Multitasking and Real-time Scheduling

Transcription

1 Multtaskng and Real-tme Schedulng EE8205: Embedded Computer Systems Dr. Gul N. Khan Electrcal and Computer Engneerng Ryerson Unversty Overvew RTX - Preemptve Schedulng Real-tme Schedulng Technques Fxed-Prorty and Earlest Deadlne Frst Schedulng Utlzaton and Response-tme Analyss Prorty Inverson Sporadc and Aperodc Process Schedulng Chapter 6 of the Text by Wolf, Chapter 13 of Text by Burns and Wellngs and Kel-RTX documents G. Khan Embedded Computer Systems EE8205: Real-tme Schedulng Page: 1

2 Prorty-drven Schedulng Rules: each process has a fxed prorty (1 lowest); hghest-prorty ready process gets CPU; process contnues untl done. Processes P1: prorty 3, executon tme 10 P2: prorty 2, executon tme 30 P3: prorty 1, executon tme 20 P3 ready t=18 P2 ready t=0 P1 ready t=15 P2 P1 P2 P G. Khan Embedded Computer Systems EE8205: Real-tme Schedulng Page: 2 tme

3 RTX Schedulng Optons RTX allows us to buld an applcaton wth three dfferent kernelschedulng optons: Pre-Emptve schedulng Each task has a dfferent prorty and wll run untl t s preempted or has reached a blockng OS call. Round-Robn schedulng Each task has the same prorty and wll run for a fxed perod, or tme slce, or untl has reached a blockng OS call. Co-operatve mult-taskng Each task has the same prorty and the Round-Robn s dsabled. Each task wll run untl t reached a blockng OS call or uses the os_tsk_pass() call. The default schedulng opton for RTX s Round-Robn Preemptve. G. Khan Embedded Computer Systems EE8205: Real-tme Schedulng Page: 3

4 RTX: Preemptve Schedulng When a task wth a hgher prorty than the currently runnng task becomes ready to run, RTX suspends the currently runnng task. A preemptve task swtch occurs when: the task scheduler s executed from the system tck tmer nterrupt. Task scheduler processes the delays of tasks. If the delay for a task wth a hgher prorty has expred, then the hgher prorty task starts to execute nstead of the current task. an event s set for a hgher prorty task by the currently runnng task or by an nterrupt servce routne. The currently runnng task s suspended, and the hgher prorty task starts to run. a token s returned to a semaphore, and a hgher prorty task s watng for the semaphore token. The currently runnng task s suspended, and the hgher prorty task starts to run. The token can be returned by the currently runnng task or by an nterrupt servce routne. a mutex s released and a hgher prorty task s watng for the mutex. The currently runnng task s suspended, and the hgher prorty task starts to run. G. Khan Embedded Computer Systems EE8205: Real-tme Schedulng Page: 4

5 Pre-emptve Schedulng A preemptve task swtch occurs when: a message s posted to a malbox, and a hgher prorty task s watng for the malbox message. The currently runnng task s suspended, and the hgher prorty task starts to run. The message can be posted by the currently runnng task or by an nterrupt servce routne. a malbox s full, and a hgher prorty task s watng to post a message to a malbox. As soon as the currently runnng task or an nterrupt servce routne takes a message out from the malbox, the hgher prorty task starts to run. the prorty of the currently runnng task s reduced. If another task s ready to run and has a hgher prorty than the new prorty of the currently runnng task, then the current task s suspended mmedately, and the hgher prorty task resumes ts executon. os_tsk_pro ( ) functon changes the executon prorty of the task dentfed by the argument task_d e.g. os_tsk_pro_self (5); os_tsk_pro(tsk2, 10); G. Khan Embedded Computer Systems EE8205: Real-tme Schedulng Page: 5

