- thephoneiswokenupandisnotputtosleepduetoamistakein
ing the exotic components. Their semantics of power control of manipulating power control APIsinanapp. Thecomponent that
thesecomponentsissimilartotheplainwakelocksin§2.1,i.e.,no iswoken upcontinuestodrainthebatteryforaprolonged period
referencecountingortimer-basedrelease. oftime,resultinginsevereandunexpectedbatterydrain.Typically
Insummary,thedevelopersareburdenedwithexplicitlymanip- thebatterydraincontinuesuntiltheappisforcefullykilled4orthe
ulating power control APIsfor both traditional and exotic smart- systemisrebooted.
phone components to ensure the correct operations of the apps. No-sleepbugsformoneofthemostimportantcategoriesofsoft-
We call this new smartphone programming paradigm as power- wareenergybugsinsmartphoneapps[4]. Unlikeregularsoftware
encumberedprogramming. bugsinapps,energybugsdonotleadtoanappcrashorOSblue
screenofdeath[4]. Anapphitbyanenergybugcontinuestopro-
2.3 Issues from Event-based Programming videtheintendedfunctionality,withasingledifference: thephone
The complexity of power-encumbered programming is exacer- suffersasevere,unexpectedbatterydrain. Theseverityoftheen-
bated by the event-based nature of smartphone apps. Compared ergy drain due to the bug depends on the component that is not
toprogrammingindesktop/server environments, smartphonepro- put tosleep. Asshown inTable1, for eachof the3components
grammingisevent-orientedbecauseoftheinheritinteractivenature (GPS,Screenwithfullorlowbrightnessandcamera),theimpact
ofano-sleepbugcanbeseverewiththebatterydrainingatarate
ofphoneapps. Atypicaluser-facingsmartphoneappiswrittenas
asetofeventhandlerswitheventsbeinguserorexternalactivities. of10-25%everyhour5withoutanyuserinteraction.
Thedeveloperneedstokeeptrackofeachpossibleeventandwhen Forothercomponents, e.g., theCPUandproximitysensor, the
itmaybetriggered,andmanipulatethewakelocksaccordingly. batterydrainsatarelativelylowrate-upto5%everyhour. When
Weillustratehowthelevelofcomplexityintroducedbypower- a CPU wakelock (PARTIAL_WAKE_LOCK)is held, it prevents the
encumberedprogrammingisexacerbatedbyevent-basedprogram- CPUfrom‘freezing’,astatewhereitwouldconsumezeropower
ming through a concrete example from the Dialer app [14] that (IDLE state). In a wakelock held state, the CPU draws minimal
comeswiththeAndroidframework. power(dependingontheCPUspecificationsofthehandset).How-
The Dialer app implements the dialing functionality of the ever,astheCPUremainson,otheractivitiescontinuetorun,e.g.,
phone. The app is triggered when the user receives an incoming WiFi NIC chatters, background periodic OS processes, hardware
call or when the user clicks the phone icon to make an outgo- interruptshandlingbyOS,etc. Theseactivitiestogetherconsume,
ing call. To implement itsfunctionality, the app explicitly main- asmeasuredonGoogleNexusOne,about5%ofthebatteryevery
tainsthreewakelocks:FULL_WAKE_LOCKforkeepingthescreenon hour. Anyadditionaluseractivityisnotaccountedforinthis. As
(e.g.,insituationslikewhentheuserisdialingthenumberstocall), a result, over a long period of time, say 12 hours, an only-CPU
PARTIAL_WAKE_LOCKforkeepingtheCPUon(e.g.,incaseofan wakelock bugcandrainabout 50-60% ofthebatterywithout any
incoming call when the phone is switched off), and PROXIMITY userinteractionorperformingnecessaryactivities.
_SCREEN_OFF_WAKE_LOCK which switches the proximity sensor
onandoff(todetectuser’sproximitytothephone). 3. METHODOLOGY
To manage the three wakelocks, the app explicitly maintains a
Tocharacterizetherootcauseofno-sleepbugsobservedincur-
state machine where the states represent the lock behavior, i.e.,
rent mobileapps, wecollectedno-sleepbugsinsmartphone apps
which lockneeds tobeacquired and whichneeds tobereleased,
infourways. (a) Mobileforums: Wecrawled4popular mobile
and the “condition” of the phone represents the state transitions.
Internetforums(thesameasin[4]): onegeneral forumwithdis-
The conditions are diverse and include events such as (a) if the
cussionscoveringallmobiledevicesandOSes,andthreeOS/com-
phonegetsacall,(b)ifthephoneispressedagainsttheuser’sear
panyspecificmobileforums.Intotalwecrawled1.2Mposts,from
in which case the proximity sensor triggers the screen to go off,
whichwefilteredoutpostsrelatedtono-sleepbugsinsmartphone
(c)ifthecallends, (d)ifawiredorbluetoothheadset isplugged
apps. Foreachappreportedbytheusertocontainano-sleepbug,
in(e.g.,inthemiddleofacall),(e)ifthephonespeakeristurned
wedownloadedthebinaryinstallersoftheversionoftheappthat
on,(f)ifthephonesliderisopenedinbetweencalls,and(g)ifthe
wasreportedtocontainthebugandthefirstversionwhichhadthe
userclickedhomebuttoninthemiddleofacall. Foreachofthese
problemsolved.Wethendecompiledtheappfrombinaryinstallers
triggering events, the phone changes the state of wakelock state
machine,acquiringoneandreleasinganother.
4InAndroid,apps(esp. backgroundservices)arenotkilled; they
Inadditiontowakelocks intheDialerapp, theRadioInterface
runinbackground onceauser stopsinteractingwiththem. They
Layer(RIL)inAndroidmaintainsadditional5wakelockstohan- areusuallykilledbythesystemonlyincaseofmemorypressure.
dleincomingandoutgoingcalls. Usingexplicitcomponentaccess 5TheserateswerecalculatedusingHTCnexusandmagichandsets
tracingthroughaninstrumentedAndroidframeworkrunningona runningAndroid.Afullychargedbatteryholdsbetween1100mAH
GoogleNexusOnehandset,weobservedthatperformingasingle to1500mAHofcharge.
