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Flexible Scheduling of Software with Logical Execution Time Constraints*

Flexible Scheduling of Software with Logical Execution Time Constraints*. Stefan Resmerita and Patricia Derler University of Salzburg, Austria *UC Berkeley, USA. Introduction. Scope: Embedded software aplications Set of periodic tasks with predictable timing behavior Preemptive scheduling

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Flexible Scheduling of Software with Logical Execution Time Constraints*

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  1. Flexible Scheduling of Software with Logical Execution Time Constraints* Stefan Resmerita and Patricia Derler University of Salzburg, Austria *UC Berkeley, USA

  2. Introduction • Scope: Embedded software aplications • Set of periodic tasks with predictable timing behavior • Preemptive scheduling • Event-triggered tasks • Problem: Predictability is achieved by restricting the set of feasible schedules • Aim: Relax scheduling restrictions while preserving predictability 2

  3. The LET Programming Model • Specification of logical execution times for tasks • Giotto, TDL, HTL, xGiotto, FTOS • Implementation • Dedicated runtime system 3

  4. Main Runtime Operations • Update outputs at LET end • Invoke task at LET start • Update inputs • Release task for execution 4

  5. Scheduling • High-level: scheduling of operations • Static schedule compiled into a „timing program“ • Platform independent • Low-level: scheduling of task executions • Platform dependent • May use any policy (e.g., FPS, EDF) • Schedulability test uses WCET information • What if the system is not schedulable? 5

  6. Trade-offs • Increased predictability • Separation of timing from functionality • Separation of reactivity from scheduling • Platform independence • Portable timing program • Performance costs • Application performance (response time) • Platform requirements (memory/time) • Processor utilization (idle time) 6

  7. This Work • Provides a methodology for obtaining more flexible high-level schedules • Keep predictability • Increase processor utilization • Cost: Portability is reduced • Provide tool support 7

  8. Enlarging Scheduling Margins • Use more information about • Execution times of tasks • Predictability of inputs • Low-level scheduling 8

  9. PowerOn 4ms 8ms_A 8ms_B 16ms Common Task Structures • Shared memory • Internal dispatching • Offsets • Example (Two tasks) • Periods: 4 and 8 • Offsets: 2 and 4 0 2 4 6 8 10 12 9

  10. Case 1: Reading from Sensors • Internal port p is connected to a sensor • The variable p is updated at tLs(T) • δ(T,p): minimun execution time of T up to accessing p • Task T can be started at time tr = tLs(T) – δ(T,p) 10

  11. Case 2: Reading from LET-Based Tasks • Ports p1 and p2 are updated from tasks T1 and T2, respectively, at the end of their LETs tr =max{tLe(T1) – δ(T,p1), tLe(T2) – δ(T,p2)} 11

  12. Modified Operational Requirements (O1) Update task outputs at LET end (O2) Update inputs connected to sensors at LET start (O3) Update input ports connected to LET tasks at the end of the source task‘s LET (O4) Release task T at time tr tLs(T) such that • No input port connected to a sensor is accessed before tLs(T) • Every port that is accessed before tLs(T) has a constant value between the moment of the access and tLs(T) 12

  13. Computation of Early Release Times • Formally: (1) 13

  14. Main Result 14

  15. Schedulability • Assumption: The system with classical release times is schedulable. • Question: Is the system with release margins schedulable? • Answer: Depends on the underlying scheduling algorithm 15

  16. Scheduling with Earliest Deadline First 16

  17. Fixed-Priority Scheduling • Counter-example (T1 has higher priority): • Conservative solution: use only the minimum margin Classical case: No missed deadline Early release of T1 leads to a missed deadline for T2 17

  18. Our Approach: Dual-Priority Scheduling • Assign a dual priority to each LET-based task • All dual priorities are lower than all nominal ones • A task is scheduled by FPS: • With nominal priority inside its LET • With dual priority outside its LET • Effect: a task is executed outside its LET only if the CPU would be otherwise idle! • DP scheduling is as predictable as FPS 18

  19. Mixing Events in • DPS can be used in systems containing also event-triggered tasks • Event-triggered tasks are always scheduled with nominal priorities Theorem 3: If event triggered tasks have lower priorities than LET-based tasks, then their response times remain the same or decrease when using release margins with DPS instead of classical release times with FPS. 19

  20. Application Example Inverted pendulum: 20

  21. Evaluation of DPS 21

  22. Conclusions • Approach for relaxed scheduling contraints • Usage of execution time information beyond just WCET • Employ timing predictability offered by LET to improve scheduling of the application • Static scheduling, fully automatic • Further work: dynamic scheduling, evaluation 22

  23. Thank you!

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