6 RTX - Preemptve Swtchng Example Task job1 has a hgher prorty than task job2. When job1 starts, t creates task job2 and then enters the os_evt_wat_or functon. The RTX kernel suspends job1 at ths pont, and job2 starts executng. As soon as job2 sets an event flag for job1, the RTX kernel suspends job2 and then resumes job1. Task job1 then ncrements counter cnt1 and calls the os_evt_wat_or functon, whch suspends t agan. The kernel resumes job2, whch ncrements counter cnt2 and sets an event flag for job1. os_evt_set Ths process of task swtchng contnues ndefntely. #nclude <rtl.h> OS_TID tsk1,tsk2; nt cnt1,cnt2; task vod job1 (vod); task vod job2 (vod); G. Khan Embedded Computer Systems EE8205: Real-tme Schedulng Page: 6

7 RTX - Task Swtchng Example task vod job1 (vod) { os_tsk_pro (2); tsk1 = os_tsk_self (); os_tsk_create (job2, 1); whle (1) { os_evt_wat_or (0x0001, 0xffff); cnt1++; } } task vod job2 (vod) { whle (1) { os_evt_set (0x0001, tsk1); cnt2++; } } vod man (vod) { os_sys_nt (job1); whle (1); } G. Khan Embedded Computer Systems EE8205: Real-tme Schedulng Page: 7

8 RTX and Interrupt Functons RTX can work wth nterrupt functons n parallel. However, t s better to avod IRQ nestng. An IRQ functon can send a sgnal/ or message to start a hgh prorty task. Interrupt functons are added to an ARM applcaton n the same way as n any other non-rtx projects. FIQ nterrupts are never dsabled by the RTX kernel. We cannot call the sr_ lbrary functons from the FIQ nterrupt functon. G. Khan Embedded Computer Systems EE8205: Real-tme Schedulng Page: 8

9 Threads - Sgnals/Wats Threads support s avalable wth for RTX wth CMSIS-RTOS API vod led_thread1 (vod const *argument); osthreaddef(led_thread1, osprortynormal, 1, 0); osthreadid Thr_led_ID1; nt man(vod){... Thr_led_ID1 = osthreadcreate(osthread(led_thread1), NULL);...} A wat flag may be set n the code as follows: ossgnalwat (0x03,osWatForever); The thread s watng for the sgnal flag 0x03 to be asserted. oswatforever parameter ndcates the maxmum duraton n msecs that the thread should wat to be sgnaled. A sgnal may be send to a thread or cleared usng: ossgnalset(thr_led_id2,0x01); or ossgnalclear(thr_led_id2, 0x01); G. Khan Embedded Computer Systems EE8205: Real-tme Schedulng Page: 9

10 The Schedulng Problem Can we meet all deadlnes? Must be able to meet deadlnes n all cases. How much CPU tme, we need to meet the deadlnes? Process Intaton Perodc process: executes on (almost) every perod. Aperodc process: executes on demand. Analyzng aperodc process set s harder---must consder worst-case combnatons of process actvatons. G. Khan Embedded Computer Systems EE8205: Real-tme Schedulng Page: 10

11 Process Tmng Requrements Perod: nterval between process actvatons. Intaton nterval: recprocal of perod. Intaton tme: tme at whch process becomes ready. Deadlne: tme at whch process must fnsh. Tmng volatonswhat happens f a process doesn t fnsh by ts deadlne? Hard deadlne: system fals f mssed. Soft deadlne: user may notce, but system doesn t necessarly fal. Example: Space Shuttle software error A software tmng error delayed shuttle s frst launch: Prmary control system PASS and backup system BFS. BFS faled to synchronze wth PASS. Change to one routne added delay that threw off start tme calculaton. 1 n 67 chance of tmng problem. G. Khan Embedded Computer Systems EE8205: Real-tme Schedulng Page: 11

12 Process Model The applcaton s assumed to consst of a fxed set of processes. Processes are completely ndependent of each other. All system's overheads, context-swtchng tmes and so on are gnored (.e. assumed to have zero cost) All processes are perodc, wth known perods. All processes have a deadlne equal to ther perod (that s, each process must complete before t s next released) All processes have a fxed worst-case executon tme. G. Khan Embedded Computer Systems EE8205: Real-tme Schedulng Page: 12

