Scheduling Motivation

Transcription

1 76 eal-me & Embedded Systems 7 Uwe. Zmmer - he Australan Natonal Unversty 78 Motvaton n eal-me Systems Concurrency may lead to non-determnsm. Non-determnsm may make t harder to predct the tmng behavour. eal-me- schemes reduce non-determnsm. Uwe. Zmmer, he Australan Natonal Unversty page 78 of 96 (chapter 7: up to page 8) 77 eferences for ths chapter [Burns7] Alan Burns and Andy Wellngs Concurrent and eal-me Programmng n Ada Cambrdge Unversty Press (7) [Murthy] C S Murthy, G Manmaran esource Management n eal Systems and Networks MI Press, Cambrdge, Massachuetts, London, England () Uwe. Zmmer, he Australan Natonal Unversty page 77 of 96 (chapter 7: up to page 8) 79 Deployment n eal-me Systems A schedulng scheme provdes two features: Orderng the use of resources (e.g. CPUs, networks) n a lve system. Predctng the worst-case behavour of the system when the schedulng algorthm s appled. he predcton can then be used: at comple-run: to confrm the overall temporal requrements of the applcaton. at run-: to permt acceptance of addtonal usage/reservaton requests. Uwe. Zmmer, he Australan Natonal Unversty page 79 of 96 (chapter 7: up to page 8)

2 74 Statc versus Dynamc n eal-me Systems gd: All schedules are set off-lne. Full predctablty (many hgh ntegrty real- systems). Statc: Schedule relatons are statcally ordered off-lne. Predctable response to dsturbances (many real- systems). Dynamc: Schedules depend on run- stuaton. More flexble, more effcent (most soft real- systems). Uwe. Zmmer, he Australan Natonal Unversty page 74 of 96 (chapter 7: up to page 8) 74 phases (real-) pre-empton or cycle done batch creaton ready admtted dspatch executng CPU termnaton unblock Admttance accordng to schedulablty Dspatchng and Pre-Empton accordng to deadlnes, prortes, or utlttes blocked block or synchronze Uwe. Zmmer, he Australan Natonal Unversty page 74 of 96 (chapter 7: up to page 8) 74 phases (non real-) pre-empton or cycle done batch creaton ready admtted dspatch executng CPU termnaton unblock suspend (swap-out) ready, suspended suspend (swap-out) swap-n blocked, suspended unblock swap-out blocked block or synchronze Uwe. Zmmer, he Australan Natonal Unversty page 74 of 96 (chapter 7: up to page 8) 74 Assumptons A smple process model he number of processes n the system s fxed. All processes are perodc and all perods are known. All processes are ndependent. he task-swtchng overhead s neglgble. All deadlnes are dentcal wth the process cycle s (perods). he worst case executon s known for all processes. All processes are released at once. hs model can only be appled to a specfc group of hard real- systems. (Extensons to ths model wll be dscussed later n ths chapter). Uwe. Zmmer, he Australan Natonal Unversty page 74 of 96 (chapter 7: up to page 8)

3 744 eal- schedulng ask set (6, 8) (, ), ) (, C ) 4 4 Uwe. Zmmer, he Australan Natonal Unversty page 744 of 96 (chapter 7: up to page 8) 746 eal- schedulng Earlest Deadlne Frst (EDF). Determne (one of) the process(es) wth the earlest deadlne.. Execute ths process a. untl t fnshes. b. untl another process deadlne s found earler then the current one. Pre-emptve scheme. Dynamc scheme, snce the dspatched process s selected at run-, due to the current deadlnes. Uwe. Zmmer, he Australan Natonal Unversty page 746 of 96 (chapter 7: up to page 8) 74 eal- schedulng Deadlnes (6, 8) (, ), ) (, C ) 4 4 Uwe. Zmmer, he Australan Natonal Unversty page 74 of 96 (chapter 7: up to page 8) 747 eal- schedulng: Earlest Deadlne Frst Execute EDF schedule Works! (6, 8) (, ), ) (, C ) 4 4 If multple deadlnes concde, other means are needed to select a process,.e. Avod unnecessary task swtches. Dspatch by task d (out of the currently qualfyng processes). Uwe. Zmmer, he Australan Natonal Unversty page 747 of 96 (chapter 7: up to page 8),

