# Real-Time Scheduling and Threads: Basics

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1 Real-Time and Threads: Basics Luca Abeni RTS-LiKe 2014 Luca Abeni 1 / 44

2 Real-Time Applications Real-Time in Setting the The time when a result is produced matters A correct result produced too late is equivalent to a wrong result (or to no result) What does too late mean, here? Applications characterised by temporal constraints that have to be respected Temporal constraints are modelled using the concept of deadline Some activity has to finish before a specified time (deadline) Some data has to be generated before a deadline Some process/thread must terminate before a deadline... RTS-LiKe 2014 Luca Abeni 2 / 44

3 Processes, Threads, and Tasks Real-Time in Setting the Algorithm logical procedure used to solve a problem Program formal description of an algorithm, using a programming language Process instance of a program (program in execution) Thread flow of execution Task process or thread A task can be seen as a sequence of actions and a deadline must be associated to each one of them! Some kind of formal model is needed to identify these actions and associate deadlines to them RTS-LiKe 2014 Luca Abeni 3 / 44

4 Mathematical of a Task - 1 Real-Time in Setting the Real-Time taskτ i : stream of jobs (or instances)j i,k Each jobj i,k = (r i,k,c i,k,d i,k ): Arrives at timer i,k (activation time) Executes for a timec i,k Finishes at timef i,k Should finish within an absolute deadlined i,k c i,k r i,k f i,k d i,k RTS-LiKe 2014 Luca Abeni 4 / 44

5 Mathematical of a Task - 2 Real-Time in Setting the Summing up: a job is an abstraction used to associate deadlines (temporal constraints) to activities r i,k is the time when jobj i,k is activated (by an external event, a timer, an explicit activation, etc...) c i,k is the computation time needed by jobj i,k to complete d i,k is the absolute time instant by which jobj i,k must complete jobj i,k respects its deadline iff i,k d i,k Response time of jobj i,k : ρ i,k = f i,k r i,k RTS-LiKe 2014 Luca Abeni 5 / 44

6 Periodic and Sporadic Tasks Real-Time in Setting the Periodic / Sporadic taskτ i = (C i,d i,t i ): stream of jobsj i,k, with r i,k+1 = (or ) r i,k +T i d i,k = r i,k +D i C i = max i,k} k T i is the task period (or Minimum Inter-arrival Time) D i is the task relative deadline C i is the task worst-case execution time (WCET) R i is the worst-case response time: R i = max k {ρ i,k } = max k {f i,k r i,k } for the task to be correctly scheduled, it must ber i D i RTS-LiKe 2014 Luca Abeni 6 / 44

7 Example: Periodic Task Real-Time in Setting the A periodic task has a regular structure (cycle): activate periodically (periodt i ) execute a computation suspend waiting for the next period void PeriodicTask ( void arg ) { <i n i t i a l i z a t i o n >; } <s t a r t p e r i o d i c timer, period = T>; while ( cond ) { } <do something...> ; <wait next a c t i v a t i o n >; RTS-LiKe 2014 Luca Abeni 7 / 44

8 Graphical Representation Tasks are graphically represented by using a scheduling diagram. For example, the following picture shows a schedule of a periodic task τ 1 = (3,6,8) (withwcet 1 = 3,D 1 = 6,T 1 = 8) Real-Time in Setting the τ Notice that, while jobj 1,1 andj 1,3 execute for 3 units of time (WCET), job J 1,2 executes for only 2 units of time. RTS-LiKe 2014 Luca Abeni 8 / 44

9 Real-Time Real-Time in Setting the A real-time taskτ i = (C i,d i,t i ) (orτ i = (C i,t i ) ifd i = T i ) is properly served if all jobs respect their deadline......appropriate scheduling is important! The scheduler must somehow know the temporal constraints of the tasks......in order to schedule them so that such temporal constraints are respected How should real-time tasks be scheduled? (scheduling algorithm?) Is it possible to schedule them so that all deadlines are respected? Does provide an appropriate scheduling algorithm? RTS-LiKe 2014 Luca Abeni 9 / 44

10 Real-Time : Example The task sett = {(1,3),(4,8)} is not schedulable by FCFS Real-Time in Setting the τ 1 τ T = {(1,3),(4,8)} is schedulable using fixed priorities τ 1 τ RTS-LiKe 2014 Luca Abeni 10 / 44