270
to Java source code using ded6 [16]. For apps that were suc-
Listing2:No-sleepbug:differentcodepaths.
cessfully decompiled (e.g., FaceBook), westudied the root cause
of no-sleep bugs. (b) Bug lists: We crawled mobile bug repos- 1 try{
2 wl.acquire(); //CPU should not sleep
itories of open source mobile frameworks like Android [17] and
3 net_sync(); //Throws Exception(s)
Maemo[18]. Weextractedbugsreportedwithno-sleepconditions 4 wl.release(); //CPU is free to sleep
and extracted the source code (open source) of the versions that 5 } catch(Exception e) {
actuallycontainedthebugs(e.g.,no-sleepbuginAndroidSIPSer- 6 System.out.println(e); //Print the error
vice[19])anditspatch(ifavailable).(c)Opensourcecoderepos- 7 } finally {
8 } //End try-catch block
itories: Wescrapedthecommitlogsofopen-sourceAndroidapps
hostedononlinecoderepositorieslikegithub[20]. Weextracted
thecommitlogsofno-sleepbugfixesanddownloadedtheversions
thatrepresentsatypicaltemplateofano-sleepbugwhereanapp
bothbeforeandafterthefix. (d)Runningourno-sleepbugde-
takes adifferent, somewhat unanticipated code pathafter waking
tectiontool:Finally,weranoursolutionofautomaticallydetecting
upacomponentandthereforedoesnotputitbacktosleep. Asin
no-sleepbugsdevelopedinthispaperon86Androidappsandthe
Listing1,thecriticaltaskintheapp,net_sync(),isprotected
stockframeworkanddiscovered42appswithno-sleepcode-paths
byacquiringandreleasingtheCPUwakelockinstructingtheCPU
asdetailedin§6,§7and§9(labeledwith“*”inTable2). These
not to sleep during the remote syncing phase. However, routine
appsareusedinthecharacterizationstudyin§4.
net_sync()maythrowexceptions[41],aJavalanguagemech-
anismfornotifyingappsofsomefailureconditions,suchasacon-
4. CHARACTERIZINGNO-SLEEP BUGS
nect to a remote end host failed, a string could not be parsed to
Usingthebug-collectionmethodologydescribedabove,wenow integer, or a specific file to be opened does not exist. A thrown
presentacase-studyofno-sleepbugsobservedinsmartphoneapps. exception is explicitly caught by the try’s catch block which
We characterize the root cause and impact of the bugs. Table 2 simplyprintstheexceptionfordebuggingpurposes. Nowtheno-
gives asummaryof thethreegeneral categoriesof no-sleep bugs sleepbugcanmanifestitselfinthefollowingcodepath. Firstthe
wehaveidentifiedandtheirimpact. Draintimeshowstheamount tryblockexecutesandacquiresthewakelock. Nextacallisis-
of time it will take to drain a fully charged battery under typical suedtothecriticaltask, net_sync(). Ifanexception israised
usage.7 Withoutthebugs,ittakesabout15hourstodrainafully insidenet_sync(),thecontroldirectlyjumpstothecatchblock,
charged battery. The bug references in Table 2 refer to both the thedebugoutputisprinted,andthecodeexitsthetrycatchblock.
bug fix commit logs and user complaints about specific apps on Consequently, the code-path followed does not release the wake-
Androidbugrepositories,allofwhichindicatetherealimpactand lock, keeping the CPU on indefinitely. To fix this problem, the
user frustration caused by the no-sleep bugs. The first two cate- wakelock should be released in the finally block so that it is
gories,No-SleepCodePathsandNo-SleepRaceCondition,exhibit alwaysexecuted.
typicalsymptomsofno-sleepbugswhereacomponentisnotputto Alargenumberofno-sleepbugsarecausedbythissecondrea-
sleepatall,whereasthethirdcategory,no-sleepdilation,represents son. Theseincludepopular appssuchasFaceBook[22],Agenda
thescenariowhereacomponentwasheldonmuchlongerthanthe widget [21] (another version), MyTrack [25] (no sleep of GPS),
programmer’sintention(ontheorderofhours). Belowwepresent BabbleSink [26], CommonsWare and apps [27], as well as ser-
anin-depthanalysisofthesethreecategoriesofno-sleepbugs. vicesthatcomewiththeAndroidframework,suchasAndroidTele-
phony[28],AndroidExchange[29,30],andWifiService[31].For
4.1 No-Sleep Code Path
example,inSIPservice[19],thewakelockwasnotreleasedsince
Therootcauseformostoftheobservedno-sleepbugsinasin- theobjectscontainingthewakelocksweredeletedandsothelock
gle threaded activity was the existence of a code path in the app handlersweredeletedalongwiththem.
that wakes up a component, e.g., by acquiring the wakelock for The third cause for a no-sleep code path is that a higher level
thecomponent,butdoesnotputthecomponentbacktosleep,e.g., condition(likeanappleveldeadlock)preventedtheexecutionfrom
thereisnoreleaseofthelock.Thiscategorycapturesamajorityof reachingthepointwherethewakelockwastobereleased. Thisis
theno-sleepbugswehaveobservedinourbugcollection. likelytohappeninsmartphoneprogrammingbecauseevent-based
Weobservedthreecausesfortheexistenceofacodepathwhere programmingofsmartphonescanleadtomanypossiblecodebranches
thecomponentwasswitchedonbutnotputtosleep. (asintheexample of the Android Emailapp) that makes itdiffi-
Thefirstcauseisthat theprogrammer simplyforgot torelease cult for the programmer to anticipate all the possible code paths
thewakelockthroughoutthecode,ortheprogrammerreleasedthe andkeeptrackofthewakelockstate. LocationListener[33,
lockintheifbranchofaconditionbutnotintheelsebranch. 34]intheAndroidframeworkcontainedsuchano-sleepbug. The
Althoughitseemslikeasimplemistake, thisdoeshappeninreal developer did release the wakelock, however, a higher level app
apps. Forexample,aversionoftheAgendawidget[21]contained deadlockpreventedthecodefromenteringthereleasephaseofthe
suchano-sleepbug. app.