4 4 4 4 Uwe. Zmmer, he Australan Natonal Unversty page 7 of 96 (chapter 7: up to page 8) page 749 of 96 (chapter 7: up to page 8) 4 Uwe. Zmmer, he Australan Natonal Unversty page 7 of 96 (chapter 7: up to page 8) G ate monotonc orderng s optmal (n the framework of fxed prorty schedulers): f a process set s schedulable under an FPS-scheme, then t s also schedulable under FPS wth rate monotonc prortes. G Statc scheme, snce the order dspatch order of processes s fxed and calculated off-lne. G Pre-emptve scheme. At run-: dspatch the runnable process wth the hghest prorty.. Each process s assgned a fxed prorty accordng to ts cycle : & P P Fxed Prorty (FPS), rate monotonc 7 Uwe. Zmmer, he Australan Natonal Unversty G In the example: Worst case response s are dentcal to cycle s. 4 Maxmal utlzaton eal- schedulng 4 page 748 of 96 (chapter 7: up to page 8) esponse : me from schedule request to process completon. eal- schedulng: Earlest Deadlne Frst G If deadlnes D are dentcal to cycle s for each task then: n C he maxmal utlzaton for EDF becomes: / # (suffcent and necessary test) = (, C), 4, ) 4, ( (, ) (, (6, ( 66, 8) 7 Uwe. Zmmer, he Australan Natonal Unversty Gves an dea how crtcal the schedule s., 4, ) (, C) (, C) (, (, ), ) ( 6, 8) (6, (, ) Worst case response s mes to deadlnes eal- schedulng: Earlest Deadlne Frst eal- schedulng: Earlest Deadlne Frst 749 (6, 8) 748

5 7 eal- schedulng: Fxed Prorty ask set (6, 8) (, ), ) (, C ) 4 4 Uwe. Zmmer, he Australan Natonal Unversty page 7 of 96 (chapter 7: up to page 8) 74 eal- schedulng: Fxed Prorty Execute FPS schedule Fals! (6, 8) (, ), ) (, C ) 4 4 Uwe. Zmmer, he Australan Natonal Unversty page 74 of 96 (chapter 7: up to page 8) 7 eal- schedulng: Fxed Prorty Execute FPS schedule Fals! (6, 8) (, ), ) (, C ) 4 4 Uwe. Zmmer, he Australan Natonal Unversty page 7 of 96 (chapter 7: up to page 8) 7 eal- schedulng: Fxed Prorty Maxmal utlzaton.9.8 Maxmal utlty.7.6. U n C / / # N( N - ) / Umax = wth C the computaton and the length of the perod for task out of N tasks and assumng that the deadlne D = Suffcent, yet not necessary test Number of processes N Uwe. Zmmer, he Australan Natonal Unversty page 7 of 96 (chapter 7: up to page 8)

6 76 eal- schedulng: Fxed Prorty Execute FPS schedule Fals! (6, 8) (, ), ) (, C ) 4 4 U n C / / = >.779. N( N - ) / Umax = Utlzaton test fals, schedulablty not guaranteed. Uwe. Zmmer, he Australan Natonal Unversty page 76 of 96 (chapter 7: up to page 8) 78 eal- schedulng: Fxed Prorty Execute FPS schedule Works! (6, 6) (, ), ) (, C ) 4 4 U n C N( / / = + + = >. N - ) / Umax = Utlzaton test fals, schedulablty not guaranteed. Uwe. Zmmer, he Australan Natonal Unversty page 78 of 96 (chapter 7: up to page 8) 77 eal- schedulng: Fxed Prorty educed task set (6, 6) (, ), ) (, C ) 4 4 U n C N( / / = + + = >. N - ) / Umax = Utlzaton test fals, schedulablty not guaranteed. Uwe. Zmmer, he Australan Natonal Unversty page 77 of 96 (chapter 7: up to page 8) 79 eal- schedulng: Fxed Prorty Execute FPS schedule Works! (6, 6) (, ), ) (, C ) 4 4 U n C N( / / = + + = >. N - ) / Umax = Utlzaton test fals, schedulablty not guaranteed. Uwe. Zmmer, he Australan Natonal Unversty page 79 of 96 (chapter 7: up to page 8) (,, )

7 76 eal- schedulng: Fxed Prorty Further reduced task set (6, 4) (, ), ) (, C ) 4 4 U n C N( / / = + + =. N - ) / U = max Utlzaton test succeeds, schedulablty guaranteed. Uwe. Zmmer, he Australan Natonal Unversty page 76 of 96 (chapter 7: up to page 8) 76 eal- schedulng: Fxed Prorty Execute FPS schedule Works! (6, 4) (, ), ) (, C ) 4 4 U n C N( / / = + + =. N - ) / U = max Utlzaton test succeeds, schedulablty guaranteed. Uwe. Zmmer, he Australan Natonal Unversty page 76 of 96 (chapter 7: up to page 8) 76 eal- schedulng: Fxed Prorty Execute FPS schedule Works! (6, 4) (, ), ) (, C ) 4 4 n C U / / = = N( N - ) / U = max Utlzaton test succeeds, schedulablty guaranteed. Uwe. Zmmer, he Australan Natonal Unversty page 76 of 96 (chapter 7: up to page 8) 76 eal- schedulng: Fxed Prorty Worst case response s (6, 4) (, ), ) (, C ) 4 4 for the hghest prorty task: = C Uwe. Zmmer, he Australan Natonal Unversty page 76 of 96 (chapter 7: up to page 8),