11 So... Are Enough? Real-Time in Setting the According to the previous example, a fixed-priority scheduler can be appropriate for scheduling real-time tasks......is this true in general, or only for some (theoretical) examples? Given a set of real-time tasksγ = {τ i }, can a fixed priority scheduler allow to respect all the deadlines? Is it possible to know in advance if some deadline will be missed? How to assign the priorities? If fixed priorities are enough, thesched FIFO andsched RR policies can be used! RTS-LiKe 2014 Luca Abeni 11 / 44

12 Optimal Priority Assignment Real-Time in Setting the Given a periodic task set T with all tasks having relative deadlined i equal to the periodt i ( i, D i = T i ) The best assignment is the Rate Monotonic (RM) assignment Shorter period higher priority Given a periodic task set with deadline different from periods: The best assignment is the Deadline Monotonic assignment Shorter relative deadline higher priority For sporadic tasks, the same rules are valid as for periodic tasks RTS-LiKe 2014 Luca Abeni 12 / 44

13 Utilisation-Based Analysis Real-Time in Setting the Given a task set, is it possible to check if it is schedulable of not? In many cases it is useful to have a very simple test to see if the task set is schedulable. A sufficient test is based on the Utilisation bound: The utilisation least upper bound for scheduling algorithm A is the smallest possible utilisationu lub such that, for any task set T, if the task set s utilisationu is not greater thanu lub (U U lub ), then the task set is schedulable by algorithma RTS-LiKe 2014 Luca Abeni 13 / 44

14 Utilisation Each task uses the processor for a fraction of time U i = C i T i Real-Time in Setting the The total processor utilisation is U = i C i T i This is a measure of the processor s load RTS-LiKe 2014 Luca Abeni 14 / 44

15 Necessary Condition Real-Time in Setting the IfU > 1 the task set is surely not schedulable However, ifu < 1 the task set may or may not be schedulable... IfU < U lub, the task set is schedulable!!! Gray Area betweenu lub and1 We would like to haveu lub near to1 U lub = 1 would be optimal!!! RTS-LiKe 2014 Luca Abeni 15 / 44

16 Utilisation Bound for RM Real-Time in Setting the We considernperiodic (or sporadic) tasks with relative deadline equal to periods. Priorities are assigned with Rate Monotonic; U lub = n(2 1/n 1) U lub is a decreasing function ofn; For largen: U lub 0.69 n U lub n U lub RTS-LiKe 2014 Luca Abeni 16 / 44

17 Utilisation Bound for DM Real-Time in Setting the If relative deadlines are less than or equal to periods, instead of consideringu = n i=1 C i T i, we can consider: U = Then the test is the same as the one for RM (or DM), except that we must useu instead ofu. n i=1 C i D i Idea: τ = (C,D,T) τ = (C,D,D) τ is a worst case forτ Ifτ can be guaranteed,τ can be guaranteed too RTS-LiKe 2014 Luca Abeni 17 / 44

18 Pessimism of the Analysis: Example τ 1 = (3,6),τ 2 = (3,12),τ 3 = (6,24); U = 1 ; Real-Time in Setting the τ 1 τ 2 τ RTS-LiKe 2014 Luca Abeni 18 / 44

19 Dynamic Priorities - Earliest Deadline First Real-Time in Setting the RM and DM are optimal fixed priority assignments Maybe we can improve schedulability by using dynamic priorities? Fixed priority scheduling: a taskτ always has the same priority Dynamic priority scheduling: τ s priority can change during time... Let s assume that the priority changes from job to job (a job J i,j always has the same priorityp h,k ) Simplest idea: give priority to tasks with the earliest absolute deadline: d i,j < d h,k p i,j > p h,k Earliest Deadline First (EDF) DM relative deadlines; EDF absolute deadlines RTS-LiKe 2014 Luca Abeni 19 / 44

20 Can We Do any Better than RM? Real-Time in Setting the Yes (of course!): EDF can get full processor utilisation Consider a system of periodic tasks with relative deadline equal to the period. The system is schedulable by EDF if and only if U lub = 1!!! i C i T i 1 IfD i T i : Processor demand approach or response time analysis can be applied to EDF too But it is not obvious! RTS-LiKe 2014 Luca Abeni 20 / 44