The second cause isthat the programmer did put code that re- Finally, themostcommonpatternofno-sleepcode-pathbugis
leasesthecomponent wakelockonmanycodepaths,butthecode aresultofthefactthatdevelopersdonotproperlyunderstandthe
tookanunanticipatedcodepathduringexecutionalongwhichthe life-cycleofAndroidprocesses. InAndroid, anappactivityonce
component was not put to sleep. Listing 2 shows a code-snippet startedisalwaysalive.Whentheuserexitsanyapp,Androidsaves
thestateoftheappandpassesitbacktotheappiftheuserreturns
6Not all app binaries could be transformed into (meaningful) toit.Theappisonlycompletelykilledwhenthephoneiscritically
source code, especially the ones that have been obfuscated dur-
lowonRAMorwhentheappkillsitself.Thismethodologyisused
ingcompilation(usingtoolslikeproguard[15]),e.g.,theNYTimes
toreducethestartuptimeoftheappandtomaintainitsstate.
Androidapp.
7Typicalusageassumesthatthephoneisusedactivelybyauserfor Thisessentiallymeansthattheappmaynotactuallybedestroyed
20%ofthetotaltime. Draintimesarecalculatedusingtheenergy for very longperiods of time. But many appdevelopers only re-
drainrateinthebugstatemeasuredontheHTCNexusphone. leasethewakelockintheonDestroy()call-back,insteadofin
271
Table2:No-sleepbugcasestudy.Entrieswith(F)representbugsintheAndroidframework,andwith(*)representnewbugsfoundbyourtechnique.
App Description BugDescription Drain Ref.
No-SleepCodePaths
AgendaWidget Popular Android widget Twobugswerereportedindifferenceversions(a)notallbranchesreleasewakelocksin 9hrs [21]
managingnews/calendar AlarmService;(b)programmerforgottocallwakelockreleaseafteracquiringit.
FaceBook The default FaceBook facebook.katana.HomeActivity, the central Activity, acquires wakelock to 9hrs [22]
Appv 1.3.0 runFaceBookService.Notallpossiblebranchesintheservicereleasewakelocks.
k9mail Oneofthemostpopular Per-threadwakelockmaintained. WakelockacquiredwhenIMAPDONEwassent,but 9hrs [23]
emailclientforAndroid wasnotreleasedinMessagingControllerPushReceiverduringIDLEstat
CheckinMaps Visualstoriesonmaps GPSremainson,evenwhenuserclosestheapp(callingtheonPause()handler). 5hrs [24]
(*)MyTrack TrackUserPathonline GPSremainson,evenafterusernavigatesawayfromtheappdrainingbattery. 5hrs [25]
BabbleSink Findphone’slocation ANullPointerExceptioncausesthethreadtoexitwithoutreleasingwakelock. 9hrs [26]
CommonsWare AndroidTrainingBook Wakelockreleasedwithoutfinalize. 9hrs [27]
SipService(F) Std. voice protocol im- ASiphandler(object)wasdeletedwhichhadwakelocksacquiredbeforereleasingthe 9hrs [19]
plementationinAndroid wakelock.Thedeletedhandlerscannotperformrelease()toreleasethewakelocks.
Telephony(F) TelephonyHandler: RIL Androidtelephonydoesnotreleasethepartialwakelockrightawayifthereisanerrorin 9hrs [28]
serviceinAndroidcode sendingtheRILrequest,preventingthephonetogoinpowercollapsedrainingbattery.
(*) Android Ex- Thedefaultemailappin During background syncing of mailboxes in an exchange account, the app acquires 9hrs [29]
change(F) Androidframework wakelockanddoesnotreleaseinallfailureconditions,specificallyinIOExceptions. [30]
WifiService(F) AndroidWiFiHandlers CPUdoesnotgotosleepduringamessageremovalandwakelockwasheldforever. 9hrs [31]
PowerManager(F) PowerServiceAndroid TwoinstancesofwakelocksarenotreleasedinPowerManagerServiceinAndroid. 9hrs [32]
LocationListener GPS handling library in AdeadlockinLocationManagerServiceforreleasingwakelocksafterclientno- 9hrs [33]
(F) Androidframework tificationshavebeenreceivedpreventedthereleaseofthewakelocksdrainingbattery. [34]
No-SleepRaceCondition
(*)AndroidEmail Default Android email Raceconditionbetweenemailsynchronizingthreadandthemainthreadwhichkillsthe 9hrs [35]
app(F) appperformingsync synchronizingthreadresultedinasharedwakelocktoremaininacquiredstateafterexit. [36]
No-SleepDilation
MyTrack TrackUserPathonline Wakelockacquiredandreleasedmuchbeforeandaftertherequiredfunctionalityinapp. 9hrs [25]
GoogleBackup(F) Cloudbackup[37] Wakelockreportedtobeheldforalongdurationoftime(uptoanhour)inpoornetwork. 11hrs [38]
GPSDriver(F) AndroidGPShandler WakelocksarebeingheldforlongerthanneededinlowlevelGPSdrivercode. 15hrs [39]
GoogleMaps AndroidGooglemaps Appwasreportedtoholdwakelockforseveralhoursevenwhenitwasnotused. 10hrs [40]
droid framework’s class CdmaConnection. This class uses a
Listing3:WakelockComplexity
PARTIAL_WAKE_LOCK for managing the connection and re-
1 @Override protected void finalize(){ leases the wakelock when the connection is disconnected. How-
2 /** ever, there are many different possible program paths arising
3 * It is understood that This finializer is not
4 * guaranteed to be called and the release lock from different patterns of user interactions, hardware states de-
5 * call is here just in case there is some path pendentontheexternalenvironment,etc. Thedeveloperincluded
6 * that doesn’t call onDisconnect and or releaseWakeLockinfinalizeasanadditionalsafetymea-
7 * onConnectedInOrOut. sure,eventhoughfinalizeisnotguaranteedtobecalled. This
8 */
9 if (mPartialWakeLock.isHeld()) { exampleshowstheneedforanautomatedtoolthatcanaiddevel-
