Real-Time Operating Systems M. 11. Real-Time: Periodic Task Scheduling

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1 Real-Tme Operatng Systems M 11. Real-Tme: Perodc Task Schedulng

2 Notce The course materal ncludes sldes downloaded from:! and! (sldes by Slberschatz, Galvn, and Gagne, assocated wth Operatng System oncepts, 9th Edton, Wley, 2013)! (sldes by Buttazzo, assocated wth Hard Real-Tme omputng Systems, 3rd Edton, Sprnger, 2011)! whch has been edted to sut the needs of ths course.! The sldes are authorzed for personal use only.! Any other use, redstrbuton, and any for proft sale of the sldes (n any form) requres the consent of the copyrght owners.! 11.2! Torron, Real-Tme Operatng Systems M 2013!

3 Problem Formulaton (, T ) job k r k d k For each perodc task, guarantee that:! Each job τ k s actvated at r j = (k-1)t! Each job τ k completes wthn d k = r j + D! 11.3! Buttazzo, Hard Real-Tme omputng Systems 2013!

4 A Farm Schedulng Problem Feed cow for 25 mn / 50 mn Feed pg for 10 mn / 20 mn 11.4! Buttazzo, Hard Real-Tme omputng Systems 2013!

5 Frst Algorthm Alternate pg wth cow! Pg ow Evaluaton:! Pg gets hungry! ow gets fat! 11.5! Buttazzo, Hard Real-Tme omputng Systems 2013!

6 Second Algorthm Feed pg and cow 10 mn each! Pg ow Evaluaton:! Pg s OK! ow s not happy! 11.6! Buttazzo, Hard Real-Tme omputng Systems 2013!

7 Thrd Algorthm Feed pg and cow 5 mn each! Pg ow Evaluaton:! Pg s OK, ow s OK! Farmer s tred! 11.7! Buttazzo, Hard Real-Tme omputng Systems 2013!

8 Optmal Algorthm Feed the most starvng anmal! Pg ow Evaluaton:! Everybody s happy! 11.8! Buttazzo, Hard Real-Tme omputng Systems 2013!

9 What Do We Learn? Reducng the executon tme wndow, we get closer to a feasble soluton! The tme s splt proportonally between the anmals! In the example, each anmal requred food for 50% of the tme! How can we generalze the soluton f the anmals requre dfferent fractons of tme?! 11.9! Buttazzo, Hard Real-Tme omputng Systems 2013!

10 A New Schedulng Problem Feed cow for 4 mn / 16 mn Feed pg for 20 mn / 40 mn 11.10! Buttazzo, Hard Real-Tme omputng Systems 2013!

11 Proportonal Share Proportonal Share algorthm:! 1. Dvde the tmelne nto slots of equal length! 2. Wthn each slot serve each task for a tme proportonal to ts utlzaton:! ow utlzaton factor = 4/16 = ¼! Pg utlzaton factor = 20/40 = ½! Pg 4/16 ow 20/ ! Buttazzo, Hard Real-Tme omputng Systems 2013!

12 Proportonal Share In general:! Let: U = requred feedng fracton! Δ = GD(T 1,T 2 ) = 8! Execute each task for δ = U Δ n each slot Δ! Pg 4/16 ow 20/ Note: U Δ ensures n T, n fact: δ(t /Δ)=! Feasblty test: Σδ Δ,.e., ΣU 1! 11.12! Buttazzo, Hard Real-Tme omputng Systems 2013!

13 Tmelne Schedulng (yclc Executve) It has been used for 30 years n mltary systems, navgaton, and montorng systems.! Examples:! Ar traffc control! Space Shuttle! Boeng 777! Idea:! Dvde tme axs n slots of equal length! Desgn statc schedulng (by hand)! Allocate each task n a slot, so as to meet the desred request rate! Actvate executon of each slot by a tmer! 11.13! Buttazzo, Hard Real-Tme omputng Systems 2013!

