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On the Optimality of RM and EDF for Non-Preemptive Real-Time Harmonic Tasks. Mitra Nasri Sanjoy Baruah Gerhard Fohler Mehdi Kargahi. October 2014. Non-preemptive Scheduling. Benefits No context switches Cache and pipelines are not affected by other tasks

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On the Optimality of RM and EDF for Non-Preemptive Real-Time Harmonic Tasks


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    1. On the Optimality of RM and EDF for Non-Preemptive Real-Time Harmonic Tasks Mitra Nasri Sanjoy Baruah Gerhard Fohler Mehdi Kargahi October 2014

    2. Non-preemptive Scheduling • Benefits • No context switches • Cache and pipelines are not affected by other tasks • More precise estimation of WCET • Simpler mechanisms to protect critical sections • In some systems, preemption is either not allowed or too expensive • Benefits from the application’s point of view • Minimum I/O delay • (the delays between sampling and actuation)

    3. Existing Results • Non-Preemptive Scheduling is NP-Hard • For Periodic Tasks[Jeffay 1991] • For Harmonic Tasks[Cai 1996] • For Harmonic Tasks with Binary Period Ratio [Nawrocki 1998] ki = {1, 2, 4, 8, 16, …} Ti τi Ti -1 τi -1 Period Ratio

    4. Related Works • Schedulability test for npEDF, npRM, and Fixed Priority • [Kim 1980, Jeffay 1991, George 1996, Park 2007, Andersson 2009, Marouf 2010,…] • Heuristic scheduling algorithms • Clairvoyant EDF [Ekelin 2006], Group-Based EDF [Li 2007] • Optimal scheduling algorithms for special cases • [Deogun 1986, Cai 1996, Nasri 2014]

    5. Optimal Algorithms for Harmonic Tasks • [Deogun 1986]: An optimal algorithm if tasks have • Constant integer period ratio K ≥ 3 • [Cai 1996]:An optimal algorithm if tasks have • Constant period ratio K = 2, or • Integer period ratio ki ≥ 3 • Limitations • Linear time in the number of jobs, exponential time in the number of tasks! • Constructs an offline time table with exponential number of entries • An optimal scheduling algorithm is the one which guarantees all deadlines whenever a feasible schedule exists Period Ratio

    6. Optimal Algorithms, cont. • Precautious-RM [Nasri 2014]is optimal if tasks have • Constant period ratio K = 2 • Integer period ratio ki ≥ 3 • Arbitrary integer period ratio ki ≥ 1 and enough vacant intervals • Precautious-RMis online and has O(n) computational and memory complexity (in the number of tasks)

    7. Contributions • We study • The existence of a utilization-based test • The pessimism in the existing necessary and sufficient test • The efficiency of the recent processor speedup approach • Then for special cases of harmonic tasks we present • The schedulability conditions • A more efficient speedup factor

    8. System Model • Task set τ= {τ1, τ2, …, τn} • Ti is the period of τi • ci is the WCET of τi • ki is the period ratio of τi to τi-1 • Deadlines are implicit; Di = Ti Ti τi Ti -1 τi -1 Period Ratio

    9. Existence of a Utilization-Based Test • Is it possible to have a utilization-based test for non-preemptive scheduling of periodic tasks? [Bini 2003] RM (Hyperbolic Bound) [Liu 1973] EDF

    10. Impossibility of Finding a Utilization-Based Test • Utilization of a non-preemptive task set which cannot be scheduled by any clairvoyant scheduling algorithm, can be arbitrarily close to zero. 2 τ2:(2,1/ε) ε … ε~0 ⇒ U~0 1/ε missed τ1:(ε,1) 1

    11. Impossibility of Finding a Utilization-Based Test • It is impossible to find any relation between utilizations such that if it holds, schedulability of any scheduling algorithm is guaranteed. cn = 2(T1-cr) +ε At least one deadline miss τn We build an infeasible task set with those utilizations c1+ c2+ … + cn-1 c1+ c2+ … + cn-1 … … τ1 to τn-1 T1-cr T1-cr +ε • τ1 to τn-1: • T1= T1= … = Tn-1=an arbitrary value • c1= u1T1, c2= u2T2, … , cn-1= un-1Tn-1 τn: cn = 2(T1-cr)+ε Tn = cn/un cr = c1+ c2+ … + cn-1

    12. Is There Any Necessary & Sufficient Test? • [Jeffay 1991]: Necessary and sufficient conditions for the schedulability of periodic tasks with unknown release offsets (with npEDF): Yes! (for npEDF) Does it provide necessary conditions for task sets with known release offset?