13 Real-tme Schedulng Technques Fxed-Prorty Schedulng (FPS) Earlest Deadlne Frst (EDF) FPS: Fxed-Prorty Schedulng Ths s the most wdely used approach. Each process has a fxed, (statc) prorty that s computed before executon. The runnable processes are executed n the order determned by ther prorty. In real-tme systems, the prorty of a process s derved from ts temporal requrements, not ts mportance to the correct functonng of the system or ts ntegrty. G. Khan Embedded Computer Systems EE8205: Real-tme Schedulng Page: 13

14 FPS: Fxed-Prorty Schedulng Rate Monotonc Prorty Assgnment Each process s assgned a (unque) prorty based on ts perod; the shorter the perod, the hgher the prorty For two processes and j: T < T j P > An optmal prorty assgnment means: f any process set can be scheduled (usng preemptve prorty-based schedulng) wth a fxed-prorty assgnment scheme, then the gven process set can also be scheduled wth a rate monotonc assgnment scheme Prorty 1 s the lowest (least) prorty P j G. Khan Embedded Computer Systems EE8205: Real-tme Schedulng Page: 14

15 Prorty Assgnment: An Example Perod T: Mnmum tme between process releases. C: Worst-case computaton tme (WCET) of the process. U: The utlzaton of each process (equal to C/T). R: Worst-case response tme of the process. B: Worst-case blockng tme for the process. D: Deadlne of the process. N: Number of process. The nterference tme of the process. Release jtter of the process. Process Perod, T Prorty, P a 25 5 b 60 3 c 42 4 d e 75 2 G. Khan Embedded Computer Systems EE8205: Real-tme Schedulng Page: 15

16 Utlzaton-Based Analyss For D=T process sets, a suffcent but not necessary schedulablty test exsts. U N = 1 C T ( 2 N Utlzaton bound % % % % % % N 1 / N 1) G. Khan Embedded Computer Systems EE8205: Real-tme Schedulng Page: 16

17 Process Set A Utlzaton-Based Analyss Process Perod, T Computaton Tme, C Prorty, P Utlzaton, U a b c The combned utlzaton s 0.82 (or 82%) Ths s above the threshold for three processes (0.78) and, hence, ths process set fals the utlzaton test. c b a c b Tme G. Khan Embedded Computer Systems EE8205: Real-tme Schedulng Page: 17

18 Tme-Lne for Process Set A Process a b c Process Release Tme Process Completon Tme Deadlne Met Process Completon Tme Deadlne Mssed Preempted Executng Tme c b a c b G. Khan Embedded Computer Systems EE8205: Real-tme Schedulng Page: 18

19 Utlzaton-Based Analyss Process Set B Process Perod Computaton Tme Prorty Utlzaton T C P U a b c The combned utlzaton s (or 77.5%) Ths s below the threshold for three processes (0.78) and, hence, ths process set wll meet all ts deadlnes. G. Khan Embedded Computer Systems EE8205: Real-tme Schedulng Page: 19

20 Utlzaton-Based Analyss Process Set C Process Perod Computaton Tme Prorty Utlzaton T C P U a b c The combned utlzaton s 1.0 Ths s above the threshold for three processes (0.78) but the process set wll meet all ts deadlnes. The Utlzaton test s sad to be suffcent but not necessary G. Khan Embedded Computer Systems EE8205: Real-tme Schedulng Page: 20

21 Process Tme-Lne for Process Set C a b c Tme Utlzaton-based test s nether exact nor general but ts O(N) G. Khan Embedded Computer Systems EE8205: Real-tme Schedulng Page: 21

22 Earlest Deadlne Frst (EDF) Schedulng The runnable processes are executed n the order determned by the absolute deadlnes of the processes. The next process to run beng the one wth the shortest (nearest) deadlne. It s possble to know the relatve deadlnes of each process e.g. 25ms after release. The absolute deadlnes are computed at run tme and hence the scheme s descrbed as dynamc. Value Based (VBS) Schedulng If a system can become overloaded then smple statc prortes or deadlnes are not suffcent; a more adaptve scheme s needed. Ths often takes the form of assgnng a value to each process and employng an on-lne value-based schedulng algorthm to decde whch process to run next. G. Khan Embedded Computer Systems EE8205: Real-tme Schedulng Page: 22