14 Example task A B f T 40 Hz 25 ms 20 Hz 50 ms 10 Hz 100 ms = GD (mnor cycle) T = lcm (major cycle) T Guarantee s very smple (wthn each mnor cycle):! A + B Δ = 25 ms! A + c Δ = 25 ms! 11.14! Buttazzo, Hard Real-Tme omputng Systems 2013!

15 yclc Executve: Implementaton The task sequence s not decded by a schedulng algorthm n the kernel, but t s trggered by calls made by the man program (no context swtches)! A B A A B tmer tmer tmer mnor cycle major cycle A tmer ! Buttazzo, Hard Real-Tme omputng Systems 2013!

16 yclc Executve: PROs and ONs Advantages: lghtweghtness, regularty! Smple mplementaton (does not requre RTOS)! Low run-tme overhead! Allows jtter control! Dsadvantages: rgdty! Fragle durng overloads! Dffcult to expand the schedule! Dffcult to handle aperodc actvtes! 11.16! Buttazzo, Hard Real-Tme omputng Systems 2013!

17 Problems Durng Overloads What do we do durng task overruns?! Let the task contnue! May have domno effect on all the other tasks (tmelne break)! Abort the task! The system can reman n nconsstent states! 11.17! Buttazzo, Hard Real-Tme omputng Systems 2013!

18 Expandblty If one or more tasks need to be upgraded, we! may have to re-desgn the whole schedule agan! task A B f T 40 Hz 25 ms 20 Hz 50 ms 10 Hz 100 ms Example:! Stuaton: B s updated but A + B > Δ! (prevous schedule) A B 0 25 Acton: splt B n two subtasks, B 1 and B 2, and re-buld the schedule! A B 1 A B 2 A B 1 A B ! Buttazzo, Hard Real-Tme omputng Systems 2013!

19 Expandblty task A B f T 40 Hz 25 ms 20 Hz 50 ms If the frequency of some task s changed, the! mpact can even be more sgnfcant:! 10 Hz 100 ms Example:! Stuaton: B s cycle changes from 50 ms to 40 ms! (prevous schedule) task A B T 25 ms 50 ms T 25 ms 40 ms 100 ms 100 ms before after mnor cycle: major cycle: = 25 = 5 T = 100 T = sync. per cycle! Acton: re-buld the schedule usng dfferent major/mnor cycle length! 11.19! Buttazzo, Hard Real-Tme omputng Systems 2013!

20 Expandblty task A B f T 40 Hz 25 ms 20 Hz 50 ms task A B T T 25 ms 25 ms 50 ms 40 ms 100 ms 100 ms before after mnor cycle: = 25 = 5 major cycle: T = 100 T = sync. per cycle! 10 Hz 100 ms (prevous schedule) T T 11.20! Buttazzo, Hard Real-Tme omputng Systems 2013!

21 Prorty Schedulng Idea:! Assgn each task a prorty based on ts tmng constrants! Verfy the feasblty of the schedule usng analyss technques! Execute tasks on a prorty-based kernel! Prortes could be statc or dynamc! Examples:! RM: assgn fxed prorty to tasks, proportonal to task rate! EDF: at all tmes, assgn top prorty to job wth earlest absolute deadlne! 11.21! Buttazzo, Hard Real-Tme omputng Systems 2013!

22 Assumptons The nstances of a perodc task are regularly actvated at a constant rate wth perod T.! All tasks are released as soon as they arrve.! All nstances of a perodc task have:! the same worst-case executon tme! the same relatve deadlne D = T! Independent tasks (no precedence relatons, no resource constrants)! No task can suspend tself (trap)! Neglgble kernel overheads! T 11.22! Buttazzo, Hard Real-Tme omputng Systems 2013!

23 Statc Prorty Schedulng

24 How to assgn prortes? Typcally, task prortes are assgned based on the tasks relatve mportance! Example: Solars schedulng! hghest global prorty nterrupt threads schedulng order frst However, dfferent assgnments can lead to dfferent utlzaton bounds! realtme (RT) threads system (SYS) threads far share (FSS) threads fxed prorty (FX) threads tmeshare (TS) threads lowest 0 nteractve (IA) threads last 11.24! Buttazzo, Hard Real-Tme omputng Systems 2013!