21 An Example RM τ 1 = (3,8,8),τ 2 = (6,11,11) U = 0.92 Real-Time in Setting the τ 1 τ RTS-LiKe 2014 Luca Abeni 21 / 44

22 An Example RM τ 1 = (3,8,8),τ 2 = (6,11,11) U = 0.92 Real-Time in Setting the τ 1 τ RTS-LiKe 2014 Luca Abeni 21 / 44

23 An Example RM τ 1 = (3,8,8),τ 2 = (6,11,11) U = 0.92 Real-Time in Setting the τ 1 τ RTS-LiKe 2014 Luca Abeni 21 / 44

24 An Example RM τ 1 = (3,8,8),τ 2 = (6,11,11) U = 0.92 Real-Time in Setting the τ 1 τ RTS-LiKe 2014 Luca Abeni 21 / 44

25 An Example RM τ 1 = (3,8,8),τ 2 = (6,11,11) U = 0.92 Real-Time in Setting the τ 1 τ RTS-LiKe 2014 Luca Abeni 21 / 44

26 The Same Example EDF τ 1 = (3,8,8),τ 2 = (6,11,11) U = 0.92 Real-Time in Setting the τ 1 τ RTS-LiKe 2014 Luca Abeni 22 / 44

27 The Same Example EDF τ 1 = (3,8,8),τ 2 = (6,11,11) U = 0.92 Real-Time in Setting the τ 1 τ RTS-LiKe 2014 Luca Abeni 22 / 44

28 The Same Example EDF τ 1 = (3,8,8),τ 2 = (6,11,11) U = 0.92 Real-Time in Setting the τ 1 τ RTS-LiKe 2014 Luca Abeni 22 / 44

29 The Same Example EDF τ 1 = (3,8,8),τ 2 = (6,11,11) U = 0.92 Real-Time in Setting the τ 1 τ RTS-LiKe 2014 Luca Abeni 22 / 44

30 The Same Example EDF τ 1 = (3,8,8),τ 2 = (6,11,11) U = 0.92 Real-Time in Setting the τ 1 τ RTS-LiKe 2014 Luca Abeni 22 / 44

31 The Same Example EDF τ 1 = (3,8,8),τ 2 = (6,11,11) U = 0.92 Real-Time in Setting the τ 1 τ RTS-LiKe 2014 Luca Abeni 22 / 44

32 The Same Example EDF τ 1 = (3,8,8),τ 2 = (6,11,11) U = 0.92 Real-Time in Setting the τ 1 τ RTS-LiKe 2014 Luca Abeni 22 / 44

33 What About Multiple CPUs? Real-Time in Setting the UniProcessor Systems A scheduleσ(t) is a function mapping timetinto an executing taskσ : t T {τ idle } wheret is the set of tasks running in the system τ idle is the idle task: when it is scheduled, the CPU becomes idle For a multiprocessor system withm CPUs,σ(t) is extended to map t in vectorsτ (T {τ idle }) M How to implement a Real-Time scheduler form > 1 processors? Partitioned scheduling Global scheduling RTS-LiKe 2014 Luca Abeni 23 / 44

34 The Quest for Optimality Real-Time in Setting the UP : N periodic tasks withd i = T i : (C i,t i,t i ) Optimal scheduler: if C i T i 1, then the task set is schedulable EDF is optimal Multiprocessor scheduling: Goal: schedule periodic task sets with C i T i M Is this possible? Optimal algorithms RTS-LiKe 2014 Luca Abeni 24 / 44

35 Partitioned - 1 Real-Time in Setting the Reduceσ : t (T {τ idle }) M tom uniprocessor schedules σ p : t T {τ idle },0 p < M Statically assign tasks to CPUs Reduce the problem of scheduling onm CPUs tom instances of uniprocessor scheduling Problem: system underutilisation M CPU CPU CPU CPU RTS-LiKe 2014 Luca Abeni 25 / 44

36 Partitioned - 2 Real-Time in Setting the Reduce anm CPUs scheduling problem tom single CPU scheduling problems and a bin-packing problem CPU schedulers: uni-processor, EDF can be used Bin-packing: assign tasks to CPUs so that every CPU has load 1 Is this possible? Think about2cpus with{(6,10,10),(6,10,10),(6,10,10)} RTS-LiKe 2014 Luca Abeni 26 / 44

37 Global Real-Time in Setting the One single task queue, shared bym CPUs The firstm ready tasks from the queue are selected What happens using fixed priorities (or EDF)? Tasks are not bound to specific CPUs Tasks can often migrate between different CPUs Problem: schedulers designed for UP do not work well M CPU CPU CPU CPU M{ RTS-LiKe 2014 Luca Abeni 27 / 44