23 Preempton and Non-Preempton Wth prorty-based schedulng, a hgh-prorty process may be released durng the executon of a lower prorty one. In a preemptve scheme, there wll be an mmedate swtch to the hgher-prorty process Wth non-preempton, the lower-prorty process wll be allowed to complete before the other executes. Preemptve schemes enable hgher-prorty processes to be more reactve, and hence they are preferred. Alternatve strateges allow a lower prorty process to contnue to execute for a bounded tme. These schemes are known as deferred preempton or cooperatve dspatchng. Schemes such as EDF and VBS can also take on a preemptve or non pre-emptve form. G. Khan Embedded Computer Systems EE8205: Real-tme Schedulng Page: 23

24 Utlzaton-based Test for EDF N = 1 C T 1 A much smpler test Superor to FPS; t can support hgh utlzatons. However, FPS s easer to mplement, as prortes are statc. EDF s dynamc and requres a more complex run-tme system that wll have hgher overhead. It s easer to ncorporate processes wthout deadlnes nto FPS; gvng a process an arbtrary deadlne s more artfcal It s easer to ncorporate other factors nto the noton of prorty than t s nto the noton of deadlne. Durng overload stuatons: FPS s more predctable; Low prorty process mss ther deadlnes frst EDF s unpredctable; a domno effect can occur n whch a large number of processes mss deadlnes G. Khan Embedded Computer Systems EE8205: Real-tme Schedulng Page: 24

25 Response-Tme Analyss Task 's worst-case response tme, R s calculated frst and then checked (trvally) wth ts deadlne. R D R = C + I where I s the nterference from hgher prorty tasks Durng R, each hgher prorty task j wll execute a no. of tmes. Number of Releases = Total nterference = R T j R C j T j Celng functon gves the smallest nteger greater than the fractonal number on whch t acts. Celng of 1/3 = 2, 6/5 = 2 And 6/3 =2 G. Khan Embedded Computer Systems EE8205: Real-tme Schedulng Page: 25

26 Response Tme R R = C + j hp ( ) T j C j where hp() s the set of tasks wth prorty hgher than task Solve by formng a recurrence relatonshp: n n + 1 w = C + j hp ( ) T j n The set of values w, w, w,..., w,.. s monotoncally non-decreasng n When = n+1 0 w w the soluton to the equaton has been found, w must not be greater than R (e.g. 0 or C ) w C j G. Khan Embedded Computer Systems EE8205: Real-tme Schedulng Page: 26

27 Response Tme Calculaton Algorthm for n 1..N loop -- for each process n turn n := 0 n w : = C loop n+1 calculate new w n+1 n f w = w then n R = w ext value found end f n+1 f w > T then ext value not found end f n := n + 1 end loop end loop G. Khan Embedded Computer Systems EE8205: Real-tme Schedulng Page: 27

28 Response Tme Calculaton Example Process Set D Process Perod, T Computaton Tme, C Prorty, P a b c R a = 3 w w w 0 b 1 b 2 b = 3 3 = = 6 = = 6 7 R b = 6 G. Khan Embedded Computer Systems EE8205: Real-tme Schedulng Page: 28

29 G. Khan Embedded Computer Systems EE8205: Real-tme Schedulng Page: 29 Response Tme Calculaton Process c = + + = = + + = = + + = = c c c c w w w w = = + + = = + + = c c c R w w

30 Process Set C Process Perod, T Computaton Tme, C Prorty, P Response Tme, R a b c The combned utlzaton s 1.0. Ths was above the utlzaton threshold for three processes (0.78) therefore t faled the test. The response tme analyss shows that the process set wll meet all ts deadlnes. RTA s necessary and suffcent. If the process set passes the test they wll meet all ther deadlnes; f they fal the test then, at run-tme, a process wll mss ts deadlne. (unless computaton tme estmatons themselves turn out to be pessmstc) G. Khan Embedded Computer Systems EE8205: Real-tme Schedulng Page: 30