    13. Pessimism in the existing schedulability test • For task sets with no or known release offset, Jeffay’s conditions are only sufficient. This task set is feasible by npEDF, but it is rejected in Jeffay’s test

    14. Processor Speedup Approach ci Processor with speed 1 … … τi τi Ti ci/S Processor with speedS [Thekkilakattil 2013]

    15. Applying the Existing Speedup Factor (in Harmonic Task Sets) • The speed S that guarantees the feasibility of a non-preemptive execution of a harmonic task set is upper bounded by • The proof can be done by finding the bound on the maximum possible execution time of a non-preemptive task! S ≤ 8 It may reduce U=1toU’=0.125

    16. We Focus on Harmonic Tasks • Can we find cases where npEDF and npRM are optimal? • Can we find better speedup factor?

    17. Schedulable and Non-schedulable Tasks Schedulable! Non-Schedulable! What if the execution times are limited to ci≤ T1 – c1?

    18. Is a Task Set with Limited Execution Times, Schedulable with npEDF? • If we have U ≤ 1 andci≤T1 – c1 can we guarantee schedulability? • Intuition: maximum blocking will be bounded to T1 – c1 • Is it enough?

    19. Limiting the Execution Times Is Not Enough • npRM and npEDF are not optimal for harmonic tasks with U≤ 1 and ci≤T1 – c1 This task set is infeasible The relation between periods cannot be ignored easily!

    20. Next Step: Finding Feasible Cases for npEDF • We can count the vacant intervals to make sure each task has its own place to be scheduled!

    21. Definition of Vacant Intervals c3= T1 – c1 • A vacant interval is constructed by the slack of τ1 • The number of vacant intervals is defined as V3 = 2 τ3 c2= T1 – c1 V2 = 3 τ2 c1 τ1

    22. Sufficient Schedulability Conditions for npRM and npEDF • npRM and npEDF have no deadline miss if in the harmonic task set we have • U ≤ 1 • ci≤T1 – c1 • Vi ≥1, 1 < i < n; and Vn ≥0 ki > 1 ci= T1 – c1 … … τi c3= T1 – c1 τ3 c2= T1 – c1 τ2 c1 τ1

    23. The New Speedup Factor • npEDF and npRM guarantee schedulability if • ci≤T1 – c1 and ki> 1, or • ci≤T1 – c1and Vi≥ 1 • [Deogun 1986] • ci≤ 2(T1 – c1) and K ≥ 3 • [Cai 1996] • ci≤ 2(T1 – c1) and K = 2 or ki≥ 3 Sis bounded to 2

    24. npEDF and npRM with speedup factor 2 are optimal for task sets with enough vacant interval or integer period ratio greater than 1 In ECRTS 2014, we have introduced a framework to construct customized harmonic periods. It can be used to increase the applicability of our results.

    25. Experimental Results

    26. Scheduling Algorithms • Non-preemptive RM (npRM) • Precautious-RM (pRM)[Nasri 2014] • Cai’s Algorithm (GSSP)[Cai 1996] • Group-Based EDF (gEDF) [Li 2007] • npEDF + Speedup Factor of [Thekkilakattil 2013] (TSP-EDF) • npEDF + Our Speedup Factor (OSP-EDF)

    27. Task Sets with ci≤ 2(T1 – c1) • Task sets • ci≤ 2(T1 – c1) • Vi≥ 1 • ki ∊ {1, 2, …, 6} • Parameter: u1 from 0.1 to 0.9 • OSP-EDF, TSP-EDF, and Precautious-RM have no misses. • gEDF has the highest amount of miss ratio. In this case, it is worse than npRM. • The goal is to show the efficiency of the speedup of TSP- and OSP-EDF.