38 Global - Problems Real-Time in Setting the Dhall s effect: U lub for global multiprocessor scheduling can be quite low (for RM or EDF, converges to1) Pathological case: M CPUs,M +1tasks. M tasks (ǫ,t 1,T 1), a task(t,t,t). U = M ǫ T ǫ 0 U 1 However, global EDF guarantees an upper bound for the tardiness! Deadlines can be missed, but by a limited amount of time Global scheduling can cause a lot of useless migrations Migrations are overhead! Decrease in the throughput Migrations are not accounted in admission tests... RTS-LiKe 2014 Luca Abeni 28 / 44

39 Using in Real-Time in Setting the SCHED FIFO andsched RR use fixed priorities They can be used for real-time tasks, to implement RM and DM Real-time tasks have priority over non real-time (SCHED OTHER) tasks The difference between the two policies is visible when more tasks have the same priority In real-time applications, try to avoid multiple tasks with the same priority RTS-LiKe 2014 Luca Abeni 29 / 44

40 Setting the Real-Time in Setting the int sched_get_priority_max(int policy); int sched_get_priority_min(int policy); int sched_setscheduler(pid_t pid, int policy, const struct sched_param * param); int sched_setparam( pid_t pid, const struct sched_param * param); Ifpid == 0, then the parameters of the running task are changed The only meaningful field ofstruct sched param is sched priority RTS-LiKe 2014 Luca Abeni 30 / 44

41 Problems with Real-Time Priorities Real-Time in Setting the In general, regular (SCHED OTHER) tasks are scheduled in background respect to real-time ones Real-time tasks can preempt / starve other applications Example: the following task scheduled at high priority can make a CPU / core unusable void bad bad task ( ) { while ( 1 ) ; } Real-time computation have to be limited (use real-time priorities only when really needed!) On sane systems, running applications with real-time priorities requires root privileges (or part of them!) RTS-LiKe 2014 Luca Abeni 31 / 44

42 Real-Time Throttling Real-Time in Setting the A bad high-priority task can make a CPU / core unusable provides the real-time throttling mechanism to address this problem How does real-time throttling interfere with real-time guarantees? Given a priority assignment, a taskset is guaranteed all the deadlines if no throttling mechanism is used......but, what happens in case of throttling? Very useful idea, but something more theoretically founded might be needed... RTS-LiKe 2014 Luca Abeni 32 / 44

43 What About EDF? Real-Time in Setting the Can EDF (or something similar) be supported in? Problem: the kernel is (was?) not aware of tasks deadlines......but deadlines are needed in order to schedule the tasks EDF assigns dynamic priorities based on absolute deadlines So, a more advanced API for the scheduler is needed... Assign at least a relative deadlined i to the task... We will see that we need a runtime and a period too Moreover,d i,j = r i,j +D i......however, how can the scheduler knowr i,j? The scheduler is not aware of jobs... RTS-LiKe 2014 Luca Abeni 33 / 44

44 Tasks and Jobs... And Deadlines! Real-Time in Setting the To use EDF, the scheduler must know when a job starts / finishes Applications must be modified to signal the beginning / end of a job (some kind of startjob() / endjob() system call)......or the scheduler can assume that a new job arrives each time a task wakes up! Or, some other algorithm can be used to assign dynamic scheduling deadlines to tasks deadlined s i : assigned by the kernel to taskτ i If the scheduling deadlined s i matches the absolute deadline d i,j of a job, then the scheduler can respectd i,j!!! RTS-LiKe 2014 Luca Abeni 34 / 44

46 CBS: Some More Details Real-Time in Setting the Based on CPU reservations(q i,p i ) Ifτ i tries to execute for more thanq i everyp i, the algorithm decreases its priority, or throttles it τ i consumes the same amount of CPU time consumed by a periodic task with WCETQ i and periodp i Q i /P i : fraction of CPU time reserved toτ i If EDF is used (based on the scheduling deadlines assigned by the CBS), thenτ i is guaranteed to receiveq i time units everyp i if j Q j/p j 1!!! Only on uni-processor / partitioned systems... M CPUs/cores with global scheduling: if j Q j/p j M each task is guaranteed to receiveq i everyp i with a maximum delay RTS-LiKe 2014 Luca Abeni 36 / 44

