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Missed Deadline Notification in Best-Effort Schedulers

Missed Deadline Notification in Best-Effort Schedulers. Balaji Raman. Outline. Motivation Methodology Implementation Details Results Conclusion. Motivation – What are Best Effort Schedulers?. Best effort scheduling Does not provide facilities to specify or meet performance guarantees.

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Missed Deadline Notification in Best-Effort Schedulers

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  1. Missed Deadline Notification in Best-Effort Schedulers Balaji Raman

  2. Outline • Motivation • Methodology • Implementation Details • Results • Conclusion

  3. Motivation – What are Best Effort Schedulers? • Best effort scheduling • Does not provide facilities to specify or meet performance guarantees. • Advantages • Simple • Applications do not require system interfaces • Suitability • Soft real time systems • Disadvantages • Not suitable for hard real time systems • Under Utilization

  4. Methodology-MDN • Indicate deadline misses to scheduler. • Problems in all existing general purpose system schedulers- Linux, BEST.. • Advantages of MDN • Minimal interface

  5. Implementation Details • Implemented in three schedulers • Linux Scheduler • BEST Scheduler • Be-Rate Scheduler • We will look MDN implementation in Linux Scheduler in detail.

  6. Linux Time-Share Scheduler • Scheduling based on Quantum Counter Variable • A process can block its task if it is periodic. Q=4 C=1 Tid=D Q=3 C=1 Tid=C Q=2 C=1 Tid=B Q=1 C=5 Tid=A Scheduler Q=4 C=4 Tid=A Q=3 C=1 Tid=D Q=2 C=1 Tid=C Q=1 C=1 Tid=B Scheduler

  7. Linux Time-Share Scheduler • When all counters in queue are zero • Counter=Counter/2+nice • Problem: Deadlines could be missed for tasks with counter=0. Q=4 C=1 Tid=A Q=3 C=0 Tid=D Q=2 C=0 Tid=C Q=1 C=0 Tid=B Scheduler

  8. MDN + Linux • Counter=counter/2+(nice+mdn_nice) • Initially mdn_nice=0. • If MDN received mdn_nice++ • Priority boosted for not missing future deadlines. • Anti-Cheat Policy

  9. Best Scheduler • Detect Periodic tasks • Improves performance of soft real-time tasks Tasks entering Run Queue Run Q Period of the task ~ Y-X Scheduler (Task D Entering Times: Rec_Time=X Prev_Time=Y) Tid=D Curr_Time=X Q=2 Tid=B Q=1 Tid=A

  10. Best Scheduler Scheduling Policy • Earliest Deadline First. • Problems BEST Detects Period BEST Detects Period BEST Detects Period BEST Detects Period BEST Detects Period Deadline

  11. BEST + MDN • MDN Detects Misses in deadline. • Anti-cheat Policy • Based on block ratio MDN: Deadline Missed BEST Detects Period BEST Detects Period BEST Detects Period Deadline

  12. Be-Rate Scheduler • Rate based algorithm. • MDN Enhancement. • Anti-Cheat Policy.

  13. Experimental Evaluation • Measure performance by observing the number of deadlines missed with & without MDN. • Benchmark: 2 synthetic workload application • CPU bound, Soft real-time process. • Experiments done on Intel Pentium P3 and P4 processors.

  14. Results (Linux) Progress (CPU seconds) xxxxxxx xx xx xxx xx xxxxxxxx xxxx xxxx xxxxxx CPU-bound SRT-50% xx: Deadline Missed Time (seconds)

  15. Results (Linux + MDN) Progress (CPU seconds) xx x CPU-bound SRT-50% xx: Deadline Missed Time (seconds)

  16. Results (Linux) Progress (CPU seconds) xxxxxxx xx xx xxx xx xxxxxxxx xxxx xxxx xxxxxx CPU-bound SRT(25)-33% xx: Deadline Missed SRT(33)-33% xx: Deadline Missed xxxxxxx xx xx xxx xx xxxxxxxx xxxx xxxx xxxxxx Time (seconds)

  17. Results (Anti Cheating Policy) Progress (CPU seconds) CPU-bound Cheating Process Time (seconds)

  18. Results (Linux + MDN) Progress (CPU seconds) xx x CPU-bound SRT(25)-33% xx: Deadline Missed SRT(33)-33% xx: Deadline Missed xx x Time (seconds)

  19. Conclusion • Best-effort schedulers an attractive advantage. • MDN makes Best-effort schedulers more efficient. • Future work • Tuned to workloads. • Rate controlled MDN.

  20. Back Up Slides • BeRate • Rate based algorithm • Two Parameters • Task should complete e units of computation over p interval of time. • Schedules so that they can continue processing in the same rate as in the past. • Uses EDF scheduler.

  21. Back Up slides • Berate Scheduler • Given a fair share of processing Berate estimates period of the task so that task would meet its deadline using EDF scheduler • MDN is added. • If estmated deadline is earlier then the task will be serviced quicker.

  22. Backup Slides • Results of BEST, BeRate • BeRate performance improvement slighter • Best tasks misses many deadlines because of overload, Best cannot detect period but using MDN it is able to alleviate its effect.

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