25 Prorty Importance If τ 2 s more mportant than τ 1 and s assgned a hgher prorty!! the schedule may not be feaslble...! 1 P 1 > P 2 2 deadlne mss 1 P 2 > P ! Buttazzo, Hard Real-Tme omputng Systems 2013!

26 Prorty Importance!!!!!!! whle the utlzaton upper bound can be arbtrarly small.! P 2 > P deadlne mss U = ε/ /! 0! An applcaton can be unfeasble even f the processor s almost empty!! 11.26! Buttazzo, Hard Real-Tme omputng Systems 2013!

27 Rate Monotonc (RM)! Assgn each task a fxed prorty proportonal to ts request rate! (Lu & Layland 73)! A B ! Buttazzo, Hard Real-Tme omputng Systems 2013!

28 Rate Monotonc s Optmal RM s optmal among all fxed prorty algorthms (f D = T )! RM optmalty (n the sense of feasblty):! If there exsts a fxed prorty assgnment whch leads to a feaslbe schedule for Γ, then the RM assgnment s feasble for Γ.! If Γ s not schedulable by RM, then t cannot be scheduled by any fxed prorty assgnment.! 11.28! Buttazzo, Hard Real-Tme omputng Systems 2013!

29 An RM-Unfeasble Schedule D = T U pp deadlne mss 11.29! Buttazzo, Hard Real-Tme omputng Systems 2013!

30 Identfyng the Worst ase Feasblty may depend on the ntal actvatons (phases):! U p deadlne mss ! Buttazzo, Hard Real-Tme omputng Systems 2013!

31 rtcal Instant The longest response tme occurs when a task arrves together wth all hgher prorty tasks! 1 2 R R ! Buttazzo, Hard Real-Tme omputng Systems 2013!

32 rtcal Instant For ndependent preemptve tasks under fxed prortes, the crtcal nstant of τ occurs when t arrves together wth all hgher prorty tasks.! 1 1/6 2 2/8 3 2/12 Idle tme 2/ ! Buttazzo, Hard Real-Tme omputng Systems 2013!

33 How can we verfy feasblty? Each task uses the processor for a fracton of tme:! U T Hence the total processor utlzaton s:! U p n 1 T U p s a measure of the processor load! 11.33! Buttazzo, Hard Real-Tme omputng Systems 2013!

34 A Necessary ondton If U p > 1 the processor s overloaded hence the task set cannot be schedulable! However, there are cases where:! U p < 1! but the task set s not schedulable by RM! U p U p deadlne mss Utlzaton upper bound: f 1 or 2 s ncreased, τ 2 wll mss ts deadlne!! 11.34! Buttazzo, Hard Real-Tme omputng Systems 2013!

35 and a Dfferent Upper Bound 2 4 U ub The upper bound U ub depends on the specfc task set.! 11.35! Buttazzo, Hard Real-Tme omputng Systems 2013!

36 and Yet Another One 2 4 U 1 4 p The upper bound U ub depends on the specfc task set.! In these examples: U ub = 0.833, 0.9, 1,.! Is there anythng more we can tell about U ub?! 11.36! Buttazzo, Hard Real-Tme omputng Systems 2013!

37 The Least Upper Bound U ub 1 U lub ! Buttazzo, Hard Real-Tme omputng Systems 2013!

38 A Suffcent ondton Gven a task set Γ:! If U p U lub, Γ s certanly schedulable wth RM! If U lub < U p 1, we cannot say anythng about Γ s feasblty! 11.38! Buttazzo, Hard Real-Tme omputng Systems 2013!

39 Least Upper Bound for RM Lu & Layland, 1973! Gven a set of n perodc tasks:! U RM lub n 2 1/ n 1 for n U lub ln 2 Used for RM guarantee test:! ompute processor utlzaton! Verfy that t does not exceed the least upper bound! 11.39! Buttazzo, Hard Real-Tme omputng Systems 2013!

