Gang FTP scheduling of periodic and parallel rigid real-time tasks - PowerPoint PPT Presentation

Gang FTP scheduling of periodic and parallel rigid real-time tasks Jo¨ Vandy B erten el G oossens Universit´ e libre de Bruxelles RTNS 2010 V. Berten, J. Goossens (ULB) Gang FTP scheduling of real-time tasks RTNS 2010 1 / 26

Table of contents Introduction 1 Predictability 2 Schedulability test 3 Conclusions 4 V. Berten, J. Goossens (ULB) Gang FTP scheduling of real-time tasks RTNS 2010 2 / 26

Introduction Table of contents I Introduction 1 Model Predictability Problem Predictability 2 3 Schedulability test 4 Conclusions V. Berten, J. Goossens (ULB) Gang FTP scheduling of real-time tasks RTNS 2010 3 / 26

Introduction Introduction Our objectives: Explore the theory of parallel tasks , and Gang scheduling Provide schedulability tests for various kinds of Gang schedulers Why? Parallel tasks are coming on real-time/embedded systems (energy efficiency) Very few results in the literature V. Berten, J. Goossens (ULB) Gang FTP scheduling of real-time tasks RTNS 2010 4 / 26

Introduction Model Task model C v d r D n (rigid) parallel ( v ), periodic (T), constrained deadline (D ≤ T) tasks m identical processors V. Berten, J. Goossens (ULB) Gang FTP scheduling of real-time tasks RTNS 2010 5 / 26

Introduction Model Task model Preemptive Migrations Rigid (not malleable/moldable) V. Berten, J. Goossens (ULB) Gang FTP scheduling of real-time tasks RTNS 2010 6 / 26

Introduction Model Gang FJP scheduler 1 2 3 4 5 6 1 5 2 3 4 4 6 Gang FJP scheduler Pick the highest priority job 1 If it fits, start it now 2 Start again with the remaining jobs 3 V. Berten, J. Goossens (ULB) Gang FTP scheduling of real-time tasks RTNS 2010 7 / 26

Introduction Model Schedulability test Predictability Predictability Schedulable for WCET ⇒ Schedulable + Feasibility interval [ A , B ] Feasibility Interval Schedulable in [ A , B ] ⇒ Schedulable forever = Schedulability test Schedulability test Simulate the system in [ A , B ] with WCET V. Berten, J. Goossens (ULB) Gang FTP scheduling of real-time tasks RTNS 2010 8 / 26

Introduction Predictability Problem Deadline miss 4 1 5 1 2 2 6 5 3 4 4 6 3 3 ⇒ Gang FJP not predictable! One of the problems: priority inversion (slack introduces new preemptions) V. Berten, J. Goossens (ULB) Gang FTP scheduling of real-time tasks RTNS 2010 9 / 26

Predictability Table of contents I Introduction 1 Predictability 2 Possibilities Parallel Monotonic Idling FJP Scheduler Limited Gang FJP Scheduler Limited Slack Reclaiming Schedulability test 3 Conclusions 4 V. Berten, J. Goossens (ULB) Gang FTP scheduling of real-time tasks RTNS 2010 10 / 26

Predictability Possibilities Making the system predictable We propose several solutions making the system predictable: Avoiding priority inversion Not using the slack Using the slack “smartly” Two ways of doing so: Constraint the task system Constraint the scheduler V. Berten, J. Goossens (ULB) Gang FTP scheduling of real-time tasks RTNS 2010 11 / 26

Predictability Parallel Monotonic Parallel monotonic Parallel monotonic FJP assignment Larger job ⇒ Lower priority → High priority to small jobs 1 2 3 4 5 6 1 2 3 4 5 6 V. Berten, J. Goossens (ULB) Gang FTP scheduling of real-time tasks RTNS 2010 12 / 26

Predictability Parallel Monotonic Parallel monotonic 1 1 4 5 5 2 6 2 6 4 3 3 We avoid priority inversion! Theorem Parallel Monotonic systems are predictable V. Berten, J. Goossens (ULB) Gang FTP scheduling of real-time tasks RTNS 2010 13 / 26

Predictability Idling FJP Scheduler Idling FJP scheduler Idling FJP scheduler Just don’t use the slack! 1 1 5 1 5 2 2 3 4 4 6 3 4 4 4 6 Still priority inversions, but same behavior as in the WCET case Not work conserving! Theorem Idling FJP schedulers are predictable V. Berten, J. Goossens (ULB) Gang FTP scheduling of real-time tasks RTNS 2010 14 / 26