    28. Conclusion Negative Results Non-existence of any utilization-based test npEDF npRM Pessimism in the existing test For tasks with known release time Inefficiency of the recent processor speedup approach for many cases of harmonic tasks

    29. Conclusion New Results Schedulability conditions with limited execution time npEDF npRM Extending those conditions to task sets with ki> 1 Deriving more efficient speedup factor when the execution time is not limited

    30. Questions Thank you

    31. An Efficient Speedup Factor • The speedup factor that guarantees feasibility of npRM and npEDF for task sets with U ≤ 1 andci≤ 2(T1 – c1) and Vi ≥1 (for 1 < i < n), andVn ≥0 is bounded to Sis bounded to 2

    32. Necessary Conditions of Schedulability of Non-preemptive Task Sets • U ≤ 1 • ci≤ 2(T1 – c1) We call it the Slack Rule

    33. Future Works • We can use a sort of packing in the tasks so that in the formula, each vacant interval can be occupied by different subset of tasks. • We might be able to show that the problem of finding the minimum number of Vis is NP-Complete because it can reduces to subset sum problem. T2 c2+ c5+ … + cj … … τ2,τ5, … , τj c1 … τ1 T1- c1

    34. Schedulability Analysis for Periodic Tasks • [Kim 1980]: Exact schedulability analysis for npEDF • [Jeffay 1991]: • Necessary and sufficient conditions for schedulability of npEDF for periodic tasks with unknown release phase • npEDF is optimal among non-work conserving algorithms • [George 1996, Park 2007, Andersson 2009]: Sufficient conditions for RM and FP algorithms • [Marouf 2010]: Schedulability analysis for strictly periodic tasks

    35. Heuristic Algorithms • Clairvoyant EDF [Ekelin 2006] • It looks ahead in the schedule and tries to … • Not optimal • Group-Based EDF[Li 2007] • Creates groups of tasks with close deadlines • Selects a task with the shortest execution time from a group with the earliest deadline • Efficient for soft real-time tasks • Not optimal

    36. Experiment Setup • General task sets with limited execution time • ci≤ 2(T1 – c1 ) • ki ∊ {1, 2, …, 6} • Parameter: Ufrom 0.1 to 0.9

    37. Task Sets with Limited Execution Times • TSP-EDF is only optimal algorithm (it uses S ≤ 8). • OSP-EDF has in average only 1% miss ratio, however, it cannot guarantee schedulability because of not having Vi ≥ 1 condition. • Precautious-RM is very efficient among other algorithms, yet it is not optimal. • Note: some of those task sets are infeasible.

    38. Experiment Setup • General task sets • ki ∊ {1, 2, …, 6} • Parameter: Ufrom 0.1 to 0.9 • (with uUniFast)

    39. General Task Sets • TSP-EDF is only optimal algorithm (it uses speedup). • OSP-EDF has in average only 0.02 miss ratio, however, it cannot guarantee schedulability • Precautious-RM is very efficient among other algorithms, yet it is not optimal. • Note: many of those task sets are infeasible.

    40. Experiment Setup • Feasible task sets • ci≤ T1 – c1 • Vi≥ 1 • ki ∊ {1, 2, …, 6} • Parameter: u1 from 0.1 to 0.9

    41. Feasible Task Sets • gEDF has a lot of misses. • GSSP cannot handle ki =1 • Others have no misses • Beforeu1=0.5, Cmaxis usually from other tasks, after thatc1becomes larger than others [Thekkilakattil 1013]

    42. The speed S that guarantees the feasibility of a non-preemptive execution of a harmonic task set is upper bounded by • For the proof we use necessary condition ci≤ 2(T1 – c1), thus: • Cmax is either 2(T1 – c1) or c1 • Dmin is T1 S ≤ 8 It may reduce U=1toU’=0.125

    43. In Harmonic Task Sets • npRM and npEDF are identical if the period is the tie breaker (for npEDF) RM and EDF are also identical From now on, any result for npRM is applicable on npEDF as well