8 764 eal- schedulng: Fxed Prorty Worst case response s (6, 4) (, ), ) (, C ) 4 4 for others tasks: = C + I (nterference from hgher prorty tasks) Uwe. Zmmer, he Australan Natonal Unversty page 764 of 96 (chapter 7: up to page 8) 766 eal- schedulng: Fxed Prorty esponse analyss = C+ / e o $ k > k C k Fxed -pont equaton t + ecurrent form: = C + / f p $ C k wth: = C k > k t Iterate the recurrent form untl: + = t t t + or > D Uwe. Zmmer, he Australan Natonal Unversty page 766 of 96 (chapter 7: up to page 8) 76 eal- schedulng: Fxed Prorty Worst case response s (6, 4) (, ), ) (, C ) 4 4 for others tasks: = C+ / e o $ k > k C k Uwe. Zmmer, he Australan Natonal Unversty page 76 of 96 (chapter 7: up to page 8) 767 eal- schedulng: Earlest Deadlne Frst esponse analyss he worst case for Earlest Deadlne Frst s not necessarly when all tasks are released at once! All possble release combnatons n a full hyper-cycle need to be consdered! he response s are bounded by the cycle s as long as the maxmal utlzaton s #. Other tasks need to be consdered only, f ther deadlne s closer or equal to the current task. Uwe. Zmmer, he Australan Natonal Unversty page 767 of 96 (chapter 7: up to page 8)

9 768 eal- schedulng: Earlest Deadlne Frst esponse analyss () () a a C a a+ - k = ; + E + / * f p, > H + 4 k! mn k $ C k Uwe. Zmmer, he Australan Natonal Unversty page 768 of 96 (chapter 7: up to page 8) 77 eal- schedulng: Earlest Deadlne Frst esponse analyss () () a a C a a+ - k = ; + E + / * f p, *, > H + 44 k! mn k max $ C k Fxed -pont equaton = max " () a - a, a! A where A = scm ( ) Uwe. Zmmer, he Australan Natonal Unversty page 77 of 96 (chapter 7: up to page 8) 769 eal- schedulng: Earlest Deadlne Frst esponse analyss () () a a C a a+ - k = ; + E + / * f p, *, > H + 44 k! mn k max $ C k Fxed -pont equaton ecurrent form: () () a a a a t + t + - k = ; + E C + / * f p, *, > H + 44 k! mn k max $ C wth: = a + C k + Iterate untl: () a = () a t t t + or () a > a+ D Uwe. Zmmer, he Australan Natonal Unversty page 769 of 96 (chapter 7: up to page 8) 77 eal- schedulng: Fxed Prorty Worst case response s (6, 4) (, ), ) (, C ) 4 4 n C = # 4 ; = 4 # ; = # 6 ; / # N ( N - ) = Uwe. Zmmer, he Australan Natonal Unversty page 77 of 96 (chapter 7: up to page 8)

10 ; = 4 # ; Uwe. Zmmer, he Australan Natonal Unversty page 774 of 96 (chapter 7: up to page 8) Gestng all combnatons n a hyper-cycle: = max " (a) - a,a! A where A = scm ( ) ; = # Uwe. Zmmer, he Australan Natonal Unversty = 4 # 4, C) ( (, C) (, 4, ) 4,, 4, ) 4, (, ) (, ; C # / = n 4 4 page 77 of 96 (chapter 7: up to page 8) ; = 6 # 6 Worst case response s C N - ) N ( page 77 of 96 (chapter 7: up to page 8) = n / Worst case response s 77 ; = 9 6 ; eal- schedulng: Earlest Deadlne Frst 4 ; = 4 # Uwe. Zmmer, he Australan Natonal Unversty = # 4 (6, (6 6, 8) page 77 of 96 (chapter 7: up to page 8) = C N - ) N ( ( (, ) (, n / (, C) eal- schedulng: Earlest Deadlne Frst ; = # 6 (6, ( 66, 8) 774 Uwe. Zmmer, he Australan Natonal Unversty = # 4 (, C), ), ) (, ) (6, 8) Worst case response s Worst case response s ( (, ) eal- schedulng: Fxed Prorty eal- schedulng: Fxed Prorty 77 ( (6, 6) 77