40 RM Schedulablty U lub RM for n tasks! n! n(2 1/n -1)! 1! 1! 2! 0.828! 3! 0.780! 4! 0.757! 5! 0.743! 10! 0.718! 20! 0.705! 50! 0.698! 100! 0.696! 1000! 0.693! PU% n 69% 11.40! Buttazzo, Hard Real-Tme omputng Systems 2013!

41 RM Schedulablty U ub RM for 2 tasks s a functon of! F =!" T 2 T 1 #$ n! n(2 1/n -1)! 1! 1! 2! 0.828! 3! 0.780! 4! 0.757! 5! 0.743! 10! 0.718! 20! 0.705! 50! 0.698! 100! 0.696! 1000! 0.693! F U* 1! 0.828! 2! 0.899! 3! 0.928! 4! 0.944! 5! 0.954! 10! 0.976! 20! 0.988! 50! 0.995! 100! 1.000! 1000! 1.000! U * = 2( F(F +1) F) 11.41! Buttazzo, Hard Real-Tme omputng Systems 2013!

42 A Tghter Least Upper Bound for RM Bn & Buttazzo 2, 2000! Hyperbolc Bound! A set of n perodc tasks s schedulable wth RM f:! n 1 (U 1) 2 It s a tght bound: gven any set of utlzatons that volate the HB, t s always possble to produce an unfeasble task set wth those utlzaton.! 11.42! Buttazzo, Hard Real-Tme omputng Systems 2013!

43 omparson The hyperbolc bound can be compared wth the Lu-Layland bound n the task utlzaton space (U-space)! U HB vs. LL n 1/ n LL U n(2 1) 1 n HB ( 1) 2 1 U U The gan acheved by HB over LL ncreases wth n (t tends to 2)! ! Buttazzo, Hard Real-Tme omputng Systems 2013!

44 Exercse 4.1! T! τ 1! 2! 6! τ 2! 2! 8! τ 3! 2! 12! Verfy the schedulablty and construct the RM schedule.! 11.44! Buttazzo, Hard Real-Tme omputng Systems 2013!

45 Exercse 4.2! T! τ 1! 3! 5! τ 2! 1! 8! τ 3! 1! 10! Verfy the schedulablty and construct the RM schedule.! 11.45! Buttazzo, Hard Real-Tme omputng Systems 2013!

46 Extenson to Tasks wth D < T More modelng possbltes. For nstance:! Tasks wth jtter constrants;! Actvtes wth shorter response tme wth respect to ther perod.! T D r,k d,k r,k+1 t Deadlne Monotonc (DM):! P 1/D (statc)! Earlest Deadlne Frst (EDF):! P 1/d,k (dynamc)! 11.46! Buttazzo, Hard Real-Tme omputng Systems 2013!

47 Deadlne Monotonc! Assgn each task a fxed prorty nversely proportonal to ts relatve deadlne! (Leung & Whtehead 1982)!! ! Buttazzo, Hard Real-Tme omputng Systems 2013!

48 Deadlne Monotonc s Optmal If D < T, f a task set s schedulable by some fxed prorty assgnment, then t s also schedulable by DM.! DM P 2 > P RM P 1 > P ! Buttazzo, Hard Real-Tme omputng Systems 2013!

49 Good News, Bad News Good news! DM gves optmal prorty assgnment.! Bad news! problem wth the utlzaton bound:!! U p n 1 D but the task set s schedulable! PU workload overestmated! RM guarantee test too pessmstc for DM! 11.49! Buttazzo, Hard Real-Tme omputng Systems 2013!

50 Seen so far Problem defnton (perodc task schedulng)! oncepts (processor utlzaton, crtcal nstant, upper bound)! Schedulng Algorthms! Theoretcal (Proportonal Share)! Paper & pencl (Tmelne Schedulng)! Fxed Prorty (optmal)! Rate Monotonc f D=T! Deadlne Monotonc f D<T! Shedulablty Analyss! Least Upper Bound! Lu-Layland! Hyperbolc Bound! Next?! 11.50! Torron, Real-Tme Operatng Systems M 2013!