31 Worst-Case Executon Tme WCET Obtaned by ether measurement or analyss The problem wth measurement s that t s dffcult to be sure when the worst case has been observed. The drawback of analyss s that an effectve model of the processor (ncludng caches, ppelnes, memory wat states and so on) must be avalable. Most analyss technques nvolve two dstnct actvtes. The frst takes the process and decomposes ts code nto a drected graph of basc blocks. These basc blocks represent straght-lne code. The second component of the analyss takes the machne code correspondng to a basc block and uses the processor model to estmate ts worst-case executon tme. Once the tmes for all the basc blocks are known, the drected graph can be collapsed. G. Khan Embedded Computer Systems EE8205: Real-tme Schedulng Page: 31

32 WCET Analyss Need Semantc Informaton for I n loop f Cond then -- basc block of cost 100 else -- basc block of cost 10 end f; end loop; Smple cost 10 x 100 (+overhead), say But f Cond only true on 3 occasons then cost s 375 G. Khan Embedded Computer Systems EE8205: Real-tme Schedulng Page: 32

33 Real-tme Schedulng Exercses Exercse-1: Consder three processes P, Q and S. P has a perod of 100msec n whch t requres 30msecs of processng. The correspondng values for Q and S are (6, 1) and (25, 5) respectvely. Assume that P s the most mportant process n the system, followed by Q and then S. (1) What s the behavor of the scheduler f prorty s based on mportance? (2) What s the process utlzaton of P, Q and S. (3) How should the process be scheduled so that all deadlnes are met. (4) Illustrate one of the schemes that allows these processes to be scheduled. Exercse-2: Add a fourth process R, to the set of processes gven n Exercse-1. Falure of ths process wll not lead to safety beng undermned. R has a perod of 50ms, but has a processng requrement that s data dependent and vares from 5 to 25 ms. Dscuss how ths process should be ntegrated wth P, Q and S. G. Khan Embedded Computer Systems EE8205: Real-tme Schedulng Page: 33

34 Hard and Soft Real-tme Processes Hard Real-tme Process: The deadlne must not be mssed. Soft Real-tme Process: The applcaton s tolerant of mssed deadlnes. In many stuatons the WCET (worst-case executon tme) fgures for sporadc processes are consderably hgher than the averages. Measurng schedulablty wth worst-case fgures may lead to very low processor utlzatons. Interrupts often arrve n bursts e.g. an abnormal sensor readng may lead to sgnfcant addtonal computaton. G. Khan Embedded Computer Systems EE8205: Real-tme Schedulng Page: 34

35 Sporadc Processes A Sporadc process s that whch has hard real-tme applcatons. Sporadc processes have a mnmum nter-arrval tme. They also requre D < T The response tme algorthm for fxed prortyschedulng works perfectly for values of D less than T as long as the stoppng crtera becomes W n+1 > D It also works perfectly well wth any prorty orderng, hp() always gves the set of hgher-prorty processes G. Khan Embedded Computer Systems EE8205: Real-tme Schedulng Page: 35

36 Hard/Soft Process Schedulng Gudelnes Rule 1 all processes should be schedulable usng average executon tmes and average arrval rates. Rule 2 all hard real-tme processes should be schedulable usng worst-case executon tmes and worst-case arrval rates of all processes (ncludng soft) A consequent of Rule 1 s that there may be stuatons n whch t s not possble to meet all current deadlnes Ths condton s known as a transent overload Rule 2 ensures that no hard process wll mss ts deadlne If Rule 2 gves rse to unacceptably low utlzatons for normal executon then acton must be taken to reduce the worst-case executon tmes (or arrval rates) G. Khan Embedded Computer Systems EE8205: Real-tme Schedulng Page: 36

37 Aperodc Processes Aperodc processes have soft real-tme jobs. They do not have mnmum nter-arrval tmes. Can run aperodc processes at a prorty below the prortes assgned to hard processes, therefore, they cannot steal, n a pre-emptve system, resources from the hard processes. Ths does not provde adequate support to soft processes, whch wll often mss ther deadlnes. To mprove the stuaton for soft processes, a server (sporadc) can be employed. Servers protect the processng resources needed by hard processes but otherwse allow soft processes to run as soon as possble. POSIX support Sporadc Servers G. Khan Embedded Computer Systems EE8205: Real-tme Schedulng Page: 37