11 776 eal- schedulng: Earlest Deadlne Frst Worst case response s (6, 6) (, ), ) (, C ) 4 4 = # 4 ; = 8 # ; = # 6 ; n C / # = Uwe. Zmmer, he Australan Natonal Unversty page 776 of 96 (chapter 7: up to page 8) 778 eal- schedulng: Comparson esponse me Analyss Fxed Prorty Earlest Deadlne Frst Utlzaton est n / = C # N ( N - ) C esponse Utlzaton mes ", est esponse mes ", n C + / e o$ C / k k > k # $ () a a max -. a! A = " ^, C h, = " ^6,8 h, ^, h, ^4, h, "#, 4,, " 6,, 4, " ^, C h, = " ^6, 6 h, ^, h, ^4, h, ", 4,, " 8,,, " ^, C h, = " ^6, 4 h, ^, h, ^4, h, ", 4,, " 6,,, Uwe. Zmmer, he Australan Natonal Unversty page 778 of 96 (chapter 7: up to page 8) 777 eal- schedulng: Earlest Deadlne Frst Worst case response s (6, 4) (, ), ) (, C ) 4 4 # 4 = ; = 6 # ; = # 6 ; n C / # = Uwe. Zmmer, he Australan Natonal Unversty page 777 of 96 (chapter 7: up to page 8) 779 eal- schedulng: Comparson Fxed Prorty Earlest Deadlne Frst EDF can handle hgher (full) utlzaton than FPS. FPS s easer to mplement and mples less run- overhead Graceful degradaton (resource s over-booked): FPS: processes wth lower prortes wll always mss ther deadlnes frst. EDF: any process can mss ts deadlne and can trgger a cascade of faled deadlnes. esponse analyss and utlzaton tests: FPS: O(n) utlzaton test response analyss: fxed pont equaton EDS: O(n) utlzaton test response analyss: fxed pont equaton n hyper-cycle Uwe. Zmmer, he Australan Natonal Unversty page 779 of 96 (chapter 7: up to page 8)

12 78 eal-world Extenson Smplstc Assumptons asks are perodc Deadlnes are dentcal wth task s perod (D = ) asks are ndependent Pre-emptve schedulng Worst case executon s are known Uwe. Zmmer, he Australan Natonal Unversty page 78 of 96 (chapter 7: up to page 8) 78 eal-world Extenson More ealstc Assumptons asks are perodc we wll ntroduce sporadc and aperodc processes Deadlnes are dentcal wth task s perod (D = ) asks are ndependent Pre-emptve schedulng Worst case executon s are known we wll ntroduce arbtrary deadlnes we wll ntroduce schedules for nteractng tasks we wll ntroduce (brefly) cooperatve schedulng we wll ntroduce fault tolerant schedulng Uwe. Zmmer, he Australan Natonal Unversty page 78 of 96 (chapter 7: up to page 8) 78 eal-world Extenson More ealstc Assumptons asks are perodc we wll ntroduce sporadc and aperodc processes Deadlnes are dentcal wth task s perod (D = ) asks are ndependent Pre-emptve schedulng Worst case executon s are known we wll ntroduce arbtrary deadlnes we wll ntroduce schedules for nteractng tasks we wll ntroduce (brefly) cooperatve schedulng we wll ntroduce fault tolerant schedulng Uwe. Zmmer, he Australan Natonal Unversty page 78 of 96 (chapter 7: up to page 8) 78 Sporadc and Aperodc Processes Hard real- tasks (6, 7), ) (, C ) 4 4 Uwe. Zmmer, he Australan Natonal Unversty page 78 of 96 (chapter 7: up to page 8)

13 784 Sporadc and Aperodc Processes FPS for hard real- tasks (6, 7), ) (, C ) 4 4 Uwe. Zmmer, he Australan Natonal Unversty page 784 of 96 (chapter 7: up to page 8) 786 Sporadc and Aperodc Processes FPS lowest prorty for soft real- task (, ) (6, 7), ) (, C ) 4 4 Sporadc / aperodc task does not nterfere wth hard real- tasks. esponse s for sporadc / aperodc task can be large. Uwe. Zmmer, he Australan Natonal Unversty page 786 of 96 (chapter 7: up to page 8) 78 Sporadc and Aperodc Processes Introducng a soft real- task (, ) (6, 7), ) (, C ) 4 4 Sporadc / aperodc task set to lowest prorty. Uwe. Zmmer, he Australan Natonal Unversty page 78 of 96 (chapter 7: up to page 8) 787 Sporadc and Aperodc Processes Introducng a server task on hghest prorty (6, 7), ) (, C ) 4 4 Settng a deferrable server task as a proxy for sporadc / aperodc tasks on hghest prorty level. Uwe. Zmmer, he Australan Natonal Unversty page 787 of 96 (chapter 7: up to page 8) -