51 Response Tme Analyss A suffcent and necessary schedulablty test for DM (Audsley et al., 1990)!! For each task τ, compute the nterference (preempton) due to hgher prorty tasks:! I k D k D ompute ts response tme as:! Verfy whether! R D R = + I! 11.51! Buttazzo, Hard Real-Tme omputng Systems 2013!

52 omputng the Interference Assume tasks are ordered by ncreasng relatve deadlnes! < j f and only f D D j! k 0 R Interference of k on n the nterval [0, [, R ] ]: Interference of hgh prorty tasks on : I I k 1 k 1 R T T k R T k k k 11.52! Buttazzo, Hard Real-Tme omputng Systems 2013!

53 11.53! Buttazzo, Hard Real-Tme omputng Systems 2013! omputng the Response Tme R R 1 k k k T R 1 Iteratve soluton: R 0 67 k k s k s T R R 1) ( 1 1 R terate whle ( 1) s s R R k 1 1 k k s k s T R R 1) ( 1 1 R terate whle ( 1) s s R R omputng Response Tme R R 1 k k k T R 1 Iteratve soluton: R 0 67 k k s k s T R R 1) ( 1 1 R terate whle ( 1) s s R R 14/11/2012 omputng Response Tme R R 1 k k k T R 1 Iteratve soluton: R 0 67 k k s k s T R R 1) ( 1 1 R terate whle ( 1) s s R R

54 Exercse 4.7! D! T! τ 1! 2! 5! 6! τ 2! 2! 4! 8! τ 3! 4! 8! 12! Verfy the schedulablty and construct the DM schedule.! 11.54! Buttazzo, Hard Real-Tme omputng Systems 2013!

55 Exercse 4.3! T! τ 1! 1! 4! τ 2! 2! 6! τ 3! 3! 10! Verfy the schedulablty and construct the RM schedule.! 11.55! Buttazzo, Hard Real-Tme omputng Systems 2013!

56 Exercse 4.4! T! τ 1! 1! 4! τ 2! 2! 6! τ 3! 3! 8! Verfy the schedulablty and construct the RM schedule.! 11.56! Buttazzo, Hard Real-Tme omputng Systems 2013!

57 Dynamc Prorty Schedulng

58 EDF D = T! No constrants! U p RM: unfeasble.! EDF: feasble!! Wth EDF, any task set can utlze the processor up to 100%! 1 2 D = T U p deadlne mss 11.58! Buttazzo, Hard Real-Tme omputng Systems 2013!

59 EDF Optmalty & Schedulablty Optmalty. EDF s optmal among all algorthms (Dertouzos 1974)! If there exsts a feasble schedule for Γ, then EDF wll fnd a feasble schedule! If Γ s not schedulable by EDF, then t cannot be scheduled by any algorthm! (result ndependent of perodcty)! Schedulablty. For a set of n perodc tasks,! U EDF lub 1!(Lu & Layland 1973)! In other words, a task set Γ s EDF-schedulable f and only f U p 1! 11.59! Buttazzo, Hard Real-Tme omputng Systems 2013!

60 EDF Schedulablty Necessty: schedulable! U p 1 s trval! To prove suffcency: U p 1! schedulable! 1. We fnd any algorthm for whch the above condton holds! 2. Then, for the EDF optmalty, we can say that the above condton also holds for EDF.! t onsder the algorthm whch schedules n every nterval of length Δ a fracton of task: δ = U t Δ! Proportonal Share Algorthm! n Feasblty s ensured f, that s, f U p 1.! ! Buttazzo, Hard Real-Tme omputng Systems 2013!

61 Exercse 4.5! T! τ 1! 1! 4! τ 2! 2! 6! τ 3! 3! 8! Verfy the schedulablty and construct the EDF schedule.! 11.61! Buttazzo, Hard Real-Tme omputng Systems 2013!