10 Log.e(LOG_TAG, "[CdmaConn] UNEXPECTED; mPartialWakeLock opersincheckingallpossibleprogrampathsforno-sleepbugs.
is held when finalizing.");
11 }
12 releaseWakeLock();
13 } 4.2 No-Sleep RaceCondition
Thesecondcategoryofno-sleepbugsweobservedwascauseby
raceconditionsinmulti-threadedapps. Specifically,weobserved
onPause(). onDestroy()isonlycalledwhentheappcom- thatthepowermanagementofaparticularcomponentwascarried
ponentisabouttobedestroyed. Asaresult,onceanappwiththis out(i.e.,switchedonandoff)bydifferentthreadsintheapp.Inthe
bugisstarted,thephonewillonlysleepwhenitisrunningcritically common case, one thread switches the component on, and some
lowonmemory(whichmaytakealongperiodoftime). timelateranotherthreadswitchesthecomponent off, resultingin
Manually tracking all possible code paths for wakelock ac- thenormalbehaviorofcomponentutilization.However,inacorner
quire/release appears to be a daunting task for app developers. case condition, it canhappen that the threadthat switches onthe
Listing3showsanexampleofthecomplexityinvolvedinpower- componentgetsscheduledtorunafterthethreadthatswitchesthe
encumbered programming. This is a code-snippet from the An- componentoff,resultinginano-sleepbugwiththecomponentleft
272
foranunexpectedlengthoftimebeforeitreturns,thethebatteryis
Listing4:No-sleepbug:racecondition.
drainedforthatprolongedperiodoftime.
1 public void Main_Thread(){ Whileitisarguablethatkeepingthesystemonduringtheexecu-
2 mKill = false; //Unset kill flag
tionofacriticaltask,nomatterhowlongittakes,wasindeedthe
3 wl.acquire(); //CPU should not sleep
4 start(worker_thread); //Start worker intentionbytheappdeveloper, wecharacterizesuchsituationsas
5 //....Do Something thethirdcategoryofno-sleepenergybugs,no-sleepdilation.These
6 mKill = true; //Set kill flag areconsideredno-sleepenergybugsforthefollowingreasons: (a)
7 stop(worker_thread); //Signal worker suchinstancesofprolongedcomponentwakeupareusuallyunex-
8 wl.release(); //CPU can sleep now
9 } //End Main_Thread(); pected,evenbytheappdeveloper,aswefoundbyreadingthelog
10 public void Worker_Thread(){ ofcodecommits;(b)themobileprogrammingAPIdocumentation
11 while(true){ strictlywarndevelopersnottokeepthecomponentsawakeforpro-
12 if(mKill) break; //Break if flagged longed periods of time unless it is actually required, e.g., in the
13 net_sync(); //Critical task
Skypeapp,whereauserperformingavideocallrequiresthecom-
14 wl.release(); //Rel. wl before sleep
15 sleep(180000);//Sleep for 3 minutes ponents(screen,CPU)tobeswitchedonfromthestarttilltheend
16 wl.acquire(); //CPU should not sleep ofthecallirrespectiveofhowlongthecallpersists;(c)instancesof
17 } //End while loop; no-sleepbugsinthiscategorywereobservedtocauseseverefrus-
18 } //End Worker_Thread();
tration among smartphone users since the energy drain was both
severe and unexpected; and (d) the root cause of such prolonged
on. Effectivelythereisaraceconditionbetweenthemanipulation completion timeof critical taskswas usually because of ahigher
ofthewakelockbythetwothreads. level bug (i.e., programming mistake) in the code, which signifi-
Listing4showsacodesnippetofano-sleepbugcausedbyarace cantlyinflatedtherunningtimeofcriticaltask.
condition. Main_Threadrunsfirst,acquiringawakelock(waking Wefoundtwocausesforno-sleepdilationinsmartphoneapps:
up a component, e.g., the CPU), and then fires Worker_Thread app delay and app optimizations. We first discuss the dilation
whichperiodicallyexecutesacriticaltask,e.g.,syncingstockup- causedbyappdelayintheGPSdriver[39]inAndroid. Thedriver
dates.AftereverysynchWorker_Threadgivesupthelock,sleeps8 heldwakelocksforlongerthanneeded.Insomecircumstances,af-
for3minutes(allowingtheCPUtosleep),andre-acquiresthelock ter holding thewakelock, thedriver issuedawait, waitingfor an
after waking up. This process is repeated in an infinite loop un- event. However, after being signaled, a second wait was issued
tilWorker_ThreadisnotifiedbyMain_ThreadusingthemKill causing another wait untilthedriver wassignaledagain. Allthis
flagtobreakoutoftheloop. Toinitiatetheterminationoftheapp, was done while holding a wakelock. As a result, a higher level
Main_ThreadsetsthemKillflag,andcallstheAPIstoptosig- buginhandlingsignalsextendedthetimeperiodthewakelockwas
nalWorker_ThreadtoinitiatethehaltwhichwakesMain_Thread beingheld.