14 4 4 4 Uwe. Zmmer, he Australan Natonal Unversty G Mnmal nter-arrval-s knowledge s employed. G Interference level less or equal to a deferrable server. page 79 of 96 (chapter 7: up to page 8) A sporadc server only replenshes after a fxed after ts actual deployment., 4, ), 4,, ) 4 Uwe. Zmmer, he Australan Natonal Unversty G Pushes the hard real- tasks to ther deadlnes page 79 of 96 (chapter 7: up to page 8) Start the sporadc / aperodc tasks on hgh prorty and demote them n for the hard real- tasks to complete (G dynamc schedulng scheme)., C) ( (, (, ) (, (, ) (6, 6 7) 7 (6, - FPS wth dual prortes 79 Sporadc task utlzng sporadc server 4 page 789 of 96 (chapter 7: up to page 8) Sporadc and Aperodc Processes 4 Uwe. Zmmer, he Australan Natonal Unversty Deferrable server task only deployng f there are requests from the sporadc / aperodc task. (( 66, 7) (6, (, C) ( G Schedule must also work wll less nterference., C) ( Sporadc and Aperodc Processes page 788 of 96 (chapter 7: up to page 8) ((, (, ), 79 Uwe. Zmmer, he Australan Natonal Unversty G Hard real- tasks are stll schedulable wth the server task deployng ts full length., C) ( (, 4, ), 4,, ) (, (, ) (6 6, 7) (6, - Sporadc task utlzng deferrable server FPS server task as normal task Sporadc and Aperodc Processes Sporadc and Aperodc Processes 789 ( (6 6, 7) (6, 788

15 Introducng g an EDF server Sporadc and Aperodc Processes 4 4 EDL EDL EDL Uwe. Zmmer, he Australan Natonal Unversty 4 4 page 794 of 96 (chapter 7: up to page 8) G Deadlnes explctly pushed to ther lmts durng the EDL phases. EDL Earlest Deadlne Last (EDL) for sporadc tasks Sporadc and Aperodc Processes page 79 of 96 (chapter 7: up to page 8) Earlest Deadlne Last schedulng (whle stll keepng all deadlnes) when sporadc / aperodc tasks are to be scheduled. (, C) ( (, 4,, ) ((6, (6 ( 66, 7)) (, ( (, ) 794 Uwe. Zmmer, he Australan Natonal Unversty he EDF equvalent to a deferrable server: a perodc server task wth an mmedate deadlne. (, C), ) (6, 7) 79 Server Hard Sporadc orad o ra ad a dc d d c c Sporadc Hard Spo 4 Sporadc task utlzng EDF server Sporadc and Aperodc Processes 4 eal-world Extenson page 79 of 96 (chapter 7: up to page 8) Uwe. Zmmer, he Australan Natonal Unversty G we wll ntroduce arbtrary deadlnes G we wll ntroduce (brefly) cooperatve schedulng G we wll ntroduce schedules for nteractng tasks page 79 of 96 (chapter 7: up to page 8) G we wll ntroduce fault tolerant schedulng Worst case executon s are known Pre-emptve schedulng asks are ndependent Deadlnes are dentcal wth task s perod (D = ) G we wll ntroduce sporadc and aperodc processes G More ealstc Assumptons asks are perodc 79 Uwe. Zmmer, he Australan Natonal Unversty G Swft response s for the sporadc / aperodc tasks wth deadlnes pushed to ther lmts., C) (,, ), (6 ((6 6, 7)) (6, (, (,, ) 79 orad aad dcc Server Hard Sporadc

16 796 asks wth arbtrary deadlnes asks wth D < (Deadlne earler than cycle ) In case of fxed prorty schedulng (FPS): Change from: ate Monotonc Prorty Orderng (MPO) to: Deadlne Monotonc Prorty Orderng (DMPO) Lemma: Any task set Q whch s schedulable by a FPS scheme W, s also schedulable under DMPO. Uwe. Zmmer, he Australan Natonal Unversty page 796 of 96 (chapter 7: up to page 8) 798 Proof of DMPO optmalty Swap two prortes out of W whch volate DMPO: W D D t t n W: D D # & D Uwe. Zmmer, he Australan Natonal Unversty page 798 of 96 (chapter 7: up to page 8) 797 Proof of DMPO optmalty Swap two prortes out of W whch volate DMPO:. t, t are two tasks n Q wth P P and D D n W J DMPO. Generate Wl by swappng P and P _ Pl Pl/ _ D D DMPO. Wl schedules Q because: a. All tk! Q wth P k P or Pk P are unaffected. b. t s schedulable n Wl because Pl> P & l# # D & l # D c. t s schedulable n Wl because: n W: D D # # & meanng that t nterfered only once wth t also: t released once n, and n Wl: t nterferes only once wth t l= # D D & l D Uwe. Zmmer, he Australan Natonal Unversty page 797 of 96 (chapter 7: up to page 8) 799 Proof of DMPO optmalty Swap two prortes out of W whch volate DMPO: W D D t t n W: # D D # & t nterfered only once wth t also: t released once n, and Uwe. Zmmer, he Australan Natonal Unversty page 799 of 96 (chapter 7: up to page 8)