38 Process Sets wth D < T For D = T, Rate Monotonc prorty orderng s optmal. For D < T, (DMPO) Deadlne Monotonc Prorty Orderng s optmal. D < T Example Process Set Process Perod T D < D P > Deadlne Computaton Prorty D Tme, C P a b c d Proof of DMPO s Optmal s gven n the text j P j Response Tme, R G. Khan Embedded Computer Systems EE8205: Real-tme Schedulng Page: 38

39 Deadlne Schedulng Exercses Exercse-1: Consder two jobs, A and B, n a deadlne schedulng system. The deadlne for A s before the deadlne for B. Explan why we should run A before B, that s, show that f runnng A then B fals to meet some deadlne then runnng B before A wll also fal to meet some deadlne. Exercse 2: Consder a set of 5 aperodc tasks whose executon profles are gven below. Develop the schedulng dagram of these processes employng EDF and FCFS. Process Arrval Tme Executon Tme Startng Deadlne A B C D E G. Khan Embedded Computer Systems EE8205: Real-tme Schedulng Page: 39

40 Process Interactons and Blockng If a process s suspended watng for a lower-prorty process to complete some requred computaton then the prorty model s, n some sense, beng undermned. The process s sad to suffer prorty nverson. If a process s watng for a lower-prorty process, the process s sad to be blocked. Dynamc prortes can vary durng executon. One has to avod Prorty Inverson. Bounded Prorty Inverson Duraton s not longer than that of the crtcal secton where the lower-prorty process owns the resource. Unbounded Prorty Inverson Occurs when a thrd (medum-prorty) process preempts the lowprorty process durng the nverson for an ndefnte tme. G. Khan Embedded Computer Systems EE8205: Real-tme Schedulng Page: 40

41 Prorty Inverson An extreme example of prorty nverson, consder the executons of four perodc processes: a, b, c and d; and two resources: Q and V Example of Prorty Inverson Process Prorty Executon Sequence Release Tme a 1 EQQQQE 0 b 2 EE 2 c 3 EVVE 2 d 4 EEQVE 4 G. Khan Embedded Computer Systems EE8205: Real-tme Schedulng Page: 41

42 Process Example of Prorty Inverson d c b Process Completon Tme Deadlne Met Process Release Tme a Executng Preempted Executng wth Q locked Executng wth V locked Blocked G. Khan Embedded Computer Systems EE8205: Real-tme Schedulng Page: 42

43 Prorty Inhertance If process a s blockng the process d, then t runs wth the prorty of d. Process d Process Release Tme Process Completon Tme Deadlne Met c b a G. Khan Embedded Computer Systems EE8205: Real-tme Schedulng Page: 43

44 G. Khan Embedded Computer Systems EE8205: Real-tme Schedulng Page: 44 Calculatng Blockng If a process has m crtcal sectons that can lead to ts blockng then the maxmum number of tmes t can be blocked s m. If B s the maxmum blockng tme and K s the number of crtcal sectons, the process has an upper bound on ts Response Tme and Blockng: = = K k k C k usage B 1 ) ( ), ( blockng gven by: I B C R + + = j hp j j C T R B C R + + = ) ( j hp j j n n C T w B C w + + = + ) ( 1

45 RTX - Prorty Inversons To prevent prorty nversons, RTX employs the Prorty nhertance technque. For a short tme, the lower-prorty task runs at a prorty of a hgher-prorty pendng task. The RTX mutex objects (Mutual Exclusve Lock objects) employ the Prorty nhertance. Mutex Management Routnes os_mut_nt: Intalzes a mutex object. os_mut_release: Releases a mutex object. os_mut_wat: Wats for a mutex object to become avalable Mutual excluson locks (mutexes) are an alternatve to avod synchronzaton and memory access problems. Mutexes are software objects that a task can use to lock the common resource. Only the task that locks the mutex can access the common resource. The kernel blocks all other tasks that request the mutex untl the task that locked the mutex unlocks t. G. Khan Embedded Computer Systems EE8205: Real-tme Schedulng Page: 45