upifitisinsleepstate. Main_Threadreleasesthewakelockafter Anothercauseofno-sleepdilationobservedintheappswestud-
callingstop. iedresultsfrompoorplacementofcomponentwakeupcodeinthe
In the normal scheme of things, the code in Listing 4 exe- app. For example, consider the code in Listing 1. The dilation
cutes without any energy bug. However, consider the follow- could happen if the app developer, instead of just protecting the
ing sequence of events. Main_Threadsetsthe mKillflag, sig- critical part net_sync(), wrapped a large piece of code in wake-
nals Worker_Thread to stop, and releases the wakelock. Then locks. We observed such a bug in the MyTrack app [25] where
Worker_Thread wakes up, acquires the lock and exits the loop thedeveloperacquiredtheCPUwakelockthemomenttheappwas
becauseofthemKillflag.Asaresult,thewakelockremainsheld turned onand releasedit whentheapp completed. However, the
bytheappandisneverreleased. Akeypointtonotehereisthat criticalpartofthecodewasonlytheperiodwheretheuserclicked
the semantics of the stop() API called by Main_Thread does thetrackbuttonforlocationtracking.
not guarantee that the return from the call will be synchronized,
i.e.,onlyafterWorker_Threadexits.Hadthatbeenthecase,there 5. DEBUGGINGNO-SLEEP BUGS
would havebeen noracecondition andhencenono-sleep bugin
Two general approaches exist to understanding program be-
theapp.
havior: those done at compile-time and those done at run-time.
Tracingno-sleepbugsinappsourcecodecausedbyracecondi-
Compile-time approaches incur no run-time overhead. While a
tionsisparticularlyhardsinceitrequiresenumeratingallthepos-
run-timeapproachcangather perfect, or nearperfect information
sibleexecutionorderingsofthethreads. However,usingourauto-
about a given run, a compile-time approach will (conservatively)
matic techniques for detecting no-sleep bugs presented in §7, we
determinefactsthatmaybetrueonanyrun. Becauseoftherun-
wereabletodetectaninstanceofano-sleepbugcausedbyarace
timeoverhead, compile-time approaches arepreferred when they
conditionintheAndroidEmailApp[35,36,42],whichhadasim-
are sufficientlyaccurate, asisthe case withour problem. In this
ilarpatternasshowninListing4.
paper,wepresentastatic,compile-timesolutionfordetectingno-
4.3 No-Sleep Dilation sleepenergybugsinsmartphoneapps.
Our solution treats the acquire and release of a wakelock l as
This category of no-sleep bugs differs from the first two cate- adefinitionof (assignment to)thevariable vl corresponding tol.
gories ina singleaspect: the component woken up bythe app is A definition d of a variable vl issaid to reach some point p ina
ultimately put to sleep by the app, but only after a substantially program if there exists a path from d to p that does not redefine
longer period of time than expected or necessary. For example, vl. Therefore, if a definition of vl corresponding to acquiring a
considerthecodeinListing1.Supposeroutinenet_sync()usu-
wakelock reaches the end of some code region thereexists ano-
allyfinishesinafewseconds, butduringaparticularrunithangs
sleepcodepathintheregion. Thus,detectingno-sleepcodepaths
8WeassumethatthesleepAPIcallusedinListing4registersa correspondsexactlytothereachingdefinitions(RD)dataflowprob-
wakeuptimerwiththeCPU,whichincasethephonefreezesduring lem[6],whichcanbesolvedbyastandardcompile-timedataflow
theappsleep,wakesuptheCPUattheendofthetimeout. analysis.
273
Wefirst present, in§6, our solution when only a single thread 6.1.1 TheReachingDefinitionsDataflowProblem
isbeinganalyzed,andthen,in§7,showhowtoapplytheproblem ThefirsttaskinapplyingtheseconceptstotheRDproblemisto
tomulti-threadedsmartphoneappstodetectno-sleepbugsarising constructaCFG.Next,wedefinetheGENandKILLsetsforeach
from races. We leave detecting and debugging no-sleep dilation blockB. Thelastassignmenttoavariablevintheblockcreatesa
bugsasfuturework. definitiondofvthatcanreachotherstatementsoutsidetheblock,
and therefore the definition is placed in the GEN set. Thus for
6. NO-SLEEP CODEPATHS blockB1ofFigure1,thedefinitionsatd1,d2,andd3canreach
Wefirstgiveanoverviewofdataflowanalysis,andthendescribe the other blocks, and therefore are added to B1’s GEN set. The
oursolutionasadataflowanalysisproblem. definition of the KILLset comes fromthe following observation:
anassignmenttosomevariablevinablockpreventsanydefinition
6.1 Dataflow Analysis: AnOverview ofthevariablevoutsidefromflowingthroughtheblock.Thusany
Dataflowanalysisreferstoasetoftechniquesthatascertainfacts definitionoutsidetheblockbecomemembersoftheKILLset.Thus
about program properties by analyzing the effects of statements inblockB5,definitiond8causesdefinitiond4tobeinthekillset.
alongdifferentpathsofacontrolflowgraph(CFG)onthoseprop- Wenow define theIN set. Consider theset of predecessors of
erties. There exist many useful dataflow analysis, e.g., RD (dis- someblockB.AnydefinitionthatisintheOUTsetofoneofthese
cussed above), live variable analysis (which variable values are predecessorscanreachB,andthusisinIN[B]setofB.Therefore,
used after a block), and available expressions (which subexpres- IN[B]istheunionoftheOUTsetsofallofitspredecessors, e.g.,
sionshavealreadybeencomputedandareunchangedyet). IN[B2] is OUT[B5]∪OUT[B1]. Finally, the OUT set for a
EachnodeinaCFGisabasicblockofstatements,i.e.,theblock block B must be computed. The OUT set is simply the IN set
has exactly one entry point and one exit point. There exist a di- with the effects of flowing through the block applied to it. The
rectededge(Bi,Bj)intheCFGconnecting everypairof blocks expressionIN[B]−KILL[B]givesthosedefinitionsthatreached
Bi,BjsuchthatblockBjcanexecuteimmediatelyafterblockBi. the block and can reach later blocks, and unioning this with the
There is also an edge from every exception toevery catch that GENsetgivesalldefinitionsthatcanpassthroughthisblockand
might catch it. Figure 1 shows an example of a dataflow graph. reachotherblocks. ThusfB :OUT[B]=GEN[B]∪(IN[B]−
TwospecialblocksareaddedtotheCFG:ENTERandEXIT.There KILL[B])isthetransferequationforblockB.