17 8 Proof of DMPO optmalty Swap two prortes out of W whch volate DMPO: W W' D D D D t t t t n W: # D D # & t nterfered only once wth t n Wl: also: t released once n, and Uwe. Zmmer, he Australan Natonal Unversty page 8 of 96 (chapter 7: up to page 8) 8 Proof of DMPO optmalty Swap all prortes out of W whch volate DMPO: Swap all t, t n Q, wth _ P > P/ _ D > D n W resultng n all t, t n Q wth P > P to have D < D Consttutng the DMPO scheme Snce each swappng operaton keep schedulablty, the resultng DMPO scheme s also schedulable. Deadlne monotonc orderng s optmal: (f a process set s schedulable under an FPS-scheme, then t s also schedulable under FPS wth deadlne monotonc prortes.) Uwe. Zmmer, he Australan Natonal Unversty page 8 of 96 (chapter 7: up to page 8) 8 Proof of DMPO optmalty Swap two prortes out of W whch volate DMPO: W W' D D D D ' t t t t 4 ' 4 4 n W: # D D # & t nterfered only once wth t also: t released once n, and n Wl: t nterferes only once wth t l= # D D & l D Uwe. Zmmer, he Australan Natonal Unversty page 8 of 96 (chapter 7: up to page 8) 8 asks wth arbtrary deadlnes asks wth D > (Deadlne later than cycle ) Assumpton: every task t s released only after the former release of t s completed. In case that > for a specfc schedulng stuaton, the followng release of task t s delayed by -. Mnd that > cannot hold for all release stuatons, otherwse the task s not schedulable. he worst case response ^ h mght thus be longer than but must stll be shorter than D. Uwe. Zmmer, he Australan Natonal Unversty page 8 of 96 (chapter 7: up to page 8)

18 84 asks wth arbtrary deadlnes asks wth D > (Deadlne later than cycle ) Assumpton: every task t s released only after the former release of t s completed. Snce the response can now be potentally greater than the cycle : more than one release q of the task t needs to be consdered: ( ) ( q) B qc q = + + / e o C k where 6 q ( q) -( q- ) # D k > k B s the blockng ; q s the number of releases. = % ( q) ( q ) q q max - -! " f max,/ and q q q ( q) max = ' # Uwe. Zmmer, he Australan Natonal Unversty page 84 of 96 (chapter 7: up to page 8) 86 Independent tasks L 8, 8), ) (6, ) (, C ) 4 4 Deadlnes dentcal to cycle s DMPO or MPO. Uwe. Zmmer, he Australan Natonal Unversty page 86 of 96 (chapter 7: up to page 8) 8 eal-world Extenson More ealstc Assumptons asks are perodc we wll ntroduce sporadc and aperodc processes Deadlnes are dentcal wth task s perod (D = ) asks are ndependent Pre-emptve schedulng Worst case executon s are known we wll ntroduce arbtrary deadlnes we wll ntroduce schedules for nteractng tasks we wll ntroduce (brefly) cooperatve schedulng we wll ntroduce fault tolerant schedulng Uwe. Zmmer, he Australan Natonal Unversty page 8 of 96 (chapter 7: up to page 8) 87 Independent tasks L 8, 8), ) (6, ) (, C ) 4 4 Schedulable under DMPO or MPO. Uwe. Zmmer, he Australan Natonal Unversty page 87 of 96 (chapter 7: up to page 8)

19 88 ask dependences L 8, 8), ) (6, ) (, C ) 4 4 Lock requests by two tasks. Uwe. Zmmer, he Australan Natonal Unversty page 88 of 96 (chapter 7: up to page 8) 8 Prorty nhertance ask t nherts prorty P k of task t k f:. P < P k.. ask t has locked a resource Q.. ask t k s blocked watng for the release of resource Q. Uwe. Zmmer, he Australan Natonal Unversty page 8 of 96 (chapter 7: up to page 8) 89 ask dependences L 8, 8), ) (6, ) (, C ) 4 4 he lower prorty task blocks the hgher prorty task. (note that the blue task s unaffected.) Prorty nverson Uwe. Zmmer, he Australan Natonal Unversty page 89 of 96 (chapter 7: up to page 8) 8 Prorty nhertance Maxmal blockng for task t : B = usage (,) r C() r wth: denotng the number of crtcal sectons. usage (,) r beng a boolean functon returnng for true and ndcatng the r s used by: at least one t wth P < P and at least one t k wth Pk $ P r C() r denotng the worst case computaton n crtcal secton r Each task can only be blocked once for each employed resource! Uwe. Zmmer, he Australan Natonal Unversty page 8 of 96 (chapter 7: up to page 8)

20 Uwe. Zmmer, he Australan Natonal Unversty (, C) (6, ) 4 page 84 of 96 (chapter 7: up to page 8) 4 Uwe. Zmmer, he Australan Natonal Unversty page 8 of 96 (chapter 7: up to page 8) by DMPO/MPO results n blockng for the hgher prorty tasks. (, C) (6, ), ), ) L 4 page 8 of 96 (chapter 7: up to page 8) ask dependences wth multple locks G Prorty nverson ask dependences wth multple locks 8, 8) 8 Uwe. Zmmer, he Australan Natonal Unversty 4 L he lower prorty task blocks the hgher prorty task. (note that the blue task s unaffected.) page 8 of 96 (chapter 7: up to page 8) 8, 8) 84 Uwe. Zmmer, he Australan Natonal Unversty he task on prorty s blocked due to prorty nhertance. he lower prorty task s promoted to the prorty of the blocked task. (, C) (, C) (6, ) (6, ), ) (6, ) (6 L 8, 8) Wthout prorty nhertance Prorty nhertance L 8 8, 8) 8

