Real-Time Operating Systems

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Achieving predictability: interrupts ¿ First solution: Disable all interrupts but timer interrupts ¿ all peripheral devices have to be handled by application tasks ¿ data transfer by polling ¿ great flexibility, data transfers can be estimated precisely ¿ no change of kernel needed when adding devices ¿ Problems: ¿ degradation of processor performance (busy wait) ¿ no encapsulation of low level details 15 Achieving predictability: interrupts ¿ Second solution: Disable all interrupts but timer interrupts, plus handle devices by special, timeractivated kernel routines ¿ unbounded delays due to interrupt driver eliminated ¿ periodic device routines can be estimated in advance ¿ hardware details encapsulated in dedicated routines ¿ Problems ¿ degradation of processor performance (still busy waiting - within I/0 routines) ¿ more inter-process communication than first solution ¿ kernel has to be modified when adding devices 16 8
Achieving predictability: interrupts ¿ Third solution: Enable external interrupts and reduce the drivers to the least possible size ¿ driver only activates proper task to take care of device ¿ The task executes under direct control of OS, just like any other task ¿ control tasks than have higher priority than device task 17 Achieving predictability: interrupts ¿ Advantages ¿ busy wait eliminated ¿ unbounded delays due to unexpected device handling dramatically reduced (not eliminated !) ¿ remaining unbounded overhead may be estimated relatively precisely ¿ State of the art ! 18 9
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: Real-time Operating Systems (3) ¿
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 and 30: Introduction ¿ Periodic activities
Page 31 and 32: Processor utilization factor (2) ¿
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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