46 Prorty Celng Protocols OCPP: Orgnal celng prorty protocol ICPP: Immedate celng prorty protocol OCPP Each process has a statc default prorty assgned (perhaps by the deadlne monotonc scheme) Each resource has a statc celng value defned, ths s the maxmum prorty of the processes that use t. A process has a dynamc prorty that s the maxmum of ts own statc prorty and any t nherts due to t blockng the hgherprorty processes. A process can only lock a resource f ts dynamc prorty s hgher than the celng of any currently locked resource. (excludng any that t has already locked tself) G. Khan Embedded Computer Systems EE8205: Real-tme Schedulng Page: 46

47 OCPP Inhertance Process d Executng Executng wth Q locked Executng wth V locked Process Completon Tme Deadlne Met Process Release Tme c b a G. Khan Embedded Computer Systems EE8205: Real-tme Schedulng Page: 47

48 ICPP Each process has a statc default prorty assgned (perhaps by the deadlne monotonc scheme). Each resource has a statc celng value defned, ths s the maxmum prorty of the processes that use t. A process has a dynamc prorty that s the maxmum of ts own statc prorty and the celng values of any resources t has locked. As a consequence, a process wll only suffer a block at the very begnnng of ts executon. Once the process starts actually executng, all the resources t needs must be free; f they were not, then some process would have an equal or hgher prorty and the process's executon would be postponed. G. Khan Embedded Computer Systems EE8205: Real-tme Schedulng Page: 48

49 ICPP Inhertance Process Executng Executng wth Q locked Executng wth V locked Process Completon Tme Deadlne Met Process Release Tme d c b a G. Khan Embedded Computer Systems EE8205: Real-tme Schedulng Page: 49

50 OCPP versus ICPP The worst-case behavor of the two celng schemes s dentcal (from a schedulng vew pont) A hgh-prorty process can be blocked at most once durng ts executon by lower-prorty processes Deadlocks are prevented. Transtve blockng s prevented. Ensure mutual exclusve access to resources (by protocol tself) There are some ponts of dfference: ICPP s easer to mplement than the orgnal (OCPP) as blockng relatonshps need not be montored ICPP leads to less context swtches as blockng s pror to frst executon ICPP requres more prorty movements as ths happens wth all resource usage OCPP changes prorty only f an actual block has occurred. ICPP s called Prorty Protect Protocol n POSIX G. Khan Embedded Computer Systems EE8205: Real-tme Schedulng Page: 50

51 Mars Pathfnder Suffered Unbounded Prorty Inverson Low-prorty Meteorologcal Process: Acqured the (shared) bus. Medum-prorty, Long-runnng, Communcatons Process: Woke up and preempted the meteorologcal thread. Hgh-prorty Bus Management Process: Woke up and was blocked because t couldn't acqure the bus; When t couldn't meet ts deadlne t rentalzed the computer va a hardware reset. G. Khan Embedded Computer Systems EE8205: Real-tme Schedulng Page: 51

52 Mars Pathfnder G. Khan Embedded Computer Systems EE8205: Real-tme Schedulng Page: 52

53 Duraton of Unbounded Prorty Inverson Lmtng the duraton of unbounded prorty nverson prevents low-prorty process from beng preempted by the medumprorty processes durng the prorty nverson. Technque: Manpulate process prortes at run-tme. Schedulng: Processes wth hgher prorty are scheduled to run frst. Objectve: Assgn prortes n such a way that all outputs are computed before ther deadlnes. Deadlne-Drven Assgnment: Assgn hghest prortes to processes wth shortest deadlnes. Rate Monotonc Assgnment: Assgn hghest prortes to processes that run most frequently wthout regard to deadlnes. G. Khan Embedded Computer Systems EE8205: Real-tme Schedulng Page: 53