21 86 ask dependences wth multple locks L 8, 8), ) (6, ) (, C ) 4 4 by DMPO/MPO wth prorty nhertance does not mprove the result. Uwe. Zmmer, he Australan Natonal Unversty page 86 of 96 (chapter 7: up to page 8) 88 Crcular task dependences L 8, 8), ) (6, ) (, C ) 4 4 by DMPO/MPO results n deadlock. (Prorty nhertance does not make a dfference for blocked tasks.) Uwe. Zmmer, he Australan Natonal Unversty page 88 of 96 (chapter 7: up to page 8) 87 Crcular task dependences L 8, 8), ) (6, ) (, C ) 4 4 Uwe. Zmmer, he Australan Natonal Unversty page 87 of 96 (chapter 7: up to page 8) 89 Immedate celng prorty protocol (POSIX, Ada, -Java) Each task t has a statc prorty P. Each resource k has a statc celng prorty C k : Ck = max # employ ^, kh$ P- wth employ (, k ) beng a boolean functon returnng for true f task t employs resource k. Each task t has a dynamc prorty P D : P D = max # P, max # locked ^, kh$ Ck- k - wth locked (, k ) beng a boolean functon returnng for true f task t holds resource k. Uwe. Zmmer, he Australan Natonal Unversty page 89 of 96 (chapter 7: up to page 8)

22 8 Celng Prorty Protocol L 8, 8), ) (6, ) (, C ) 4 4 Avods the deadlock! Uwe. Zmmer, he Australan Natonal Unversty page 8 of 96 (chapter 7: up to page 8) 8 Immedate celng prorty protocol (POSIX, Ada, -Java) Maxmal blockng : B = max # usage(,) r $ C() r - r = denotng the number of crtcal sectons. usage (,) r beng a boolean functon returnng for true and ndcatng that r s used by: at least one t wth P < P. C() r denotng the worst case computaton n crtcal secton r Each task can only be blocked once by one lower prorty task! Uwe. Zmmer, he Australan Natonal Unversty page 8 of 96 (chapter 7: up to page 8) 8 Immedate celng prorty protocol (POSIX, Ada, -Java) Implcatons: asks are dspatched only f all employed resources are avalable. Deadlocks are prevented (no hold and wat). Number of context swtches are reduced. Uwe. Zmmer, he Australan Natonal Unversty page 8 of 96 (chapter 7: up to page 8) 8 eal-world Extenson More ealstc Assumptons asks are perodc we wll ntroduce sporadc and aperodc processes Deadlnes are dentcal wth task s perod (D = ) asks are ndependent Pre-emptve schedulng Worst case executon s are known we wll ntroduce arbtrary deadlnes we wll ntroduce schedules for nteractng tasks we wll ntroduce (brefly) cooperatve schedulng we wll ntroduce fault tolerant schedulng Uwe. Zmmer, he Australan Natonal Unversty page 8 of 96 (chapter 7: up to page 8)

23 84 Non pre-emptve schedulng In pre-emptve schedulng: Maxmal ndvdual blockng s B can be determned for each task t by employng a prorty celng protocol. Maxmum overall blockng Bmax = max # B -. Cooperatve Every task t s dvded n k non pre-emptve blocks of C # B k max. All crtcal sectons are completely enclosed n a sngle block C k. Every task calls a de-schedulng routne at the end of each block,.e. offerng a task swtch. Uwe. Zmmer, he Australan Natonal Unversty page 84 of 96 (chapter 7: up to page 8) 86 Non pre-emptve schedulng Cooperatve esponse s: = + wth k + B C F = max / f p C > n F wth F the executon of the fnal block. k For the smplfed case of C = C = C = F = Bmax : / f p C = n k + wth = C + > k For the further smplfed case of 6 : = : = C + / C > Uwe. Zmmer, he Australan Natonal Unversty page 86 of 96 (chapter 7: up to page 8) 8 Non pre-emptve schedulng Cooperatve Implcatons: Number of task swtches s reduced. Caches, pre-fetchng, and ppelnes are more effcent. Executon s are (a bt) easer to predct. Schedules are smpler. Interdependent task sets are schedulable deadlock free by desgn. Uwe. Zmmer, he Australan Natonal Unversty page 8 of 96 (chapter 7: up to page 8) 87 Non pre-emptve schedulng Cooperatve Consderatons: Code block dvson need to be done thoroughly. Addtonal protecton aganst msbehavng (non-cooperatve) tasks: Scheduler pre-empts tasks (deferred pre-empton), whch fal to offer a de-schedule themselves. Due to a central B max, addtonal tasks need to be engneered to partcpate n a specfc cooperatve schedule. equres that a value B max can be accepted by all tasks. Short and reactve tasks are excluded or treated separately. Uwe. Zmmer, he Australan Natonal Unversty page 87 of 96 (chapter 7: up to page 8)