Predictability Limited Gang FJP Scheduler Limited Gang FJP scheduler Limited Gang FJP scheduler Pick the highest priority job 1 If it fits, start it now 2 If it fitted in step 2 , start again with the remaining jobs 3 3 3 6 1 5 1 2 2 4 4 5 6 We avoid priority inversion! Theorem Limited Gang FJP schedulers are predictable V. Berten, J. Goossens (ULB) Gang FTP scheduling of real-time tasks RTNS 2010 15 / 26

Predictability Limited Gang FJP Scheduler Limited Gang FJP scheduler 3 1 5 2 4 6 Limited Gang scheduler less efficient than “normal” Gang scheduler V. Berten, J. Goossens (ULB) Gang FTP scheduling of real-time tasks RTNS 2010 16 / 26

Predictability Limited Slack Reclaiming Limited slack reclaiming Gang FJP scheduler with limited slack reclaiming While there is no slack, behave as for Gang FJP scheduler 1 Use the slack to run ahead jobs narrower than the slack 2 4 1 5 1 5 5 5 6 2 2 3 4 4 6 3 4 4 6 Still priority inversions, but no “problematic preemptions” Theorem Gang schedulers with limited slack reclaiming are predictable V. Berten, J. Goossens (ULB) Gang FTP scheduling of real-time tasks RTNS 2010 17 / 26

Schedulability test Table of contents I Introduction 1 Predictability 2 3 Schedulability test Periodicity Feasibility interval Exact Schedulability Test Conclusions 4 V. Berten, J. Goossens (ULB) Gang FTP scheduling of real-time tasks RTNS 2010 18 / 26

Schedulability test Periodicity Periodicity of Gang FTP Theorem (Whatever) Gang FTP schedulers are periodic (using WCET) : With a period P def = lcm { T 1 , . . . , T n } Starting from S n , where: def ◮ S 1 = O 1 ; � S i − 1 − O i � � � def ◮ S i = max O i , O i + T i , ∀ i ∈ { 2 , 3 , . . . , n } . T i ⇒ Same as non-Gang systems! Going from sequential to parallel jobs did not change the periodicity V. Berten, J. Goossens (ULB) Gang FTP scheduling of real-time tasks RTNS 2010 19 / 26

Schedulability test Periodicity Periodicity O 1 = S 1 O 2 S 2 O 3 S 3 O 4 S 4 V. Berten, J. Goossens (ULB) Gang FTP scheduling of real-time tasks RTNS 2010 20 / 26

Schedulability test Feasibility interval Feasibility interval Theorem For any Gang FTP system (Parallel Monotonic, Idling scheduler, Limited Gang scheduler, Limited Slack reclaiming scheduler), we can use the following Feasibility interval : [ 0 , S n + P ] V. Berten, J. Goossens (ULB) Gang FTP scheduling of real-time tasks RTNS 2010 21 / 26

Schedulability test Exact Schedulability Test Exact schedulability test Predictability Predictability OK for Parallel Monotonic, Idling-, Limited Gang- and Limited Slack reclaiming scheduler + Feasibility Interval Feasibility interval [ 0 , S n + P ] = Schedulability test Schedulability test Simulate the system in [ 0 , S n + P ] with WCET V. Berten, J. Goossens (ULB) Gang FTP scheduling of real-time tasks RTNS 2010 22 / 26

Conclusions Table of contents I Introduction 1 Predictability 2 3 Schedulability test Conclusions 4 Future work Questions V. Berten, J. Goossens (ULB) Gang FTP scheduling of real-time tasks RTNS 2010 23 / 26

Conclusions Conclusions We strictly defined rigid , moldable and maleable reccurent tasks We provided (and proved) an exact schedulability test for several kinds of FTP Gang schedulers We studied the predictability of those schedulers V. Berten, J. Goossens (ULB) Gang FTP scheduling of real-time tasks RTNS 2010 24 / 26

Conclusions Future work Future work Extends our results to moldable and maleable tasks Sufficient RM-schedulability test for sporadic Gang scheduling . . . V. Berten, J. Goossens (ULB) Gang FTP scheduling of real-time tasks RTNS 2010 25 / 26

Conclusions Questions Questions? V. Berten, J. Goossens (ULB) Gang FTP scheduling of real-time tasks RTNS 2010 26 / 26