62 Schedulablty wth Dynamc Prorty What f D T?! Processor Demand rteron! n any nterval, the computaton demanded by the task! must be no greater than the avalable tme! (Baruah, Roser & Howell 1990)! t 1 t 2 Demand of a task τ n [t 1,t 2 ]: amount of processng tme g (t 1,t 2 ) of all nstances of τ that are actvated n [t 1,t 2 ] and must be completed n [t 1,t 2 ].! For the whole task set: g(t 1,t 2 )! g( t1, t2) d t r t ! Buttazzo, Hard Real-Tme omputng Systems 2013!

63 Processor Demand Test 0 L Processor Demand n [0,L] aka Demand Bound Functon, dbf(l)! g(0, L) n 1 L D T T Demand Test! L 0, g(0, L) L How can we bound the number of ntervals n whch the test has to be performed?! 11.63! Buttazzo, Hard Real-Tme omputng Systems 2013!

64 Example g(0, L) L 2 0 L ! Buttazzo, Hard Real-Tme omputng Systems 2013!

65 11.65! Buttazzo, Hard Real-Tme omputng Systems 2013! Boundng omplexty Some consderatons:! Snce g(0,l) s a step functon, t suffces to check feasblty only at deadlne ponts (d k )! If tasks are synchronous and U p < 1, t suffces to check feasblty only up to the hyperperod H = lcm(t 1,, T n )! g(0,l) G(0,L) and, f U < 1, there exsts an L* for whch G(0,L*) = L*.! n T D T L L G 1 ) (0, n n D T L ) ( T D T T L 1 1 ) ( n U D T LU 1 ) ( n U D T LU L G 1 ) ( ) (0, L g(0, L) G(0, L) 1 f L>L * U U D T L n 1 ) ( 1 * L L * for L > L * g(0,l) G(0,L) < L

66 Processor Demand Test A set of synchronous perodc tasks wth relatve deadlnes less than or equal to perods can be scheduled by EDF f and only f! U < 1, and!! L D, g (0, L ) L D = {d k d k mn (H, L * )} H = lcm(t 1,, T n n) L * n 1 ( T D ) U 1U g(0, L) n 1 L D T T 11.66! Buttazzo, Hard Real-Tme omputng Systems 2013!

67 Exercse 4.6! D! T! τ 1! 2! 5! 6! τ 2! 2! 4! 8! τ 3! 4! 8! 12! Verfy the schedulablty and construct the EDF schedule.! 11.67! Buttazzo, Hard Real-Tme omputng Systems 2013!

68 Quzzes True or False?! If a task set s DM-schedulable, t s EDF-schedulable! If a task set s EDF-schedulable, ts processor utlzaton U p s below the HB! If a task set s processor utlzaton U p s below the Lu-Layand bound, then U p s also below the HB! A task set consstng of two tasks, τ 1 and τ 2, wth D =T and T 1 =2T 2, s RM-feasble f and only f the total processor utlzaton s at most 1! Response Tme Analyss can be used to study schedulablty, even f relatve deadlne and pedod concde (for all τ, D =T )! The Processor Demand Test can be used to study schedulablty, even f relatve deadlne and pedod concde! If for all τ, D =T, the Processor Demand Test and Response Tme Analyss of a gven task set gve the same schedulablty result!! 11.68! Torron, Real-Tme Operatng Systems M 2013!

69 Summary

70 Perodc Task Schedulng Three schedulng approaches! Off-lne constructon (tmelne)! Fxed prorty (RM, DM)! P 1/T " P 1/D! Dynamc prorty (EDF)! P 1/d,k, d,k = r,k + D! Three analyss technques:! Processor Utlzaton Bound!!U U lub!!!o(n)! Response Tme Analyss!!for all, R D!!*! Process Demand rteron!!for all L, g(0,l) L!*!!* pseudo-polynomal complexty! 11.70! Buttazzo, Hard Real-Tme omputng Systems 2013!

71 RM vs. EDF RM! Smpler to mplement n commercal operatng systems! fxed prortes! More predctable durng! EDF! overloads! hghest prorty tasks are!! known! More effcent! Reduces context swtches! RM Better responsveness n handlng aperodc tasks! Perod rescalng durng permanent overloads! EDF deadlne mss ! Buttazzo, Hard Real-Tme omputng Systems 2013!

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