54 Modfed Process Model Untl Now: Deadlnes can be less than perod (D < T) Sporadc and aperodc processes, as well as perodc processes, can be supported Process nteractons are possble, wth the resultng blockng beng factored nto the response tme equatons. Extensons to the Orgnal Model Cooperatve Schedulng Release Jtter Arbtrary Deadlnes Fault Tolerance Offsets Optmal Prorty Assgnment G. Khan Embedded Computer Systems EE8205: Real-tme Schedulng Page: 54

55 Cooperatve Schedulng True preemptve behavor s not always acceptable for safetycrtcal systems Cooperatve or deferred preempton splts processes nto slots Mutual excluson s va non-preempton The use of deferred preempton has two mportant advantages It ncreases the schedulablty of the system, and t can lead to lower values of C(computaton tme).wth deferred preempton, no nterference can occur durng the last slot of executon. Let the executon tme of the fnal block be After the soluton converge.e. The response tme s gven by: n n w + 1 w = BMAX + C F + j hp( ) Tj n w = n+1 w n R = w + F C j G. Khan Embedded Computer Systems EE8205: Real-tme Schedulng Page: 55

56 G. Khan Embedded Computer Systems EE8205: Real-tme Schedulng Page: 56 Arbtrary Deadlnes To cater for the stuatons where D (and hence potentally R) > T The number of releases s bounded by the lowest value of q for whch the followng relaton s true: The worst-case response tme s then the maxmum value found for each q: j hp j j n n C T q w C q B q w = ) ( 1 ) ( 1) ( ) ( ) ( max 0,1,2,... q R R q = = n qt q w q R = ) ( ) ( T q R ) (

57 Fault Tolerance Fault tolerance va ether forward or backward error recovery always results n extra computaton Ths could be an excepton handler or a recovery block. In a real-tme fault tolerant system, deadlnes should stll be met even when a certan level of faults occur Ths level of fault tolerance s know as the fault model If the extra computaton tme resultng from an error n process, s f C R = C + B + R C j max j hp( ) T k hep( ) j where hep() s set of processes wth prorty equal to or hgher than + C f k G. Khan Embedded Computer Systems EE8205: Real-tme Schedulng Page: 57

58 G. Khan Embedded Computer Systems EE8205: Real-tme Schedulng Page: 58 Fault Tolerance If F s the number of faults allows If there s a mnmum arrval nterval = f k f hep k j hp j j C T R C T R B C R max ) ( ) ( f k hep k j hp j j FC C T R B C R max ) ( ) ( =

59 Dynamc Systems and Onlne Analyss There are dynamc soft real-tme applcatons n whch arrval patterns and computaton tmes are not known a pror. Although some level of off-lne analyss may stll be applcable, ths can no longer be complete and hence some form of on-lne analyss s requred. The man task of an on-lne schedulng scheme s to manage any overload that s lkely to occur due to the dynamcs of the system's envronment. EDF s a dynamc schedulng scheme that s an optmal. Durng transent overloads EDF performs very badly. It s possble to get a cascade effect n whch each process msses ts deadlne but uses suffcent resources to result n the next process also mssng ts deadlne. G. Khan Embedded Computer Systems EE8205: Real-tme Schedulng Page: 59

60 Admsson Schemes To counter ths detrmental domno effect many on-lne schemes have two mechansms: An admssons control module that lmts the number of processes that are allowed to compete for the processors, and An EDF dspatchng routne for those processes that are admtted. An deal admssons algorthm prevents the processors gettng overloaded so that the EDF routne works effectvely. G. Khan Embedded Computer Systems EE8205: Real-tme Schedulng Page: 60

61 Values If some processes are to be admtted, whlst others rejected, relatve mportance of each process must be known. Ths s usually acheved by assgnng value. Values can be classfed as: Statc: a process always has the same value whenever t s released. Dynamc: the process's value can only be computed at the tme the process s released (because t s dependent on ether envronmental factors or the current state of the system) Adaptve: here the dynamc nature of the system s such that the value of the process wll change durng ts executon. To assgn statc values requres the doman specalsts to artculate ther understandng of the desrable behavor of the system. G. Khan Embedded Computer Systems EE8205: Real-tme Schedulng Page: 61