47 CBS vs Other Reservation Algorithms Real-Time in Setting the The CBS is based on EDF Assigns scheduling deadlinesd s i EDF ond s i good CPU utilisation (optimal on UP!) The CBS allows to serve non periodic tasks Some reservation-based schedulers have problems with aperiodic job arrivals - due to the (in)famous deferrable server problem The CBS explicitly supports aperiodic arrivals (see the rule for assigning deadlines when a task wakes up) Allows to support self-suspending tasks No need to strictly respect the Liu&Layland task model No need to explicitly signal job arrivals / terminations RTS-LiKe 2014 Luca Abeni 37 / 44

48 CBS: the Algorithm Each taskτ i is associated a scheduling deadlined s i runtimeq i and a current Real-Time in Setting the Both initialised to0when the task is created When a job arrives: If the previous job is not finished yet, queue the activation Otherwise, check if the current scheduling deadline can be used (d s i > t andq i/(d s i t) < Q i/p i ) If not,d s i = t+p i,q i = Q i Whenτ i executes for a timeδ,q i = q i δ Whenq i = 0,τ i cannot be scheduled (until timed s i ) At timed s i,ds i = ds i +P i andq i = q i +Q i RTS-LiKe 2014 Luca Abeni 38 / 44

49 Real-Time in Setting the New scheduling policy Foreground respect to all of the other policies Uses the CBS to assign scheduling deadline to tasks Assign a (maximum) runtimeq i and a (reservation) periodp i to tasks Additional parameter: relative deadlined i The check if the current scheduling deadline can be used rule is used at task wake-up Then uses EDF to schedule them Both global EDF and partitioned EDF are possible Configurable through the cpuset mechanism RTS-LiKe 2014 Luca Abeni 39 / 44

50 Using Real-Time in Setting the Juri will talk about the API, but......how to dimension the scheduling parameters? (Maximum) runtimeq i (Reservation) periodp i also provides a (relative) deadlined i Obviously, it must be i Q i P i M The kernel can do this admission control Better to use a limit smaller thanm (so that other tasks are not starved!) RTS-LiKe 2014 Luca Abeni 40 / 44

51 Assigning Runtime and Period Real-Time in Setting the Temporal isolation Each task can be guaranteed independently from the others Hard Schedulability property IfQ i C i andp i T i (maximum runtime larger than WCET, and server period smaller than task period)......then the scheduling deadlines are equal to the jobs deadlines!!! All deadlines are guaranteed to be respected (on UP / partitioned systems), or an upper bound for the tardiness is provided (if global scheduling is used)!!! So, can be used to serve hard real-time tasks! RTS-LiKe 2014 Luca Abeni 41 / 44

52 What About Soft Real-Time? Real-Time in Setting the What happens ifq i < C i, orp i > T i? Q i P i must be larger than the ratio between average execution timec i and average inter-arrival timet i... and there will be no control on the task s response times...otherwise,d s i Possible to do some stochastic analysis (Markov chains, etc...) Givenc i < Q i < C i,t i = np i, and the probability distributions of execution and inter-arrival times......it is possible to find the probability distribution of the response times (and the probability to miss a deadline)! RTS-LiKe 2014 Luca Abeni 42 / 44

53 Changing Parameters... Real-Time in Setting the Tasks parameters (execution and inter-arrival times) can change during the tasks lifetime... So, how to dimensionq i andp i? Short-term variations: CPU reclaiming mechanisms (GRUB,...) If a job does not consume all of the runtimeq i, maybe the residual runtime can be used by other tasks... Long-term variations: adaptive reservations Generally slower, can be implemented by a user-space daemon Monitor the difference betweend s i andd i,j Ifd s i d i,j increases,q i needs to be increased Ifd s i d i,j 0,Q i can be decreased Lot of literature for both of these approaches RTS-LiKe 2014 Luca Abeni 43 / 44

54 Things I did not Mention... Real-Time in Setting the What about interacting tasks (shared resources, IPC,...)? Inheritance (priority inheritance, deadline inheritance, BandWidth Inheritance) Juri will probably say something about this... Is the kernel able to respect the theoretical schedule? What happens if a task is scheduled later than expected? Kernel Latency!!! This is what Preempt-RT is for... Preempt-RT and: two orthogonal approaches that can (and must) be combined Optimality with multiple CPUs? RTS-LiKe 2014 Luca Abeni 44 / 44

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