Real-Time Operating Systems

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Characterization of periodic tasks ¿ A periodic task τ can be characterized (see 1-4) by: ¿ (phase φ i ) ¿ period T i ¿ worst case computation time C i ¿ Additional parameters: ¿ Response time R i = f i – r i ¿ Release jitter of a task • maximum deviation of the start time of two-consecutive instances: RRJ i = max k | (s i,k - r i,k ) - (s i,k-1 - r i,k-1 ) | ¿ Finish jitter of a task • maximum deviation of the finish time of two-consecutive instances: RRJ i = max k | (f i,k - r i,k ) - (f i,k-1 - r i,k-1 ) | ¿ Critical instant (of a task): • Release time of a task instance resulting in the largest response time 59 Processor utilization factor ¿ Given a set G of a periodic tasks the utilization factor U is the fraction of processor time spent in the execution of the task set. ¿ U determines the load of the CPU ¿ C i /T i is the fraction of processor time spent in executing task τ i U = S i=1…n C i /T i ¿ U can be improved by ¿ Increasing computation times of the tasks ¿ Decreasing the periods of the tasks up to a maximum value below which Γ is schedulable ¿ Limit depends on ¿ task set (particular relations among task’s periods) ¿ algorithm used to schedule the tasks 60 30
Processor utilization factor (2) ¿ Upper bound of U, U ub (Γ,A): ¿ Value of U (for a given task set and scheduling algorithm) for which the processor is fully utilized ¿ Γ is schedulable using A but any increase of computation time in one of the tasks makes set infeasible ¿ Least upper bound of U, U lub: ¿ Minimum of U ub over all task sets for a given algorithm U ( A) = min( U ( Γ, A)), ∀Γ. lub ¿ U lub allows to easy test for schedulability ub 61 Processor utilization factor (3) ¿ Schedulability test: ¿ U lub (A) U lub Æ Γ may be schedulable, if the periods of the tasks are suitable related. ¿ U lub (A) > 1 Æ Γ not schedulable. U ub 1 G 1 G U ub2 2 G U 3 ub3 G 4 U ub4 U G ubm m 0 1 U lub YES ? NO U 62 31
Page 1 and 2: Real-Time Operating Systems 1 Summa
Page 3 and 4: ¿ Real-time Introduction (3) ¿ Ti
Page 5 and 6: RTOS 9 Real-time Operating Systems
Page 7 and 8: Real-time Operating Systems (3) ¿
Page 9 and 10: Achieving predictability: interrupt
Page 11 and 12: Priority inversion (2) ¿ Basic pri
Page 13 and 14: Achieving predictability: memory ma
Page 15 and 16: Processes ¿ Called tasks in the RT
Page 17 and 18: Scheduling constraints ¿ The follo
Page 19 and 20: Periodic and aperiodic tasks ¿ Per
Page 21 and 22: Real-time scheduling 41 ¿ Given: S
Page 23 and 24: Scheduling metrics ¿ n tasks ¿ Av
Page 25 and 26: Jackson’s algorithm [1955] ¿ 1 |
Page 27 and 28: Horn’s algorithm ¿ Example: 53 S
Page 29: Introduction ¿ Periodic activities
Page 33 and 34: Cyclic executive approach ¿ Schedu
Page 35 and 36: Process-based scheduling ¿ Fixed-p
Page 37 and 38: RM schedulability test (2) ¿ For l
Page 39 and 40: EDF algorithm ¿ Dynamic scheduling
Page 41 and 42: Deadline monotonic (DM) scheduling
Page 43 and 44: EDF for D
Page 45 and 46: Periodic task scheduling: summary D

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