existsanedgefromENTERtoeveryblockBi (cid:2)=ENTERwithno Interprocedural analysis, whichincorporates theeffectsofrou-
predecessor, andanedgefromEXITtoeveryblockBj (cid:2)= EXIT tinecallsandroutinearguments,isbeyondthescopeofthisdiscus-
with no successor. A forward dataflow analysis propagates facts sionbutiscoveredindetailin[44].
abouttheprogramfromtheENTERtotheEXITnodewhileaback- 6.2 No-Sleep CodePathDataflow Analysis
wardanalysispropagatesinformationbackwardsthroughthegraph
fromtheEXITtotheENTERnode. We now formulate the single-thread no-sleep code path prob-
EachnodeintheCFGisannotatedwithtwosets:GENandKILL. lem as an RD problem, and show how to solve it using standard
TheKILLsetcontainsfactsintheanalysisthatbecomefalseinthis dataflowanalysis[6]. Weonlyanalyzenon-referencecounted,no-
node,andtheGENsetcontainsfactsthatbecometrue. Eachnode timerwakelocksandexoticcomponentpowerAPIs. Weleavethe
alsohasanINand anOUTset. Thesetsassociatedwithablock studyofothercategoriesasfuturework.
B canbedenotedasIN[B],OUT[B],GEN[B],andKILL[B].For
6.2.1 No-SleepCodePathtoReachingDefinitions
aforward(backward)analysistheIN(OUT)setwillcontainfacts
thataretrueimmediatelybeforethenodeisvisited,andtheOUT For no-sleep code path analysis, we are only interested in the
(IN)setwillcontainfactsthataretrueimmediatelyafterthenode pointsinthecodepathwherethesmartphonecomponentpoweris
isvisited. ThetransferfunctiondescribeshowtheOUT(IN)setis managed,e.g.,thepointsintheCFGwherewakelocksfortheCPU
computedfromtheOUT(IN),KILLandGENsets. Forsimplicity or screen are acquired or released, or points where the camera is
weonlyconsiderforwardanalysisfromthispointon. turnedonandoff. Asaresult,thedomainofthedataflowproblem
CFGs with branches contain join points where multiple paths isasetconsistingofcomponentwakelocksfortraditionalcompo-
cometogether,e.g.,theblockcontainingstatementsd8andd9in nents and component power management assignments for exotic
Figure1. A meet operation decideshow thevalues coming from components. Forbrevity, fromnowonweusewakelockstorefer
thepredecessornodearecombinedtoformthevalueoftheINset. tothepowercontrolhandlesforbothtraditionalandexoticcompo-
IftheCFGcontainscycles, aniterativealgorithmthatvisitnodes nents.
repeatedlyisuseduntiltheanalysisconvergestoafixedpointsuch Once the transformation is completed, the no-sleep code path
thatrevisitingallofthenodesdoesnotchangethevaluesofanyIN problemisreducedtofindingtheRDinthetransformedCFG,i.e.,
orOUTset. ThealgorithmworksbyaddingtheENTERnodetoa findingwhichdefinitionofawakelockreachestheEXITnodeof
worklist. Asnodesareprocessed,iftheirOUTsetchanges,their theCFG.Ifonlythosedefinitionsthatdeclareallthevariablesas
successorsareaddedtotheworklist. Whentheworklistisempty 0(i.e.,thecomponentcansleep)reachtheEXITnode,thecodeis
thealgorithmhasconvergedatafixed-point. saidtobefreeofno-sleepbugs,sinceallofthepossiblecodepaths
We note that all dataflow schemas compute approximations to putallaccessedcomponentstosleepbeforereachingtheendofthe
the actual ground truth. The actual problem being solved is un- CFG,andthereforetheendofthecode.
decidable (e.g., constant propagation [43]). This is because it is Solvingthecodepathproblem. Wenowshowhowtoapplythe
undecidable, in general, if a particular path along a CFG will be standarditerativealgorithmfordataflowanalysistosolveourno-
taken during a program’s execution. As a result, dataflow solu- sleepcodepathproblem.
tionsreturnconservativeorsafeestimatestotheactualproblem.A Forourno-sleepcodepathproblem,thesetofnon-zerovariable
conservative approach guarantees that the resultsobtained by the definitions reaching the EXIT node represents the no-sleep code
analysiswillerronthesideofsafety. ThuswhileanRDanalysis pathbugsintheapp. Table3shows,foreachblockB,theIN[B]
maysaymoredefinitionsreachsomepointthanactuallydo,itwill andOUT[B]setsattheendofthreeiterations.ItshowstheIN[B]
neverfailtofindalldefinitionsthatdoreachaprogrampoint. andOUT[B]setsarethesameattheendofthesecondandthird
274
Listing5:No-sleepcodepathduetoruntimeexceptions.
1 wake_lock_.acquire();//CPU should not sleep
2 Object b = xyz.getObject(); //b is a reference to an
object
3 b.net_sync(); //Perform critical task here
4 wake_lock_.release();//CPU is free to sleep
Listing6:Fixingno-sleepcodepathduetoruntimeexceptions.
1 wake_lock_.acquire();
2 - client = new AppengineClient(this);
3 Log.d(TAG, "onHandleIntent");
4 + try {
5 + client = new AppengineClient(this);
6 //....