24 88 eal-world Extenson More ealstc Assumptons asks are perodc we wll ntroduce sporadc and aperodc processes Deadlnes are dentcal wth task s perod (D = ) asks are ndependent Pre-emptve schedulng Worst case executon s are known we wll ntroduce arbtrary deadlnes we wll ntroduce schedules for nteractng tasks we wll ntroduce (brefly) cooperatve schedulng we wll ntroduce fault tolerant schedulng Uwe. Zmmer, he Australan Natonal Unversty page 88 of 96 (chapter 7: up to page 8) 8 eal-world Extenson More ealstc Assumptons asks are perodc we wll ntroduce sporadc and aperodc processes Deadlnes are dentcal wth task s perod (D = ) asks are ndependent Pre-emptve schedulng Worst case executon s are known we wll ntroduce arbtrary deadlnes we wll ntroduce schedules for nteractng tasks we wll ntroduce (brefly) cooperatve schedulng we wll ntroduce fault tolerant schedulng Uwe. Zmmer, he Australan Natonal Unversty page 8 of 96 (chapter 7: up to page 8) 89 Fault olerance Exceptons and ecoveres f ask t needs extra CPU- C for error recovery or excepton handlng and the mnmum nter-arrval between faults s f : = B + C + / C C f e o + ) d n max f k > k $ If error recovery s performed at the hghest prorty: = B + C + / C C f e o + ) d n max f k > k Uwe. Zmmer, he Australan Natonal Unversty page 89 of 96 (chapter 7: up to page 8) 8 General schedulng methods Some task sets can be scheduled by ntroducng offsets to the release s, yet Wthout any further restrctons ths problem s NP-hard By ntroducng further assumptons about cycle granularty and assocated deadlnes: Schedulablty analyss complexty can be reduced to polynomal. e.g. estrct cycle s to powers of two of a base. Uwe. Zmmer, he Australan Natonal Unversty page 8 of 96 (chapter 7: up to page 8)

25 8 Language support Ada provdes: ask and nterrupt prortes (statc, dynamc, actve). ask attrbutes. Prortzed entry queues. Prorty celng lockng (ICPP). Schedulers (FPS wth FIFO wthn prortes (pre-emptve), ound obn, EDF). ask executon measurements. Sporadc servers Ada does currently not provde: Uwe. Zmmer, he Australan Natonal Unversty page 8 of 96 (chapter 7: up to page 8) 84 Language support POSIX provdes: hreads and nterrupt prortes (statc, dynamc, actve). hreads can be system contented or process contented (prorty schedulng unclear n ths case). Prortzed message queues. Prorty celng lockng (ICPP). Schedulers, prorty based wth at least: FIFO, ound-obn, Sporadc Server, possbly others. mers. Uwe. Zmmer, he Australan Natonal Unversty page 84 of 96 (chapter 7: up to page 8) 8 Ada package System s subtype Any_Prorty s Integer range mplementaton-defned; subtype Prorty s Any_Prorty range Any_Prorty Frst.. mplementaton-defned; subtype Interrupt_Prorty s Any_Prorty range Prorty Last +.. Any_Prorty Last; Default_Prorty : constant Prorty := (Prorty Frst + Prorty Last) / ; end System; package Ada.Dynamc_Prortes s procedure Set_Prorty (Prorty : n System.Any_Prorty; : n Ada.ask_Identfcaton.ask_ID := Ada.ask_Identfcaton.Current_ask); functon Get_Prorty ( : Ada.ask_Identfcaton.ask_ID := Ada.ask_Identfcaton.Current_ask) return System.Any_Prorty; end Ada.Dynamc_Prortes; Uwe. Zmmer, he Australan Natonal Unversty page 8 of 96 (chapter 7: up to page 8) 8 Summary Basc real- schedulng Fxed Prorty (FPS) wth ate Monotonc (MPO) and Deadlne Monotonc Prorty Orderng (DMPO). Earlest Deadlne Frst (EDF). eal-world extensons Aperodc, sporadc, soft real- tasks. Deadlnes dfferent from perod. Synchronzed talks (prorty nhertance, prorty celng protocols). Cooperatve and deferred pre-empton schedulng. Fault tolerance n terms of excepton handlng consderatons. Language support Ada, POSIX Uwe. Zmmer, he Australan Natonal Unversty page 8 of 96 (chapter 7: up to page 8)