Real Time Systems. Part 2: Cyclic schedulers. Real Time Systems. Francisco Martín Rico. URJC. 2011

Transcripción

1 Real Time Systems Part 2: Cyclic schedulers

2 Scheduling To organise the use resources to guarantee the temporal requirements A scheduling method is composed by: An scheduling algorithm that calculates the execution of jobs in the processor An analysis method of the temporal behavior (worst case) Static scheduling method: before execution Dynamic scheduling method: after execution

3 Valid and feasible schedule Valid schedule: It assigns only one job to each processor It assigns one processor at the same time to each job, as much It does not executes any job before its release time It assigns to each job an amount of CPU time equal to its actual or maximum execution time It satisfies all the precedence relations A Feasible scheduling is a valid plan that ensures that all the jobs finish before their deadline A system is schedulable if the scheduler always produces a feasible scheduling A Optimum algorithm always produces a feasible plan for any system

4 Approaches to Real-Time scheduling Clock-drive approach: Decisions on what jobs execute at what time are made at specific time instants These instants are chosen a-priori Round-robin approach: FIFO If weighted, not all the time slices are similar Priority-drive approach: Large collection of algorithm Jobs are never idle FIFO, LIFO, SETF, LETF...

5 Clock-driven scheduling If all the tasks are periodic, it is possible to set a periodic execution plan The system is statically composed by N periodic task All the parameters of the tasks are known a-priori A task T i is defined as (φ i,p i,e i,d i ) Each job J i,k is activated at r i,k = φ i + k p i It has to finish at d i,k = r i,k + D i It is usual that p i = D i There are aperiodic tasks, but not sporadic ones

6 Static, clock-driven scheduling El plan se repite cada H = mcm(4, 5, 20) = 20 The scheduling repeats each main cycle The main El cycle plan se is repite composed cada by frames H = mcm(4, 5, 20) = 20 Tarea φ T C D Example T 1 =(4, 1) T 2 =(5, 2) T 3 = (20, 1) T 4 = (20, 2) H = mcm(4, 5, 20) Tarea φ T C D T T T T T T T T Planificación Cíclica (cont.) 44 Ejemplo Ejemplo Las decisiones se toman cada 2ms H = mcm(p i ) Ciclo Tareas Las decisiones se toman cada 2ms 1 T 1 T 3 Ciclo Tareas 2 T 2 1 T 3 T 1 T T 2 4 T 3 T T 4 T TT TT TT T T T 10 T T 2 T T 4 T 1 T 2 T 1 T 2 T 1 T 3 T 2 T1 T 3 T1 T 2 3 T 12 T 1 T T T T 1 T 2 T 1 T 1 T 2 T 1 T 3 T c2007 GSyC Sistemas de Tiempo Real: Planificadores Cíclicos 7 GSyC Sistemas de Tiempo Real: Planificadores Cíclico

7 clock-driven scheduler Task scheduler: set the next decision point i and table entry k to 0; set the timer to expire at t k ; do forever: accept timer interrupt; if and aperiodic job is executing, prompt the job; current task T = T (t k ) increment i by 1; compute the next table entry k = i%(n) set the timer to expire at [i/n] H + t k if the current task T is I, let the job at the head of the periodic job queue execute; else, let the task T execute; sleep; end scheduler

8 Period plan parameters Instead of using an ad-hoc cyclic scheduler, we want some structure All the jobs can be executed in a frame f max(e i ) The frame size divides hyperperiod [p i /f] p i /f =0 The frame size has to be small enough so that between the release time and the deadline of every job, there is at least one frame 2f gcd(p i,f) D i

9 Example Parámetros del plan cíclico (cont.) Ejemplo 1. T S max(1, 2, 1, 2) 2 H = mcm(4, 5, 20) = 20 Task Tarea φ T C D T T T T T1 T 3 T 2 T 1 T 4 T 2 T 1 T 2 T 1 T 1 T 2 T 1 T 3 T T S {2, 4, 5, 10, 20} 3. 2 T S MCD(T S, 4) 4 (para T 1 ) Para el 2: 2 2 MCD(2, 4) = 4 2=4 2 4 Vale Para el 4: 2 4 MCD(4, 4) = 8 4=4 4 4 Vale Para el 5: 2 5 MCD(5, 4) = 10 1=9 9 > 4 No vale Para el 10: 2 10 MCD(10, 4) = 20 2 = > 4 No vale Para el 20: 2 20 MCD(20, 4) = 40 4 = > 4 No vale 2 T S MCD(T S, 5) 5 (para T 2 ) Para el 2: 2 2 MCD(2, 5) = 4 1=3 3 5 Vale Para el 4: 2 4 MCD(4, 5) = 8 1=7 7 > 5 No Vale 2 T S MCD(T S, 20) 20 (T 3 y T 4 ), Valid Valid Not Valid Valid Not Valid Not Valid Not Valid El único The que only cumple value las possible tres es is T S =2

10 Job slices Segmentación (cont.) 5 Sometimes not all the constraints can be meet Ejemplo 2 (1) and (3) can be meet at the same time There is no solution cjuan Antonio de la Puente

11 Job slices Ejemplo 2 - segmentación Segmentación (cont.) 52 The solution is splitting any task into subtasks Ejemplo 2 - segmentación 4 is a correct solution cjuan Antonio de la Puente cjuan Antonio de la Puente

12 Cyclic executives Practical implementation of cyclic schedulers In these cyclic schedulers, decisions are made only at frame boundaries Each job slice is a procedure It allows aperiodic and sporadic tasks

13 Cyclic_executive Task Cyclic_executive: set time t=0; set current frame k=0; do forever: accept clock interrupt at time tf; currentblock = L(k); t = t + 1; k = t mod F; if the last job is not completed, take appropriate action; wake up the periodic task server to execute the slices in currentblock; sleep until the periodic task server completes; while the aperiodic job queue is nonempty, wake up the job at the head of the aperiodic job queue; sleep until the aperiodic job completes; remove the aperiodic job from the queue; end while; sleep until the next clock interrupt; end do; end Cyclic_executive;

14 Algorithm for constructing static schedulers (f),h x J i (f),h source (e i ),e i (e k ),e k (f),e i h (f),e k y (f),e i + e k h (f),0 sink J k (f), 0 z (f),0 T1(4, 1) T2(5, 2,7) T3(20, 5) F

15 Practical considerations Sporadic jobs Frame overruns Slack stealing Mode changes