7 + } finally {
8 wake_lock_.release();
9 + }
Figure 1: Transforming no-sleep code path into reaching definition
dataflowproblem,andtheresultingINandOUTsets.
gramming typically consists of several functions which are event
handlers,onecorrespondingtoeacheventtheapphandles. These
iteration, and hence the algorithm has reached a fixed-point and
eventscouldbeabuttonclick,anincomingcall,anotificationfrom
converged in three iterations. The value of OUT[EXIT] in the
server,aresponsefromacomponent (e.g.,GPS),etc. Eachevent
lastiterationcontainsthereachingdefinitionsattheendofthecode:
handlerisinvokedwhentheeventisfiredandthehandlermayin
alldefinitionsbutd1andd4canreachtheendofthecode,including turninvokeatreeofroutinesunderneathitbeforeexiting.
d2,d5andd8whichindicatetheexistenceofano-sleepbug.Their Handlingmultipleentrypointsofanappcreatesanewchallenge:
presence indicatesno-sleep code paths along which acomponent each handler has itsown CFG,9 and a component may be turned
(GPS, camera, and CPU wakelock_2, respectively) is woken up
oninoneeventhandler andputtosleepunder another(e.g.,start
butnotputtosleep.
camera when start button is clicked and stop camera when stop
buttonisclicked). However,theorderofexecutionofthedifferent
6.2.2 HandlingUncaughtRuntimeExceptions
events, which is needed to stitch together the CFGs of different
Java runtime exceptions (RTE) [45] (e.g., null pointer and ar- handlers, may be unknown at compile time and depends on user
rayindexoutofboundsexceptions)canbethrownduringnormal interactions.
JavaVirtualMachine(JVM)operations.RTEsthatarehandledex- Wehandlethiscomplicationasfollows. (1)Forcommonevent
plicitlybyatry-catchblockincodearehandledasbeforeby handlers (e.g., onCreate, onPause) which have known in-
addingapathfromthesourceblocktothehandlerblock.However, vocation orders, we simplyperform theRD analysis across them
RTEsareoftennothandledbyaprogramandthethreadraisingthe onthecombinedCFGobtainedfromstitchingtogetherindividual
exceptionisterminatedbytheJVMwhentheexceptionisthrown. CFGsfollowing those invocation orders. For example, if a com-
UncaughtRTEsareasourceofno-sleepbugsandmustbehan- ponent is not put to sleep when the app is paused after being
dledbyouranalysis.ConsiderthecodeinListing5.ACPUwake- firstcreated,itusuallyisasignofano-sleepbug. (2)Forthe
lockisacquired,followedbyacalltothecriticalroutinefromin- remaininghandlers,weaskthedeveloperstospecifyallexpected
stanceb.Thewakelockisreleasedafterthecall.IfaRTEisraised invocationorders, andthenperformno-sleepbugRDanalysison
(e.g.,anullpointerexceptiononline3causedbybbeingnull), thecombinedCFGfortheseorders.
thethreadishalted. Thisresultsinano-sleepenergybugsincethe
threadterminatesbeforethewakelockisreleased.Weidentifiedan
7. NO-SLEEP RACECONDITIONS
instance of this bug [26] in our characterization study. Listing 6
detailsthepatchappliedbythedevelopertofixanullpointerRTE Tostaticallydetectpossibleno-sleepraceconditionsformulti-
(codelinesappendedwith“-”or“+”indicatethattheselineswere threadapps, weadapt theRDdataflowanalysispreviouslydevel-
removedfromoraddedtothenewversion,respectively). Thede- opedforparallelprograms[49,50].
veloperaddedhandlersforthenullpointerRTEandmovedthelock A multi-threaded program typically has a repeating pattern of
releaseintoafinallyblocktoensurethatitisrunregardlessof sequentialsectionsendingwithathreadfork,interleavedexecution
anyexceptions. ofparallelthreadsfollowedbyathreadjoin,followedbythenext
OurtechniqueplacesanedgefromeachRTEthatisnothandled sequential section in the pattern. Execution is sequential within
withinaroutinetotheEXITnodeforthatroutine. Thiscreatesa each thread and so a CFG can be built for the thread. CFGs for
pathforalockacquiredefinitiontoreachtheexit,andcouldlead different threads can be stitched together by connecting the fork
tomorefalsepositives(althoughwehavenotseenthatinourtest spawning the thread withthe ENTRYnode for the thread’s CFG,
cases). Techniquessuchasnullpointeranalysis[46],ABCD[47] andtheEXITnodeoftheCFGwiththejoinnodeusingaparallel
forarrayboundscheck,andRTEanalysistechniques,e.g.,[48]can controledge[50].
beusedtomaketheanalysismorepreciseandgeneratefewerfalse TheRDanalysisisnowmodifiedforthisnewCFG.Threeobser-
positives. vations[50]motivatethesemodifications.(1)Allthreadsinaparal-
lelsectionareexecuted;(2)Anyofthedefinitionsdti,dtj,...,dtk
6.2.3 HandlingEventBasedEntryPoints tosomevariablev executingindifferentthreads,andnotordered
Android app programming is primarily event-based program- 9We extract the entry point of an event handler, i.e., the root
ming.Unliketraditionalcode,wherethemain()routinestartsthe of its CFG, from various .xml files in the build tree (e.g.,
appwiththeappexitingwhenmain()returns,Androidapppro- Manifest.xml,main.xml).
275
Table3:ComputingINandOUTforno-sleepcodepaths.
BlockB OUT[B]0 IN[B]1 OUT[B]1 IN[B]2(=IN[B]3) OUT[B]2(=OUT[B]3)
B {} {} {d ,d ,d } {} {d ,d ,d }
1 1 2 3 1 2 3
B {} {d ,d ,d } {d ,d ,d ,d ,d } {d ,d ,d ,d ,d ,d ,d ,d } {d ,d ,d ,d ,d ,d ,d }
2 1 2 3 1 2 3 4 5 1 2 3 5 6 7 8 9 1 2 3 4 5 7 9
B {} {d ,d ,d ,d ,d } {d ,d ,d ,d ,d } {d ,d ,d ,d ,d ,d ,d } {d ,d ,d ,d ,d ,d ,d }
3 1 2 3 4 5 1 2 3 4 6 1 2 3 4 5 7 9 1 2 3 4 6 7 9
B {} {d ,d ,d ,d ,d } {d ,d ,d ,d ,d } {d ,d ,d ,d ,d ,d ,d } {d ,d ,d ,d ,d ,d }
4 1 2 3 4 6 1 3 4 6 7 1 2 3 4 6 7 9 1 3 4 6 7 9
B {} {d ,d ,d ,d ,d ,d ,d } {d ,d ,d ,d ,d ,d ,d } {d ,d ,d ,d ,d ,d ,d ,d } {d ,d ,d ,d ,d ,d ,d }
5 1 2 3 4 5 6 7 2 3 5 6 7 8 9 1 2 3 4 5 6 7 9 2 3 5 6 7 8 9
EXIT {} {d ,d ,d ,d ,d ,d ,d } {d ,d ,d ,d ,d ,d ,d } {d ,d ,d ,d ,d ,d ,d } {d ,d ,d ,d ,d ,d ,d }
2 3 5 6 7 8 9 2 3 5 6 7 8 9 2 3 5 6 7 8 9 2 3 5 6 7 8 9
Listing7:Handlingsimplecodepaths.
1 wl1.acquire(); wl2.acquire(); //wakeup
2 if(wl1 != null) //if object is not null
3 wl1.release(); //release the wakelock
4 if(wl2.isHeld()) //if wl2 is acquired
5 wl2.release(); //release the wakelock
controltransfermechanismposesproblemsforstaticanalysissince
itisdifficulttodetermineatcompiletimewhichclasstheobjectis
aninstanceof,orwhichhandlerwillservicetheintent,andhence
which particular method (routine) will be called. We use a con-
servativeapproachbyanalyzingallroutines’referencesthatcould
possiblybereferredtoatruntime.
Figure2: Tracingno-sleepracebug: ParallelFlowGraph(PFG)for
HandlingSpecialCodePaths:Toreducethenumberoffalsepos-
codeinListing4.
itives, we handle the two special cases shown in Listing 7. Two
wakelocks(wl1, wl2)areacquiredbeforeifconditionsandare
bysynchronization,maybethelastdefinitionofvtoexecute,there- releasedundertheirrespectiveif. Wefoundthesetwousagesto
forenosuchdefinitionttpcanbesaidtokillanotherdefinitiondtq, becommoninmobileapps.TheRDno-sleepbuganalysisforcode
p,q∈{i,j,...,k};(3)Anykillperformedunconditionallybyany inListing7wouldflagbothlockacquiresasreachingtheendofthe
blocksincetheyarenotreleasedintheelsebranches. However,
threadinaparallelsection(i.e.,alongallsequentialpathsthrough
inbothcases,itisevidentthatthereisnobugsinceifthewakelock
the thread) kills all definitions that occur before the parallel sec-
tion’sfork; (4)Anydefinitiondperformedconditionallywithina iseithernullornotheld,itneednotbereleased.Wehandlethese
threaddoesnotkilldefinitionsd(cid:2) beforethethread’sparallelsec- twocommonusagesspecially,byinsertinganelsebranchtothe
tion’sforksinceeitherdord(cid:2)mayreachtheparallelsection’sjoin, ifconditionwhichcontainsadefinitionofrelease().
Runtime Exceptions: In our characterization study, we did not
and following statements. The details of modifying the dataflow
observeanyoccurrencesofuncaughtRTEsotherthannullpointer
analysis to account for these additional constraints and ordering
exceptions(NPEs).Hence,inourcurrentimplementation,weonly
synchronizationarediscussedindetailin[50]andarenotrepeated
handle NPEs. Weleavehandling other RTEsasfuture work. To
here.
handleNPEs,wetracethenullreachingdefinitionsateachaccess
Figure2showsthattheRDanalysisonthePFGfindsthatdefi-
nitionsd3,d4andd7canreachtheEXITnode. Sinced7turnson point of theobject forevery object declared intheprogram. Ifa
nulldefinitionreachesanobjectaccesspoint,weaddapathfrom
thecomponent,thisisano-sleepracebug.
thatpointtotheEXITnodeintheCFG.
Race Conditions: Our current implementation only implements
8. IMPLEMENTATION analysisforno-sleepraceconditionforprogramswithoutsynchro-
ProGuardExtension: Weimplementedno-sleepbugtracingasa nizationpoints. Weleaveasfutureworkextensionstohandlesyn-
chronization points, which can be implemented by adopting the
1K-LOC extension toProGuard[15]. TheProGuardtool isused
techniquesproposedin[50].
toshrink,optimizeandobfuscateAndroidcodeandhelpstomake
smaller.apkinstallerfiles. ItbuildsanintermediateRepresenta-
tion of the input source containing CFGsthat weuse. We chose
9. EVALUATION
ProGuard since it is integrated into the Android build system, it
automaticallyrunswhenanAndroidapportheframeworkiscom- Wenow present experimental resultsof no-sleepbugdetection
piled and does not require a separate, manual invocation. How- usingourdataflowanalysisbasedno-sleepbugdetectiontool. We
ever,sourcecodeisnotrequiredtoperformtheanalysissincePro- firstpresentasummaryofthedetectionresultson500appsrunning
GuardcanrundirectlyonthebytecodegeneratedbytheJavacom- onAndroidandthendiscussfalsepositivesandtheruntimeofthe
piler. If weonly have the .apk installer for an app we first use scheme.
useded[16]todecompiletheembedded.dexfiles(DalvikEx- Methodology: We collected app installers (.apk files) for 500
ecutable[51])andconvertthemtoJavabytecode(.classfiles). apps, includingpopular appslikeFacebook, Googleappssuchas
WethenrunProGuardandtheno-sleep bugdataflow analysison gtalk and stock apps in the Android framework including Email
the.classbytecodes. andDialer. TheseincludealltheappslistedinTable2. Automatic
HandlingObjectReferencesandIntentResolution:Javaobject analysis of themanifest.xmlfilefor permissions reveals that
referencesandintentresolutionsaretheonlyindirectcontroltrans- 187appsexplicitlymanipulatecomponentwake/sleepcycles. We
fermechanisms intheAndroidframework andapps. Anindirect thendecompiledthe.apkinstallersusingded[16]andobtained
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